The Ecology and Silviculture of Oaks

The Ecology and Silviculture of Oaks

Paul S. Johnson and Stephen R. ShifleyUS Department of Agriculture

Forest ServiceNorth Central Research Station

Columbia, MissouriUSA

and

Robert RogersCollege of Natural Resources

University of Wisconsin/Stevens PointStevens Point, Wisconsin

USA

CABI Publishing

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A catalogue record for this book is available from the British Library,London, UK.

Library of Congress Cataloging-in-Publication DataJohnson, Paul S.

The ecology and silviculture of oaks / Paul S. Johnson and Stephen R. Shifley, and Robert Rogers.

p. cm.Includes bibliographical references.ISBN 0-85199-570-5 (alk. paper)

1. Oak--United States. 2. Oak--Ecology--United States. I. Shifley,Stephen R. II. Rogers, Robert, 1941 - III. Title.

SD 397.O12 J64 2001634.9�721�0973--dc21 2001035883

ISBN 0 85199 570 5

Typeset in Melior by Columns Design Ltd, ReadingPrinted and bound in the UK by Biddles Ltd, Guildford and King’s Lynn

Contents

Preface xi

Acknowledgements xiii

Introduction 1Conflicting Environmental Philosophies 1Silviculture: a Consilient Discipline 5References 7

Part I. Ecology1 Oak-dominated Ecosystems 8

Introduction 8The Taxonomy of Oaks 9The Geographic Distribution of US Oaks 10Species ranges and groupings 10Distribution of oaks by hierarchically classified ecoregions 14Eastern Oak Forests 20The Northern Hardwood Region 20The Central Hardwood Region 26The Southern Pine–Hardwood Region 33The Forest–Prairie Transition Region 36Western Oak Forests 40The Southwestern Desert–Steppe Region 40The Pacific Mediterranean–Marine Region 43References 48

2 Regeneration Ecology I: Flowering, Fruiting and Reproduction Characteristics 54

Introduction 54Flowering 55Male flowers 55Female flowers 58Factors Affecting Acorn Production 61Weather 61Premature abscission 62Variation in acorn production 64

v

Acorn Predation and Dispersal 69Insects: destroyers of acorns 70Rodents: predation and dispersal 77Birds: predation and dispersal 81Oak Seedling Establishment 84Germination and initial establishment 84Early growth 86Seedling Sprouts 92Shoot dieback and root : shoot ratio 92Occurrence of shoot dieback 94Stump Sprouts and Related Growth Forms 98Definitions and origins 98Frequency of sprouting 100Sprout growth and mortality 101References 106

3 Regeneration Ecology II: Population Dynamics 117Introduction 117Regeneration Strategy 118Reproductive mechanisms: seeding and sprouting 118Accumulation of oak reproduction 121Fluctuation in population density 142Regeneration Potential 147Regeneration mode 148Modelling theory and objectives 155Stand-level regeneration models: purpose, problems and limitations 157References 158

Part II. Site Productivity and Stand Developmment4 Site Productivity 168

Introduction 168Measures of Site Productivity 169Relation of Site Productivity to Ecological Classification 171Productivity and Related Self-sustaining Properties of Oak Forests 172Effects of timber harvesting on site productivity 173Modifying site productivity through fertilization 175Methods of Evaluating Site Quality 176Site index 176Site evaluation alternatives to site index 185References 189

5 Development of Natural Stands 194Introduction 194Forest Canopy Layers 194Disturbance 195Disturbance type 196Disturbance size and frequency of occurrence 196Development of Even-aged Stands 199The stand initiation stage 201

vi Contents

The stem exclusion stage 203The understorey reinitiation stage 207The complex stage 215Development of Uneven-aged Stands 216Disturbance–Recovery Cycles 217References 224

6 Self-thinning and Stand Density 227Introduction 227Self-thinning 227Reineke’s model 227The �3/2 rule 229Stand Density and Stocking 235Terminology 235Maximum and minimum growing space 237Stand density diagrams 241References 251

Part III. Silviculture, Growth and Yield 2547 Even-aged Silvicultural Methods 254

Introduction 254Natural Regeneration Methods 255The clearcutting method 255The shelterwood method 274The seed tree method 277Artificial Regeneration Methods 278Oak nursery stock 278Oak plantation establishment 280Enrichment planting 284Intermediate Cuttings 294Definitions and theory 294Application 295Special Problems: Reducing Insect and Disease Impacts 305Gypsy moth 305Oak decline 314Oak wilt 316Economic, Environmental and Social Considerations 320The clearcutting method 320The shelterwood and seed tree methods 322References 322

8 Uneven-aged Silvicultural Methods 335Introduction 335The Single-tree Selection Method 337Principles of application 337Specifying the distribution 342Applicability to oak forests 352Group Selection Method 365Economic, Environmental and Social Considerations 372References 374

Contents vii

9 Silvicultural Methods for Multi-resource Management 380Introduction 380Oak Savannas 380Extent and characteristics 380Disturbance processes 383Managing oak savannas 385Managing Stands for Acorn Production 391Assessing and predicting acorn crops 392Effects of tree size and stand characteristics 395Guidelines for sustaining acorn production 399Old-growth Oak Forests 403Extent and characteristics 403Silvicultural options 404Forests in transition to old growth 408Old-growth forests at the landscape scale 409Aesthetics 410Stand-level aesthetics 411Landscape-level aesthetics 415References 417

10 Growth and Yield 424Introduction 424Growth of an Oak 425Annual phenology 425Diameter growth 426Height growth 432Survival rates 437Stand Growth 439Growth and yield in even-aged stands 439Growth and yield in uneven-aged stands 446Growth and Yield Models 447Modelling methods 447Stand-level models for oaks 448Stand table projection models 451Individual-tree-level models for oaks 451OAKSIM 452TWIGS 453Forest Vegetation Simulator 455Estimating ingrowth 455Model evaluation 459Volume Equations 459Regional Patterns in Oak Yield and Productivity 460References 461

AppendicesAppendix 1. Common and Scientific Names of Species 468Appendix 2. Forest Cover Types of Eastern United States

Dominated by Oaks or Oaks Mixed with Other Species 475Appendix 3. Forest Cover Types of Western United States

Dominated by Oaks or Oaks Mixed with Other Species 480

viii Contents

Appendix 4. Formulae for Converting Site Index for Indiana, Kentucky,Ohio, and West Virginia 483

Appendix 5. Converting Site Indexes for Four Regions 484Appendix 6. Converting Site Indexes, Yellow-poplar to Oak 486Appendix 7. Height/dbh Site Index Curves 487

Index 489

Contents ix

The earth is to be seen neither as anecosystem to be preserved unchanged nor asa quarry to be exploited for selfish and short-range economic reasons, but as a garden to becultivated for the development of its ownpotentialities for the human adventure.

(René Dubos, 1976)1

This book is written for forest and wildlifemanagers, ecologists, silviculturists, envi-ronmentalists, students of those fields, andothers interested in sustaining oak forestsfor their many tangible and intangible val-ues. The focus is on the oaks of the UnitedStates. Although the approach is funda-mentally silvicultural, it is based on thepremise that effective and environmentallysound management and protection of oakforests and associated landscapes shouldbe grounded in ecological understanding.Although the subject is inherently scien-tific and technical, we have striven to makeit generally accessible by minimizing theuse of technical jargon. Where technicalterms are necessary for efficient expressionof concepts, we have first defined them.

Much has been written about the ecol-ogy and silviculture of oaks. So much sothat the related body of literature repre-sents, in one sense, an informational‘embarrassment of riches’. The embarrass-ment derives primarily from the paucity ofsynthesis within and across two broadfields of study. The first is ecology, whichis the scientific study of the processes andrelations among organisms and between

organisms and their environment includingassociated energy transformations. The sec-ond is silviculture, which is the art and sci-ence of producing, tending, and sustainingforests. Although the literature on NorthAmerican oaks dates to the colonial period,most of it was written within the last 50years, and a large proportion of that withinthe last 25 years. However, much of this lit-erature resides in relatively obscure scien-tific and technical journals, proceedings ofprofessional and scientific meetings, gov-ernment publications, and other sourcesthat are often difficult to locate andretrieve. But even with ready access to thisdisparate information, its synthesis into anholistic framework of knowledge is adaunting task. This book attempts to ease,if not eliminate, those problems.

Although ecology has become a householdword, silviculture has not. Nevertheless, silvi-cultural practices have shaped the characterof the landscape wherever oaks and associ-ated forests occur, which includes much ofthe United States. Those practices often haveproduced negative public reactions and some-times even deleterious ecological conse-quences. Increasing economic demand foroak wood nevertheless makes timber harvest-ing and its aftermath an ever more conspicu-ous feature of the landscape. Moreover, thedistinction between designed silviculturalpractices and purely exploitative loggingpractices is not always apparent, especially tothe public.

Preface

1Symbiosis between the earth and humankind. Science 193(4252), 459–462 (1976).

xi

Contemporary philosophies on how oakforests and associated resources should bemanaged range from narrowly preservation-ist or narrowly utilitarian to more inclusiveand integrative multiple-value philosophies.One of the objectives of this book thereforeis to present ecological and silviculturalconcepts that can be used to address anarray of problems defined by various per-ceptions of how oak forests should betreated. The current trend in managingforests and forested landscapes is away froma narrow focus on sustaining timber andother commodity outputs and towards abroader philosophy of sustaining desiredecological states. This shift in the forestmanagement paradigm has been wrought byand is consistent with changing social val-ues, scientific advances in ecology and soci-ety’s increasing awareness of environmentalproblems and expressed concerns on howthose problems affect us collectively andindividually. Consistent with the new para-digm, this book is designed and intendednot so much as a how-to-do-it managementmanual as it is a source of ideas on how tothink about oak forests as responsiveecosystems. Armed with that understand-ing, we believe managers and conservatorsof oak forests will be better positioned toadapt to changing social values and simulta-neously to build and act on co-evolving eco-logical and silvicultural information.

The book is divided into three sections.The first contains three chapters on theecological characteristics and distributionof oak species and the various kinds of oakforests in the United States, differencesamong them and how they have been clas-sified, their natural development, and therelation of oak forests to environment andrelated environmental concerns. The nexttwo chapters on regeneration ecology pro-vide the critical interface between oak ecol-ogy and silviculture. Understanding theregeneration ecology of the oaks is para-mount to silviculturists because of wide-spread difficulties in regenerating and thussustaining oak forests.

The second section comprises threechapters covering site productivity andstand development. An understanding ofthe productive capacity of oak forests iscentral to a broad spectrum of issuesrelated to their management and potentiali-ties, not only for timber but also forwildlife and other values. The chapters onstand development, self-thinning and standdensity present concepts that are key to theapplication of silvicultural methods.

The third section comprises four chap-ters on silvicultural methods and the growthand yield of oak forests. Silivcultural meth-ods include traditional even-aged anduneven-aged methods as well as non-tradi-tional methods for multi-resource manage-ment and conservation. Regenerationmethods are discussed in relation to theapparent regeneration strategies that haveevolved in the oaks and how those strategiesvary among oak-dominated ecosystems. Theapproach to regeneration thus is less pre-scriptive and more ecologically principledthan that typically presented in silviculturetextbooks and ‘how-to’ guides. Throughoutthe book, accepted common names of treesfollow Little’s (1979) Check List of Nativeand Naturalized Trees of the United States.Scientific names of trees and other organ-isms are listed in Appendix 1.

We express our appreciation and indebt-edness to all the ecologists, foresters,wildlife biologists, soil scientists, entomol-ogists, pathologists and others, past andpresent, who have contributed to our col-lective knowledge of the oaks. We arehopeful that this compilation will makesome small contribution to a more ecosys-tem-centred approach to managing andconserving oaks in the many forests andplant communities in which they occur.

Paul S. JohnsonStephen R. Shifley

Robert RogersJuly 2001

xii Preface

The authors gratefully acknowledge the fol-lowing for their invaluable assistance, with-out which this book would not have beenpossible. For their technical assistance andfor creating a productive and congenialwork environment, we are indebted toNorth Central Research Station techniciansand support staffers Tim Bray, DianneBrooks, Kenneth Davidson, Laura Herbeck,Kevin Huen, James Lootens, MarilynMagruder, Allison Ramsey, Hoyt Richardsand Neal Sullivan. Special thanks areaccorded Lynn Roovers for countless hoursspent on graphics and other details, and toWilliam Dijak for his diligence and talent inpreparing the maps for Chapter 1. For self-lessly contributing their ideas, art work,photographs, reviews, and technical andscientific knowledge, we are deeply indebtedto USDA Forest Service foresters and scien-tists Robert Cecich, Daniel Dey, JeffreyGoelz, Gerald Gottfried, Kurt Gottschalk,David Graney, James Guldin, Jay Law,Edward Loewenstein, David Loftis, RobertMcQuilkin, Ross Melick, Paul Murphy,Felix Ponder, Ivan Sander, Susan Stout,Richard Teck, Gary Z. Wang, Dale Weigel,Daniel Yaussy and John Zasada. We alsothank all our partners in the University ofMissouri School of Natural Resources fortheir valuable advice and support. Specialthanks are accorded to Drs John Dwyer,H.E. Garrett, David Larsen, Bernie Lewis,

Nancy Loewenstein, Rose-Marie Muzika,Stephen Pallardy and Mr Dustin Walter fortheir reviews and ideas. A special thanks toDrs W. Carter Johnson, William Kearby andWalter Koenig for allowing us to reprinttheir marvellous photos, and Drs JeffreyWard (University of Connecticut), CarlRamm (Michigan State University), WilliamParker (Ontario Forestry Research Institute),Willard Carmean (Lakehead University,Thunder Bay, Ontario) and an anonymousCalifornia reviewer for their constructivereviews and suggestions. Thanks to thePioneer Forest of Salem Missouri and LeoDrey and Clint Trammel for allowing us touse their forest inventory data to evaluateuneven-aged oak silviculture. Thanks to theUSDA Forest Service and the North CentralResearch Station Assistant Directors DonaldBoelter and David Shriner, and ProjectLeader Frank Thompson, for allowing usthe freedom to pursue and complete theproject. Perhaps it goes without saying thatthe book could not have been written with-out the many researchers, past and present,who have contributed to the rich body ofliterature in oak ecology and silviculture;but we do not take their dedication andcontributions to forest science for granted.Finally, we thank our families for theirpatience, forgone summer vacations andenduring support.

Acknowledgements

xiii

Conflicting EnvironmentalPhilosophies

Ecology is the scientific study of the inter-relations among living things and theirenvironment. Ecological knowledge effectsan awareness of precarious interdependen-cies among the myriad organisms, largeand minuscule, between organisms andnon-living components of ecosystems, andthe pervasive human impacts that threatenthese relations. Ecology thus obviates ourdependency on, and our relation to, naturalprocesses and systems. Perhaps no sciencemore so than ecology has generated moreknowledge with implications relating toethics, morality and human behaviour.

In contrast, silviculture is the art andscience of tending forests to meet humanneeds. Because silviculture is usuallydirectly involved in the extraction of bio-mass, it produces disturbances along withassociated ecological side effects.Silviculture is thus based on the planneduse of controlled and directed disturbancesto achieve defined human objectives.Ideally, it should be based on scientificprinciples which ensure that specified sil-vicultural goals are consistent with pre-serving or improving a forest’s ecologicalqualities, are compatible with its naturaldynamic and thereby provide reasonableassurance of the forest’s sustainability.

Like its parent discipline, forestry, sil-viculture evolved out of 17th centuryEurope in response to purely utilitarianneeds, especially for the timber required

for sustaining the large naval armadasrequired for projecting colonial power inthe late 18th century. Paramount amongthese concerns in Britain and France wasa ready supply of pine and oak for shipmasts and hulls. However, in the UnitedStates, serious concern over a decliningforest resource did not occur until the late19th century. By then the forests of east-ern United States had been decimated byexploitative logging. A small but politi-cally influential group of conservationistsfeared the same would happen to thewestern forests. This prompted the settingaside of forest reserves in the early 1890sfrom what remained of the public domainin the west. In 1897, the Organic Act waspassed, which specified that the purposeof the reserves was ‘to improve and pro-tect the forest within the reservation, orfor the purpose of securing favourableconditions of water flows, and to furnish acontinuous supply of timber for the useand necessities of citizens of the UnitedStates’ (United States Congress, 1897).This landmark legislation specified thatthe forest reserves were intended for man-aged use, not for wilderness preservation.Following the recommendations of theAmerican Forest Congress of 1905, thereserves were transferred from theDepartment of Interior to the Departmentof Agriculture. Known as the Transfer Act,it provided that funds from the sale ofproducts or the use of land in the reservesbe used for managing and developing theforest reserve system.

Introduction

1

This change heralded the implementa-tion of an ambitious programme of scien-tific forest management under the directionof Gifford Pinchot, the first Chief of theUSDA Forest Service. At that time, forestrywas virtually an unknown discipline in theUnited States and forestry curricula in USuniversities were just emerging. Althoughpolitically controversial in its day, the con-servation movement was hailed by itsfounders as not only environmentally wise,but also economically beneficial (Pinchot,1987).

Pinchot and the founders of the earlyforest conservation movement envisioned ascientifically based forestry that would notonly provide conservation benefits butwould also result in the economic stabilityof rural communities in forested regions.Such benefits would accrue, they argued,from the application of scientificallyderived sustained yield principles, whichwould ensure for perpetuity the even flowof timber and other commodities originat-ing from the forest (Pinchot, 1987). Becausethe scientific underpinnings of sustainedyield were largely invested in silviculture,and because silviculture has historicallybeen justified on economic grounds, silvi-culture philosophically straddled agron-omy (i.e. growing trees as crops) andeconomics. However, modern silviculturehas been broadened to include not onlysustaining timber yields, but also sustain-ing non-commodity values including old-growth forests, biodiversity, wildlifehabitat and aesthetics. In this wider con-text, silviculture assumes application to apanoply of values that transcend economicutilitarianism.

Despite the differences between the twodisciplines, contemporary silviculture as ithas been applied to most North Americanforests, remains naturally allied with anddependent upon ecology for much of itsscientific underpinnings. The schismbetween silviculturists and some ecolo-gists nevertheless runs deep. One sourceof this disunion emanates from the ecolo-gists’ traditional focus on studying ecolog-ical processes in ecosystems largelyunaffected or minimally affected by

humans and drawing conclusions there-from. In contrast, silviculturists depend onscientifically based knowledge of distur-bance-mediated mechanisms to controland direct forest ecosystem processes forhuman benefit. Recovery from such distur-bances is predicated on the assumptionthat forests are inherently resilient, i.e.capable of rapidly returning to their previ-ous or other silviculturally directed state.

The silviculturist’s anthropocentricview of the forest is anathema to those whoadhere to the biocentric view, which ele-vates nature to a position superior tohuman self-interest (Devall and Sessions,1985; Chase, 1995; Ferry, 1995; Fox, 1995).The biocentrist’s agenda is centred onmaintaining ‘natural’ ecosystems, includ-ing forest, in states free from human inter-ference, and the need for establishing thepre-eminence of those states. From thatperspective, human-mediated disturbanceis seen as a disrupter of fragile ecosystemsand the intended order of things.Moreover, such disruptions can potentiallyproduce species extinctions and other irre-versible environmental effects. The bio-centric view therefore holds that the bestway to preserve nature, wherever somevestige of it remains, is to leave it alone(Devall and Sessions, 1985; Chase, 1995).Humans are viewed as just one of manyorganisms in the biosphere no moreimportant than any other – and like allcomponent organisms should be subordi-nate to the healthy functioning of theinteractive whole, i.e. the ecosystem.Biocentrism is therefore egalitarian amongorganisms and premised on an inherentright to life of all species and life forms.By extension, maintaining ecosystems intheir ‘natural’ state becomes a socialimperative. A biocentrist thus may viewsilviculture, along with other human inter-ferences in the development of forests, asecologically threatening, if not ruinous.The biocentric interpretation of the ‘mes-sage’ from ecology is thus at irreconcilableodds with the interpretation from silvicul-ture. Biocentrism nevertheless now occu-pies a position of social and politicalprominence (Chase, 1995).

2 Introduction

The connections between ecology and sil-viculture none the less are apparent andimportant, especially when silviculture isapplied to forests of natural origin. In thatsetting, silviculture by itself may not intro-duce new species or populations (i.e. newgenetic material) from outside the forest.Human energy expenditures are often limitedonly to those required in cutting and remov-ing trees. Such relatively non-intensive prac-tices have characterized the silvicultureapplied to oak forests of the United States.There, oak silviculture has largely followedan ecological model whereby forests are man-aged by directing their continually changingstates, or ecological successions, throughmanipulation of existing on-site vegetationand propagules. This approach relies on peri-odic timber harvesting and usually naturalregeneration to maintain or periodically re-create desired ecological states. It contrastswith the more intensive agronomic modelused in growing pine plantations and othermonotypes. The latter approach usuallydepends on artificial regeneration, the intro-duction of new and ‘improved’ genotypes,exotic species, and other intensive andenergy-expensive cultural methods like thoseused in agriculture, horticulture and agro-forestry (growing trees intermixed with agri-cultural or horticultural crops). Nevertheless,the silvicultural methods that have beenapplied to oaks span the entire range ofapproaches from ecological to agronomic.

In the public’s view, silviculture is anoften confusing and controversial subjectexacerbated by the claims of some environ-mentalists that it is an ecologically damag-ing enterprise that ‘seeks to accept “treefarms” in place of natural forests … Theusual approach … is to seek ever moreintensive management, which spawns evenmore problems’ (Devall and Sessions, 1985,p. 146). By comparison, there is seeminglylittle controversy and confusion over thereason to preserve something in its naturalstate free from human interference if it isotherwise threatened with extinction –even though the method or means ofpreservation may be debatable. Likewise,the reason for the cultivation and harvestof a corn field is easily understood and

accepted because of its purely utilitarianvalue, and its physical origins borne ofhuman endeavour. Socially, silviculture isa more complicated issue. It is vulnerablein appearance, conceptually and oftenphysically, seen as conforming to neitherpreservation nor agronomy. It is neitherfish nor fowl, yet is often identified as dis-ruptive if not exploitative of nature.

To the non-silviculturist, application ofthe ecological model to silviculture maysometimes be difficult to distinguish frompurely exploitative and environmentallydamaging practices. However, suchexploitation is not the intent of, nor does itconstitute, silviculture. Silviculture is notsynonymous with timber harvesting, yet isdependent upon it. The objective of modernsilviculture is to create and maintain forestsby design that produce material and non-material benefits to humans without sacri-ficing their sustainability. Silviculturalintentions nevertheless are not ecologicallyinfallible. A given silvicultural application,despite best intentions, may be inconsistentwith ecological realities because of ourincomplete knowledge and understandingof ecosystems. Poorly applied silviculturetherefore can produce unintended and neg-ative long-term ecological consequences.The possibilities for such outcomes imposeserious responsibilities on silviculturists inthe practice of their art and science.

When silviculture is applied to ‘natural’ecosystems, the intent, some would say, isto improve on nature by tinkering with it.But the biocentrist would argue thathumans cannot improve upon nature – anotion consistent with the theological viewthat ‘man cannot improve upon God’s handi-work’. And much ecological knowledge andtheory is purported to support that percep-tion. Perhaps it is the proximity of the exist-ing ‘near-natural’ state to the intendedsilviculturally created state that concernsthose whose sentiments might be to ‘leavewell enough alone’. The biocentrist mightargue that silviculture promises only ‘akinder, gentler rape of the forest’. Theseviews may be further bolstered by an aware-ness of the shrinkage of natural ecosystemsglobally and its consequences.

Introduction 3

The fragmentation of today’s landscapeinto discrete blocks of forests spatiallydetached from human development mayfurther reinforce the perception of the sep-aration of humans and forest. This outlookis reflected in the Latin origin of the wordforest, foris, which means outside. Thisetymology suggests a human view offorests evolving from deep historical andpsychological roots, and one in whichforests are functionally disconnected fromhumans. Even as late as the 18th century,the forest was perceived as something‘beyond’ the boundary of European culture(Bonney, 1996). Today, most of the US pop-ulation resides in urban areas. There,sources of basic human-sustainingresources are physically distanced fromand foreign to everyday experience. A per-ception of forests as functionally and spa-tially distant from humans may be furtherreinforced by the common acknowledge-ment that, in some cases, the physical sep-aration of humans from nature is necessaryto preserve rare or endangered species andhabitats. It is generally accepted that suchpreservation is a democratically mandatedfunction of government. The results arecommonly and favourably experiencedannually by millions of visitors to nationaland state parks, wildlife refuges, and desig-nated wilderness areas in national forestsand other federal lands. It is generallyunderstood that the role of humans there isrestricted to that of protector and spectator,but not interloper.

Contrasting with such models of theseparation of humans and nature is the his-torical relation between humans and oaks,which are characterized by connectedness.From the oak’s perspective, those connec-tions have produced both beneficial andharmful effects. The ecological evidence, aslater discussed, nevertheless indicates thatsustaining and thus preserving many oak-dominated ecosystems will require humanintervention. Humans and oaks have beenclosely associated throughout history.Before the arrival of Europeans, nativeAmericans set fires, both accidentally andintentionally, which often burned out ofcontrol over enormous areas (Grimm, 1983;

Pyne, 1982, 1997; Guyette et al., 1999).Periodic fires were repeated over centuriesin regions indigenous to the oaks, whichincludes much of North America. Their fre-quent occurrence created extensive areasof open-grown forests favourable to thesurvival of the relatively light-demandingbut fire-tolerant oaks. It was a disturbancecycle that, in time and space, is unlikely tobe repeated. Humans thus have had aprominent effect in shaping the nature andextent of the oak’s habitat, and perhapseven its evolution. But those events havebeen largely relegated to history. Much ofwhat today remains of the oak forests of theUnited States is a legacy of an earlier dis-turbance history that was partially, if notlargely, dependent on fire.

Unlike the ecologist, the silviculturisthas traditionally viewed forests from a util-itarian perspective that emphasized timberproduction. Accordingly, failure to harvestforests at their inherent sustainable capac-ity to produce wood (sustained timberyield) is deemed wasteful. A theologicalcounterpart is seemingly expressed by thebiblical admonition for man to exertdominion over the earth (Genesis 1:28). Aneconomic analogue is expressed in AdamSmith’s 1776 treatise on the inherent valueof the individual pursuit of economic self-interest (Smith, 1870). Collectively, thesebeliefs and values, largely borne of theEnlightenment, have dominated the think-ing and institutions of Western civilizationfor over 200 years.

Self-interest prevails among private for-est owners today, whether ownership goalsare economic or non-economic. To a lesserextent, economic objectives dominate themanagement of many publicly owned lands,including the national forests. In the past,agency mandates, operating budgets andincentives tied to timber sales, producedpowerful inducements to emphasize timberproduction, albeit within calculated sus-tained yield limits. Only within the lastfew decades has this philosophy beenseriously challenged. Such material utili-tarianism reduces forests to collections oftrees having only commercial value. Othervalues are consequently diminished. The

4 Introduction

American conservationist, Aldo Leopold(1966, p. 251) expressed concern for thisphilosophy by asserting that ‘… a system ofconservation based solely on economicself-interest is hopelessly lopsided. It tendsto ignore, and thus eventually to eliminate,many elements in the land community thatlack commercial value, but that are (as far as we know) essential to its healthyfunctioning.’

Silviculture: a Consilient Discipline

The practice of silviculture therefore iscaught in a web of competing values arisingfrom different philosophies, ranging frombiocentrism to economic utilitarianism.Unlike ecology, silviculture is directly con-nected to social institutions and conven-tions apart from science. Lying within itsparent discipline, forest management, it issubject to the legal and social constraints ofenvironmental law and policy operatingwithin democratic processes (at least in theUnited States and other democratic coun-tries where silviculture is practised). Withinthe context of democracy, silviculture istherefore socially integrative, i.e. in itsapplication it must consider values borne ofdiverse social and political interests.

Silviculture nevertheless lies at the coreof forest resource management because itsapplication results in direct physical actionon the forest. This is also where fundamen-tal scientific analysis is most needed.However, silviculture does not stand firmlyby itself as a scientific discipline. Thisresults in part from its strong connectionsto social and political institutions, and inpart from its interdisciplinary qualities as ascience. Within the biological domain ofscience, silviculture is most closely alliedto ecology. However, it is also heavilydependent on plant physiology and genet-ics, plant pathology, entomology, andapplied mathematics and statistics. Amongthe physical sciences, it borrows knowl-edge from geology, climatology, hydrologyand soil science. It is also closely allied toother resource management disciplinesincluding wildlife, fisheries, water and air

quality management. Silviculture thereforeis inherently scientifically integrative.

Silviculture consequently depends onlinking knowledge and theories acrossmany disciplines, both scientific and non-scientific, to form what Wilson (1998)terms ‘… a common groundwork of expla-nation’. If we accept that such linkagescomprise consilience, we might considerthat silviculture fits Wilson’s context, i.e. itcomprises a hybrid domain of knowledgein which consilience is implicit. Because ofsilviculture’s socioeconomic connections,this consilience extends to other branchesof learning including the social sciencesand humanities. These connections can berepresented by a series of concentric circlesrepresenting the social hierarchies withinwhich silviculture exists. With silvicultureat its centre, each ring of the social hierar-chy bounds all the great areas of knowl-edge, including the biological, physicaland social sciences as well as the humani-ties (Fig. I.1). This representation empha-sizes the consilient nature of silvicultureby placing it at the locus of all knowledgecomprising its context. It represents anideal, a unity of learning in which subjectsthat have been traditionally compartmen-talized are breached in Wilson’s words, to‘… provide a balanced clearer view of theworld as it really is … A balanced perspec-tive cannot be acquired by studying disci-plines in pieces but through pursuit of theconsilience among them … The enterpriseis important for yet another reason: it givesultimate purpose to intellect’ (Wilson,1998, p. 13).

Despite the complexities of silvicul-ture’s complete context, our intent in thefollowing pages is to present a synthesis ofthe ecological and silvicultural knowledgeof oak forests in the United States. It is not to resolve the environmental issues sur-rounding oak forests, which fall into thesocial, legal, political and managerialdomains represented by the concentriccircles surrounding silviculture in Fig. I.1.The silvicultural context is neverthelessbroad, and not limited to narrowly definedeconomic or commodity-production objec-tives. Consistent with the view of silviculture

Introduction 5

as a consilient discipline, we view the roleof the silviculturist as just one of manypossible players in the management of oakforests. Unlike the biocentrist, we infer nomoral imperative to create or maintain oakforests in specified states other than thosethat are perceived, as best we can discern,as sustainable, beneficial, and pleasing tohumankind, and that provide habitat forthe many plant and animal species natu-rally associated with oaks. We believethese goals are consistent with the philoso-phy of land stewardship and wise use asproposed by earlier generations of conser-vationists, from which the more recent phi-losophy of ecosystem management hasevolved. Our intention is to present infor-mation that can lead to an understandingof, and solutions to, silvicultural problems

related to oak forests. Moreover, we hopethat this information fosters an informedand amiable dialogue and trust amongforesters, land managers and owners, envi-ronmentalists, students and others inter-ested in oak forests.

The subject therefore is presented froma silvicultural perspective. The approachcomprises a comprehensive view of forestsas providing important social, spiritual andeconomic needs. Such an approachrequires anticipating and managing forchange, both predictable and unpre-dictable. This notion is consistent withBotkin’s (1990) call for a ‘new manage-ment’, wherein conservation and utiliza-tion of forest resources are compatibleparts of an integrated ecosystem approach.It contrasts with the ‘old management’ in

6 Introduction

Humanities Physicalsciences

Socialsciences

Biologicalsciences

Democracy

Environmental

law and policy

Forest

managem

ent

Forest

managem

ent

Environmental

law and policy

Democracy

Fig. I.1. Silviculture’s relation to other disciplines. Concentric circles represent the social hierarchy withinwhich silviculture exists in a democratic society. With silviculture at its centre, each ring of the hierarchybounds all the major areas of knowledge, including the biological, physical and social sciences, and thehumanities.

which conservation was too often subordi-nate to timber and other commodity pro-duction. The central concern of the newforest management, or ecosystem manage-ment (Salwasser, 1994), is the sustainabil-ity of forested ecosystems and associatedhuman values in a continually changingmosaic of landscape patterns. The resulting

management and silviculture thereforemust accommodate the complexities of theinevitably and continually changing eco-logical states that comprise a forested land-scape. It also recognizes that such changesoccur with or without human interference,and that we have both potentialities andlimitations in controlling these changes.

Introduction 7

References

Bonney, W. (1996) Troping trees. In: Schultz, K.L. and Calhoon, K.S. (eds) The Idea of the Forest.Peter Lang, New York, pp. 119–146.

Botkin, D.B. (1990) Discordant Harmonies. Oxford University Press, New York.Chase, A. (1995) In a Dark Wood: The Fight Over Forests and the Rising Tyranny of Ecology.

Houghton Miflin, Boston.Devall, B. and Sessions, G. (1985) Deep Ecology. Gibbs M. Smith, Layton, Utah.Ferry, L. (1995) The New Ecological Order. University of Chicago Press, Chicago.Fox, W. (1995) Toward a Transpersonal Ecology. State University New York Press, Albany, New York.Grimm, E.C. (1983) Chronology and dynamics of vegetation change in the prairie-woodland region of

southern Minnesota, U.S.A. New Phytologist 93, 311–350.Guyette, R., Dey, M. and Dey, D.C. (1999) An Ozark fire history. Missouri Conservationist 60, 4–7.Leopold, A. (1966). A Sand County Almanac. Ballantine, New York.Pinchot, G. ([1947] 1987) Breaking New Ground. Island Press, Washington, DC.Pyne, S.J. (1982) Fire in America. Princeton University Press, Princeton, New Jersey.Pyne, S.J. (1997) America’s Fires: Management on Wildlands and Forests. Forest History Society,

Durham, North Carolina.Salwasser, H. (1994) Ecosystem management: can it sustain diversity and productivity? Journal of

Forestry 92(8), 6–10.Smith, A. (1870) An Inquiry Into the Nature and Causes of the Wealth of Nations. London.United States Congress (1897) Surveying the public lands. US Statutes at Large 30, Ch. 2, pp. 32–36.Wilson, E.O. (1998) Consilience: The Unity of Knowledge. Knopf, New York.

1Oak-dominated Ecosystems

8

Introduction

A truly ecological perspective recognizes thathumans and their activities are part of nature,and that enhancing all aspects of their lives –including their surroundings – begins withcooperation between individuals, based onmutual trust … Rather than halting or reversingdisturbances, we should embrace change.Rather than excluding man from the garden, weshould welcome his cultivation of it.

(Alston Chase, 1995)

From earliest times, oaks have held a promi-nent place in human culture. Their useshave included wood for fuel, acorns for hogfodder and flour meal for human consump-tion, bark for tanning, wood strips for weav-ing baskets, charcoal for smelting ore,timbers for shipbuilding, mining timbers,railroad ties, pulpwood for paper, and lum-ber and laminates for furniture, panellingand flooring. Through the mid-19th century,oak was the wood of choice for shipbuildingin Europe and America. For that reason, oakforests and even individual trees weretreated as critical national assets. During theRevolutionary War, the poor condition ofthe British fleet, which lacked replacementsand repairs due to shortages of suitable oaktimbers, may have contributed to the war’soutcome (Thirgood, 1971). In the 17th cen-tury, alarm over the depletion of timber sup-plies, especially oak, prompted passage andenforcement of laws mandating the protec-tion, culture and establishment of forests inseveral European countries. In turn, thoseevents influenced the development of scien-tific silviculture, as we know it today.

Modern as well as ancient man has ben-efited from the oak’s relation to wildlife.Wherever oaks occur as a prominent featureof the landscape, wildlife populations riseand fall with the cyclic production ofacorns. Numerous species of birds andmammals are dependent on acorns duringthe food-scarce autumn and winter months.Even human cultures have relied on oaks asa staple food. Acorns were an importantpart of the diet of Native Americans inCalifornia before the 20th century (Kroeber,1925) (Fig. 1.1). Today, the ecological roleof oaks in sustaining wildlife, biodiversityand landscape aesthetics directly affects thequality of human life.

The demand for wood products fromoaks nevertheless continues to increase andcompete with other less tangible values.Some have proposed that forests, includingthose dominated by oaks, are best allowedto develop naturally, free from human dis-turbance. What should the balance beamong timber, wildlife, water, recreationand other forest values? Is there some middle ground that adequately sustainsmultiple goals? Informed answers and per-spectives require an understanding of theecology of oaks and the historical role thathumans have had in that ecology, especiallythe comparatively recent role of humans inthe ‘protection’ of oak forests from fire. Aprerequisite to such understanding is a gen-eral knowledge of the oak’s geographicaloccurrence, taxonomic diversity, adapta-tions to diverse environments, and the his-torical changes in its environment.

The Taxonomy of Oaks

Taxonomically, the oaks are in the genusQuercus in the family Fagaceae (beechfamily). The Fagaceae probably originatedin the montane tropics from which itsmembers migrated and diverged into thecurrent living genera by the late Cretaceousperiod (about 60 million years ago)

(Axelrod, 1983). By that time, mammalsand birds had only recently evolved. Rapidspeciation of oaks commenced in the mid-dle Eocene epoch (40–60 million yearsago). This was in response to the expansionof drier and colder climates, and subse-quently to increased topographic diversityin the late Cenozoic era (< 20 million yearsago) and fluctuating climates during theQuaternary period (< 2 million years ago)(Axelrod, 1983).

Their fruit, the acorn, distinguishes theoaks from other members of the beech fam-ily (e.g. the beeches and chestnuts). Withone exception, all plants that produceacorns are oaks. The exception is the genusLithocarpus, which includes the tanoak ofOregon and California. Although repre-sented by only one North Americanspecies, Lithocarpus is represented by100–200 species in Asia (Little, 1979).Lithocarpus may be an evolutionary linkbetween the chestnut and the oak (McMinn,1964; cf., Miller and Lamb, 1985, p. 200).

Worldwide there are about 400 speciesof oaks, and they are taxonomicallydivided into three groups: (i) the red oakgroup (Quercus section Lobatae1); (ii) thewhite oak group (Quercus sectionQuercus2); and (iii) the intermediate group(Quercus section Protobalanus3) (Tucker,1980; Nixon, 1997). All three groupsinclude tree and shrub species. The redoaks and white oaks include evergreen anddeciduous species, whereas the intermedi-ate oaks are all evergreen. The red oaks arefound only in the Western Hemispherewhere their north–south range extendsfrom Canada to Colombia. In contrast, thewhite oaks are widely distributed acrossthe Northern Hemisphere. The intermedi-ate group comprises only five species, all ofwhich occur within southwestern UnitedStates and northwestern Mexico. Many ofthe world’s oaks occur in regions with aridclimates, including Mexico, North Africaand Eurasia, where they are often limitedin stature to shrubs and small trees. About

Oak-dominated Ecosystems 9

Fig. 1.1. Native American collecting acorns asshown in Hutchings’ California Magazine in 1859.Acorns were a staple food of most California tribesbefore the end of the 19th century. They weregathered in conical woven baskets, which couldhold a bushel or two of the nuts. Although theacorns of many species were eaten, favouredspecies were California black and California liveoaks (Pavlik et al., 1991). After removing the shell(pericarp), acorns were ground into a flour,leached of tannins by soaking in running water,and then used to make a variety of foods includingporridge and bread. Acorns were so highly valuedthat they sometimes provoked inter-tribal ‘acornwars’. They were also widely utilized as food byNative Americans in the eastern United States.(Courtesy of the Bancroft Library, University ofCalifornia, Berkeley.)

1 Subgenus Erythrobalanus in earlier classifications.2 Subgenera Lepidobalanus and Leucobalanus in earlier classifications.3 Subgenus Protobalanus in earlier classifications.

10 Chapter 1

80% of the world’s oaks occur below 35°north latitude and fewer than 2% (six orseven species) reach 50° (Axelrod, 1983).

The most reliable distinction betweenthe white oaks and red oaks is the innersurface of the acorn shell. In the white oaksit is glabrous (hairless) or nearly so,whereas in the red oaks it is conspicuouslytomentose (hairy or velvety) (Tucker, 1980).In the intermediate group, this characteris-tic is not consistent among species. Theleaves of the white oaks are usuallyrounded and without bristle tips whereasthe leaf lobes of the red oaks are usuallypointed and often bristle-tipped. To manysilviculturists, ecologists and wildlife biol-ogists, the most important differencebetween the white oaks and red oaks is thelength of the acorn maturation period.Acorns of species in the white oak grouprequire one season to mature whereasspecies in the intermediate and most of thered oak group require two seasons. Thewhite oaks and intermediate oaks are char-acterized by the presence of tyloses (occlu-sions) in the latewood vessels (water-conducting cells) whereas tyloses areusually absent in the red oaks. These vessel-plugging materials confer greater decayresistance to the wood of the white andintermediate oaks than the red oaks. Othermorphological features that differentiatethe three groups and species within themare presented in various taxonomic treat-ments (e.g. Tucker, 1980; Jensen, 1997;Manos, 1997; Nixon and Muller, 1997) andfield identification guides (e.g. Miller andLamb, 1985; Petrides, 1988; Petrides andPetrides, 1992). These sources also includerange maps. In addition, the Silvics ofNorth America, Vol. 2 (Burns and Honkala,1990) provides information on the silvicsand geographic ranges of 25 oaks.

Of the more than 250 oak species occur-ring in the Western Hemisphere, the largestnumber occurs in Mexico and CentralAmerica. About ten species occur inCanada. For the United States species, themost complete and authoritative taxonomictreatment of the oaks is in the Flora ofNorth America North of Mexico, Vol. 3(Flora of North America Editorial

Committee, 1997), which lists 90 species ofoaks native to the continental UnitedStates. However, we follow the taxonomicnomenclature of Little’s (1979) Checklist ofUnited States Trees because of its wide-spread use in North American forestry lit-erature. This checklist recognizes 58 nativeoak species plus nine varieties. Of these,about ten species are shrubs or shrub-likeforms. More than 80 hybrids also havebeen described (Little, 1979; Tucker, 1980).

The Geographic Distribution of US Oaks

Species ranges and groupings

The oaks are widely distributed across theUnited States (Fig. 1.2). According to Little(1979), about 40 species and varieties occureast of the 100th meridian and about 30species and varieties occur to the west.Only two species, chinkapin oak and buroak, are common to both regions. Bur oakextends to the northwest whereaschinkapin extends to the southwest beyondthe 100th meridian. The western oaks fallinto three geographically distinct groups.One group is comprised of the west Texasoaks (nine species and varieties), and a sec-ond includes the southwestern oaks (16species) that occur in New Mexico,Arizona, Utah, Colorado and Nevada. Athird group is comprised of the PacificCoast oaks (about 13 tree species plus sev-eral shrubby species) occurring largely inCalifornia, Oregon and Washington.

Within the United States, numbers ofoak species vary regionally. Based on acount of the number of oak species thatoccur within 6000 square mile areas, oakspecies ‘richness’ reaches a maximum of 20species in the southeast (Aizen andPatterson, 1990) (Fig. 1.3). There, theranges of several narrowly distributedNorth American oak species overlap withthe ranges of several widely distributedspecies. Although the range of an oakspecies is positively correlated with itsacorn size, the reason for this is unknown(Aizen and Patterson, 1990).

Forest cover types (or simply covertypes) are combinations of tree species thattend to spatially reoccur at stand-levelscales (e.g. < 100 acres). The resulting cate-gories are thus silviculturally useful in dif-ferentiating among different kinds of oakstands. Categorization of United Statesforests based on defined cover types wasbegun by the Society of American Forestersin 1929. There are 145 defined cover typesin the United States and Canada (Eyre,1980). These include 31 with ‘oak’ in thecover type name or in the list of speciesthat define the type (Appendices 2 and 3).Of these, 23 oak types occur east and eightoccur west of the 100th meridian. In addi-tion, many of the non-oak cover typesinclude one or more oak species as com-mon associates.

The geographic extent of individualcover types ranges from tens of millions ofacres (e.g. the white oak–black oak–northernred oak cover type of the eastern US) to rel-

atively restricted areas (e.g. the northernpin oak cover type of the upper Lake Statesand the Mohr oak cover type of Texas andOklahoma). Other types such as the liveoak type of the South and the bur oak covertype in the Great Plains occur within longnarrow belts associated with coastal plainsand river corridors, respectively. Many ofthe western oak cover types, especiallythose in California, form belts that followthe Coastal and Sierra Nevada mountainranges and foothills surrounding theCentral Valley.

Oaks occur in environments rangingfrom extremely wet and humid (e.g. theovercup oak–water hickory cover type ofsouthern flood plains), to mesic (moist)upland forests receiving 50 or moreinches of precipitation per year (e.g. theyellow–poplar–white oak–northern redoak cover type), to Mediterranean climates that receive 10 inches or less precipitation per year (e.g. the blue

Oak-dominated Ecosystems 11

100

M241

241

M241

332

263M261

261

M262

262

323

M341

342

341

M332

M331

M341 342

311

M311

M334

341

332

M334M341

331

M334M333

M311

314

313

321

252

232

251

M222

232

231

M231

232

211

22tb

211

211M21tb

211

221a

221a

M221221b

231

411

Fig. 1.2. The distribution of oaks in conterminous United States. The shaded areas represent the aggre-gated vegetation cover types within which oaks frequently occur as important species at a scale of 1 km2.The 100th meridian demarcates the approximate division between eastern and western oaks. Generatedfrom Advanced Very High Resolution Radiometer satellite images (1990) and an associated system ofland cover classification (USDA Forest Service, 1993; Powell et al., 1994). Ecoregion boundaries are fromBailey (1997). See Tables 1.2 and 1.3 for index to numbered ecoregions and oak species found in each.(Map compiled by W.D. Dijak, USDA Forest Service, North Central Research Station, Columbia, Missouri.)

oak–digger pine cover type). Oaks occurin even drier climates where they formshrub vegetation such as the chaparral ofsouthern California and the semi-desertscrub woodland vegetation of the interiorsouthwest. Western cover types such asthe canyon live oak cover type includeclosed-canopy stands in the northern partof their range and savanna-like woodlandsin the south. Oak forests therefore rangefrom closed canopy upland and lowlandforests with trees greater than 120 ft tall toxeric (droughty) scrublands dominated bydwarf trees and shrubs.

Some oaks, such as Georgia oak andMcDonald oak are confined to very smallgeographical ranges and a narrow range ofhabitat conditions. Others such as white

oak are widely distributed and occur over abroad range of climates and habitat condi-tions. A species’ flexibility in occupyingdifferent habitats is implicit in the defini-tion of species niche. The term denotes thespecific set of environmental and habitatconditions that permit the full develop-ment and completion of the life cycle of anorganism (Helms, 1998). The oaks occupymany niches because of the wide range ofenvironmental conditions within which theycan collectively occur. However, the nicheof an individual species is more limited.Niche differentiation among the oaks andassociated species is often evident from theway species segregate along environmentalgradients such as the soil moisture gradient(Fig. 1.4). Oaks also differ in their ecologi-

12 Chapter 1

Fig. 1.3. The geographic distribution of numbers of oak species in eastern United States and Canada.The isolines were drawn from a grid comprised of 78 � 78 square mile cells within which the number ofoak species were counted based on Little’s (1971, 1977) range maps. The greatest concentration of oakspecies (15 to 20) occurs in the southeastern United States where the ranges of several narrowly distrib-uted species overlap the ranges of several widely distributed species. (Redrawn from Aizen andPatterson, 1990, used with permission.)

cal amplitude, i.e. the range of habitat con-ditions that a species can tolerate (Allaby,1994). The ecological amplitude of aspecies often forms a bell-shaped curvewhen illustrated diagrammatically (Fig.1.4). However, some species, such as buroak, occur in both bottomlands and dryuplands but are nearly absent at intermedi-ate points along the moisture gradient(Curtis, 1959; Johnson, 1990).

The species composition of forests iscontinually changing as a result of forcesboth internal (autogenic) and external (allo-genic) to the forest. Changes are often grad-ual and frequently result in thereplacement of one tree species by anotherin the process of ecological succession. Thevegetation and other organisms within theforest thus effect autogenic change. Forexample, shade-tolerant species growingbeneath the main forest canopy may gradu-ally replace dominant species of lessershade-tolerance that are unable to regener-ate under their own shade. In contrast,allogenic change occurs as a result of

changes in climate, defoliation by exoticinsects and pathogens, the movement ofsoil by wind and water, or from otherforces originating outside the forest.Autogenic and allogenic factors sometimesjointly affect the direction and rate of suc-cession. Moreover, disturbances such aswindthrow, insect and disease outbreaks,and timber harvesting can accelerate suc-cession or alter its direction.

Although the oaks are relatively intoler-ant of shade, species vary substantially inthis attribute. In some habitats, oaks arevulnerable to successional replacement bymore shade tolerant species. Compared tomany of their competitors, oak seedlingsgrow more slowly during their first fewyears after initial establishment. Whenyoung oaks are overtopped and heavilyshaded by other vegetation, few survive for very long. On the other hand, the oakstend to be relatively drought tolerant, andoften survive in habitats that limit thedevelopment of species of lesser droughttolerance. Oaks also can produce vigorous

Oak-dominated Ecosystems 13

Blackoak White

oak

Northernred oak Sugar

maple

Americanbeech

Buroak

Xeric MesicCompositional index

Impo

rtan

ce v

alue

Fig. 1.4. Changes in the relative importance of six tree species in the upland forests of southernWisconsin in relation to the regional soil moisture gradient. Species’ importance is quantitativelyexpressed by an importance value, which is an index of species’ importance based on its frequency ofoccurrence, density and basal area relative to other species within a stand. Although there is much over-lap among species’ importance value curves, no two species behave exactly the same way with respectto the moisture gradient. The length of the gradient spanned by a species’ range of importance valuestogether with the shape of its importance value curve reflects its niche with respect to the gradient.Importance value curves also define the ecological amplitude of a species, i.e. the range of conditions itcan tolerate and the magnitude of its importance in relation to the gradient under the prevailing (i.e. relativelyundisturbed) stand conditions. The moisture gradient shown is inferred from the species composition of aseries of relatively undisturbed stands (see Curtis, 1959). (Adapted from Curtis, 1959, used with permission.)

sprouts that often outgrow competitors.The balance of these factors thus determinethe relative permanence of oaks within agiven cover type.

In the eastern half of the United States,oaks are often relatively permanent membersof cover types on drier sites. In the absence ofdisturbance, many of the pine and oak–pinecover types occurring on dry habitats are suc-cessional to oaks because the oaks are some-what more shade tolerant than the pines.This successional pattern creates silviculturalproblems in maintaining pure pine stands inthe south and other regions where oaks andpines co-occur (Burns and Barber, 1989). Inbottomlands and mesic uplands, shade-tolerant or faster growing species often suc-cessionally displace the oaks. Such displace-ment creates silvicultural problems inperpetuating oaks in these forests.

The relative permanence of an oakspecies within a given cover type (i.e. itsresistance to successional replacement byother species) is likely to be highly variableif the cover type spans a broad range of envi-ronments. For example, the white oak covertype occurs across a wide range of site con-ditions from dry to moist. Whereas the typetends to be relatively permanent on dry sites,it is successional to other types on the moremesic sites. Cover type designations,although useful, largely fail to consider theseand other ecological factors that determinechanges in species composition and howthose changes vary spatially (e.g. in relationto climate and site quality), and temporally(e.g. in relation to plant succession and dis-turbance). Consequently, two or more standsrepresenting a single cover type may repre-sent quite different ecologies with respect tothe successional status of oaks, physicalenvironment, understorey vegetation, forestregeneration, fauna and other factors.

Forest inventories and satellite imageryhave been used to describe the geographicdistribution of forest types in the UnitedStates (e.g. Fig. 1.5). These maps identifybroad cover type groups that are aggregatesof the stand cover types described above.

Four groupings widely used to delineateoak forests at the regional scale are: theoak–hickory group, the oak–pine forestgroup, the oak–gum–cypress group (bot-tomland forests), and the western hard-wood group that includes the western oaksas a subset (Fig. 1.5). However, the namescommonly applied to the resulting speciesaggregations can be misleading. For exam-ple, hickory is absent throughout much ofthe northern part of the range delineated asoak–hickory (Fig. 1.5). Moreover, other for-est cover types dominated by oaks are alsoincluded within the delineated oak–hickoryarea. The term ‘oak–hickory’ neverthelessis widely used in reporting forest resourcestatistics at the regional level even thoughit is an ecologically imprecise term. Oaksalso occur as ecologically and silvicultur-ally important components of many non-oak forests (e.g. pine forests andmaple–beech–birch forests).

In the eastern United States the oak–hickory, oak–pine and the oak–gum–cypresscover type groups collectively covered 187million acres or 52% of the timberland4 in1997. That is an increase from 162 millionacres and 45% of eastern timberland in1953 (USDA Forest Service, 2000). At 124million acres, the oak–hickory group is thelargest cover type in the United States. Thewestern oaks are also significant geographi-cally and ecologically. Western hardwoodforests (including oaks, tanoak, red alderand aspen) cover 43 million acres or 12%of western forestland. Oaks comprise about23% of the cubic volume of growing stocktrees in the eastern United States and about1% in the western United States (USDAForest Service, 2000).

Distribution of oaks by hierarchicallyclassified ecoregions

Climate and landform strongly influence thedistribution of oaks. Locally, the distribu-tion of oaks is influenced by factors such asphysiography, soil moisture and geology.

14 Chapter 1

4 Timberland is forest land that is producing, or is capable of producing, more than 20 feet3 acre–1 year–1 ofindustrial wood crops under natural conditions, and that is not withdrawn from timber use, and that is notassociated with urban or rural development. Currently inaccessible and inoperable areas are included.

Oak-dominated Ecosystems 15

(a)

(b)

(c)

Oak–hickory

Oak–gum–cypress

Oak–pine

Western hardwoods

Long. 100240

260

320

340

310

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210 210

220

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240

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320

340

310

330

250

210 210

220

230

410

240

260

320

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310

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250

210 210

220

230

410

Fig. 1.5. The major areas of oak–hickory, oak–pine, oak–gum–cypress, and western hardwoods(shaded areas) by state and ecoregion Divisions. In the western US, the map shows the composite west-ern hardwood group that includes oaks, tanoak, red alder, cottonwood and aspen. Numbered ecoregionboundaries on the map are from Bailey (1997) and are summarized in Tables 1.2 and 1.3. Generatedfrom Advanced Very High Resolution Radiometer satellite images at a scale of 1 km2 (1990) and anassociated system of land cover classification (USDA Forest Service, 1993; Powell et al., 1994). (Mapcompiled by W.D. Dijak, USDA Forest Service, North Central Research Station, Columbia, Missouri.)

These and other factors have been used tostructure a hierarchical ecological classifica-tion system (McNab and Avers, 1994; Bailey,1995, 1997, 1998). This system recognizesthe increasing detail necessary to explainthe spatial arrangement of forests at increas-ingly smaller spatial scales (Table 1.1). Itthus provides an objective basis for theregional delineation of ecosystems into suc-cessively smaller and more homogeneousunits.

The hierarchical ecological units rangein size from continents to a few acres. Thelarger units are often referred to as ecore-gions; the smallest units are often equiva-lent to forest stands. Domains, Divisionsand Provinces form the larger ecoregions(Table 1.1). These are climatic and cli-matic–physiographic regions that covermillions to tens of thousands of squaremiles. Provinces are further subdivided intosmaller units termed Sections, Subsections,Landtype Associations (LTAs), Ecological

Landtypes (ELTs) and Ecological LandtypePhases (ELTPs). These units range in sizefrom thousands of square miles for Sectionsto less than 10 acres for some EcologicalLandtype Phases. Ecological Landtypes andEcological Landtype Phases are importantsilviculturally because they often corre-spond to individual stands, which are theobjects of silviculture.

The oaks occur in all three Domains(major climatic regions) of the 48 contigu-ous states: Humid Temperate, Dry andHumid Tropical (Bailey, 1997). The latteroccurs only in the southern tip of Florida.The three Domains are further subdividedinto 11 climatic Divisions. Within eachDivision, mountainous areas with eleva-tional zonation of vegetation are also iden-tified. Although oaks naturally occur in all11 of the Divisions, within each Divisionthe distribution of the four major oak foresttypes is closely related to Division bound-aries (Fig. 1.5; Tables 1.2 and 1.3).

16 Chapter 1

Table 1.1. Hierarchy of ecological units used to classify forest ecosystems in the United States.a

Ecological unit Scale (reference size)b Delineating factorsc

Domain Millions to tens of thousands of Macroclimate, ocean temperature square miles (subcontinent) and currents, geomorphology

Division Millions to tens of thousands of Geomorphology, climatesquare miles (multi-state)

Province Millions to tens of thousands of Geomorphology, climatesquare miles (multi-state, state)

Section 1000s of square miles Geomorphology, climate, vegetation(state, multi-county, National Forest)

Subsection 10s to 100s of square miles Geomorphology, climate, vegetation(multiple counties, National Forest Ranger District)

Landtype association 10s to 1000s of acres Landforms, species composition of (landscape, watershed) overstorey, soil associations

Ecological landtype 10s to 100s of acres Landform, natural vegetative (multiple stands) communities, soils

Ecological landtype 1 to 10s of acres Soils, landscape position, natural phase (stand) vegetative communities

a Adapted from McNab and Avers (1994), Bailey (1995) and Cleland et al. (1993); see also Figs 1.2, 1.5and 1.6. b Indicates a familiar unit of comparable size for reference purposes. This reference unit is not used todelineate the ecological unit.c Some of the factors used to distinguish among ecological units at a given level. Classificationcomplexity typically increases with decreasing unit size.

The 11 ecoregion Divisions within theconterminous United States are furthersubdivided into 44 Provinces (Fig. 1.2).Provinces are delineated based on broadvegetation groups and related regionallandforms. Oak forests and woodlandscommonly occur in 23 Provinces (Tables1.2 and 1.3). Province boundaries are use-ful in delineating oak distributions in some

parts of the United States. For example,Province boundaries correspond with thespatial distribution of the oak forests andwoodlands encircling California’s CentralValley. Province boundaries also separatethe oak–pine forests of the Piedmont(Province 232) from the wetter oak habitatsof the Coastal Plain and the lowerMississippi flood plain (Province 231 and

Oak-dominated Ecosystems 17

Table 1.2. The ecoregion domains, divisions and provinces in the eastern conterminous United Stateswhere oaks are found and the principal species occurring in each. Ecoregions from Bailey (1995).Division and province boundaries are shown in Fig. 1.2.

Division Province

– – – – – – – – – – – – – – – – – – – 200 Humid Temperate Domain – – – – – – – – – – – – – – – – – – –

210 Warm Continental 211 Mixed deciduous coniferous forestsM210 Warm Continental Mountains M211a Mixed forest–coniferous forest–tundra, medium

M211b Mixed forest–coniferous forest–tundra, high

10 oak species: bear, black, bur, chestnut, chinkapin, n. pin, n. red, scarlet,swamp white, white

220 Hot Continental 221a Broadleaved forests, oceanic221b Broadleaved forests, continental

M220 Hot Continental Mountains M221 Deciduous or mixed forest–coniferous forest–meadowM222 Broadleaf forest–meadow

22 oak species: basket, bear, black, blackjack, bur, cherrybark, chestnut, chinkapin,n. pin, n. red, overcup, pin, post, scarlet, shingle, Shumard, s. red,swamp chestnut, swamp white, water, willow, white

230 Subtropical 231 Broadleaved–coniferous evergreen forests232 Coniferous–broadleaved semi-evergreen forests

M230 Subtropical Mountains M231 Mixed forest–meadow province

31 oak species: Arkansas, bear, black, blackjack, bluejack, bur, Chapman, cherry-bark, chestnut, chinkapin, Durand, Georgia, laurel, live, myrtle,Ogelthorpe, n. red, Nuttall, overcup, pin, post, scarlet, shingle,Shumard, s. red, swamp chestnut, swamp white, turkey, water,white, willow

250 Prairie 251 Forest-steppes and prairies province252 Prairies and savannas province

20 oak species: black, blackjack, bluejack, bur, chinkapin, Durand, live, n. pin, n.red, overcup, laurel, pin, post, s. red, shingle, Shumard, swampchestnut, swamp white, water, white

– – – – – – – – – – – – – – – – – – – 400 Humid Tropical Domain – – – – – – – – – – – – – – – – – – –

410 Savanna 411 Open woodlands, shrubs and savanna412 Semi-evergreen forests413 Deciduous forests province

4 oak species: Chapman, live, laurel, myrtle

– – – – – – – – – – – – – – – – – – – Intrazonal Regions – – – – – – – – – – – – – – – – – – –

R Riverine forest

18 Chapter 1

Table 1.3. The ecoregion domains, divisions and provinces in the western conterminous United Stateswhere oaks are found and the principal species occurring in each. Ecoregions from Bailey (1995).Division and province boundaries are shown in Fig. 1.2.

Division Province

– – – – – – – – – – – – – – – – – – 200 Humid Temperate Domain – – – – – – – – – – – – – – – – – –

240 Marine 241 Mixed forestsM240 Marine Mountains M241 Deciduous or mixed forest–coniferous forest–meadow

M242a Forest–meadow, mediumM242b Forest–meadow, high

3 oak species: Oregon white, California black, canyon live

260 Mediterranean 261 Dry steppe262 Mediterranean hardleaved evergreen forests, open woodlandsand shrub263 Redwood forests

M260 Mediterranean Mountains M261 Mixed forest–coniferous forest–alpine meadowM262 Mediter. woodland or shrub–mixed or conif. forest–steppe ormeadowM263 Shrub or woodland–steppe–meadow

13 oak species: blue, California black, California scrub, canyon live, coast live, Dunn,Engelmann, interior live, island live, McDonald, Oregon white,turbinella, valley

– – – – – – – – – – – – – – – – – – 300 Dry Domain – – – – – – – – – – – – – – – – – –

310 Tropical/ Subtropical Steppe 311 Coniferous open woodland and semideserts 312 Steppes313 Steppes and shrubs314 Shortgrass steppes

M310 Tropical/ Subtropical M311 Steppe or semidesert–mixed forest–alpine meadow or steppe Steppe Mountains

16 oak species: Arizona, canyon live, Chisos, Dunn, Emory, Gambel, Gray, Havard,Lacey, lateleaf, Mohr, sandpaper, silverleaf, Toumey, wavyleaf, turbinella

320 Tropical/ Subtropical Desert 321 Semideserts322 Oceanic semideserts323 Deserts on sand

M320 Tropical/ Subtropical M321 Semidesert–shrub–open woodland–steppe or alpine meadowDesert Mountains M322 Desert or semidesert–open woodland or shrub–desert or steppe

22 oak species: Arizona, chinkapin, Chisos, Dunn, Durand, Emory, Gambel, Graves,gray, Havard, Lacey, lateleaf, live, Mexican blue, Mohr, netleaf, post,sandpaper, silverleaf, Toumey, turbinella, wavyleaf

330 Temperate Steppe 331 Steppes332 Dry steppes

M330 Temperate Steppe M331 Forest-steppe–coniferous forest–meadow–tundraMountains M332 Steppe–coniferous forest–tundra

M333 Steppe–coniferous forest M334 Steppe–open woodland–coniferous forest–alpine meadow

2 oak species: bur, Gambel

340 Temperate Desert 341 Semideserts 342 Semideserts and deserts

M340 Temperate Desert M341 Semidesert–open woodland–coniferous forest–alpine meadowMountains

3 oak species: Gambel, turbinella, wavyleaf

Riverine Forest). Province boundaries arealso useful in separating the regions whereoaks occur from those where they do not.

In contrast to the coarser levels of theclassification hierarchy (Domains throughSubsections), which have been delineatednationally, classification of the ELT andELTP levels is incomplete across much ofthe oak range. Even though classificationsystems down to the ELT or ELTP havebeen developed for millions of acres, theyinclude only a small fraction of the totalarea of oak forests.

ELTs or ELTPs are usually mapped inthe field based on differences in soils,physiography and vegetation (includingherbs and shrubs). The species composi-tion of the herbaceous layer is often used todistinguish among different ELT or ELTPunits because of the fidelity of some herba-ceous species (‘indicator’ species) to spe-cific biophysical conditions. Accordingly,the presence or absence of one or moreindicator species can be used to differenti-

ate among otherwise similar ELTs or ELTPs.Shrubs are also sometimes used as indica-tor species.

Compared to the herbaceous layer, thecomposition of the tree layer often recov-ers slowly from disturbances. Moreover,the tree component may not recover to itspre-disturbance composition. Joint consid-eration of physical and biological factorsand their interactions provide a basis foridentifying ecologically homogeneous landunits that are silviculturally relevant anduseful in delineating management units(Barnes et al., 1982).

Ecological classification provides abroader ecological context for understand-ing why oaks occur where they do, andhow those occurrences change with time,disturbance and other factors. At the broad-est scale the oak forests of the UnitedStates can be divided into four eastern andtwo western groups based on species asso-ciations, ecological conditions and succes-sional relations (Fig. 1.6).

Oak-dominated Ecosystems 19

SierraNevada

Pacific Mediterranean–Marine Region

CoastRange

CentralValley

Edwards Plateau

Cross Timbers

BostonMountains

OzarkHighlands

LowerMississippiValley 410

330

340

240

260

320310

250 220

210

210

230

Driftless AreaNorthern Hardwood Region

OhioValley

SouthwesternDesert–Steppe Region

Fo

rest

–Pra

irie

Tra

nsi

tio

n R

egio

n

CentralHardwood

Region

SouthernPine–Hardwood

Region Coastal PlainPiedmontHighland RimCumberland Plateau

PiedmontCoastal Plain

AppalachianMountains

AlleghenyPlateau

Fig. 1.6. The six regions where oaks commonly occur: Northern Hardwood Region; Central HardwoodRegion; Southern Hardwood–Pine Region; Forest–Prairie Transition Region, Southwest Desert–SteppeRegion; and Pacific Mediterranean–Marine Region. Numbers correspond to Ecoregion Divisions (Figs1.2 and 1.5) (Bailey, 1997). Not considered by the above regional groupings are the ranges of Gambeland bur oak, which extend into Division 330, and the ranges of Gambel, turbinella and wavyleaf oaks,which extend into Division 340. The shading shows the distribution of oaks from Fig.1.2.

Boundaries between regions followDivision boundaries in the hierarchical eco-logical classification system. These regionalgroupings are useful ecologically and silvi-culturally because they identify areas withbroadly similar macroclimates and speciesassociations. Regional differences in theapplication of silvicultural methods areclosely related to corresponding differencesin species composition, environmental fac-tors and other ecological conditions. The sixforest regions are described below in relationto the Domains, Divisions and Provinces ofthe hierarchical ecological classification sys-tem described in the preceding section.However, the regional designations do notexplicitly identify the lowland and riparianforests occurring within them. There, alongthe major rivers and streams within theSouthern Pine–Hardwood Region, the oaksattain their greatest size and growth.

Eastern Oak Forests

The Northern Hardwood Region

Geographic extent

The Northern Hardwood Region includes thenorthern halves of Minnesota, Wisconsin,Michigan and much of the northeastern UnitedStates including New Hampshire, Vermontand Maine in their entirety. It includes twoecoregion Provinces: Mixed Deciduous–Coniferous Forests Province (211) and MixedForest–Coniferous Forest–Tundra, HighProvince (M211b) within the WarmContinental Division (Figs 1.2 and 1.6; Table1.2). The Region extends 1300 miles fromwest to east and covers 123 million acres,about three-quarters of which is forested.

Braun (1972) called this area theHemlock–White Pine–Northern HardwoodRegion. She recognized two major subsec-tions, the Great Lakes–St Lawrence and theNorthern Appalachian Highlands. The west-ern and eastern portions of the NorthernHardwood Region share many of the samespecies, but they differ ecologically and

silviculturally (Godman, 1985). Those differences are due in part to the influenceof the Appalachian Mountains in the east-ern part of the Northern Hardwood Region.

More than 1.5 million non-industrialprivate forest owners own approximatelyhalf of the forests in the NorthernHardwood Region. Corporate and other pri-vate owners hold an additional 25%(Birch, 1996). There are 11 national forestsin the region (primarily in the Lake States)that cover 6.5 million acres.

Climate, physiography and soil

Precipitation typically ranges from 24 to 45inches per year although as much as 70inches occurs in some mountainous areasin the eastern part of the region. Snowfallof 60 to 100 inches per year is commonthroughout the region. More than 100inches of snowfall occurs at some of thehigher elevations, and snowfall exceeds400 inches in some locales near the GreatLakes. Mean annual temperature rangesfrom 35° to 52°F (2 to 11°C) and the grow-ing season lasts from 100 to 160 days (Fig.1.7) (McNab and Avers, 1994).

The region is characterized by low reliefwith numerous lakes, depressions, morainichills, drumlins, eskers, outwash plains andother glacial landforms. Variation in thedepth and type of glacial deposits and asso-ciated heights of water tables are importantfactors in the identification of silviculturallyrelevant ELTPs (Fig. 1.8). Elevations in themountainous areas range from 1000 to 4000feet with individual peaks exceeding 5000feet. Valleys in the mountainous areasinclude outwash plains and lakes resultingfrom glaciation (Bailey, 1995). Soils haveformed in diverse organic and mineral mate-rials including peat, muck, marl, clay, silt,sand, gravel and boulders in various combi-nations. At lower elevations in NewEngland and along the Great Lakes,Spodosols are common. Inceptisols andAlfisols dominate at lower elevations else-where. In the mountainous zones the soilsare primarily Spodosols (Bailey, 1995).

20 Chapter 1

Oak-dominated Ecosystems 21

Iron mountain, MI 42˚F 30 in. Atlanta, GA 62˚F 49 in.

Fargo, ND 41˚F 19 in.Fort Wayne, IN 50˚F 49 in.

Boone, NC 52˚F 55 in. Key West, FL 78˚F 40 in.

Fig. 1.7. Representative climates for selected ecoregion Divisions in the eastern United States. Meanmonthly precipitation is shown by the solid lines (right axis) and temperature by dashed lines (left axis).Mean annual values are given above each graph. Division boundaries are shown in Figs 1.2 and 1.5.(Ecoregion and climatic data from Bailey, 1995.)

22 Chapter 1

ELTPs on outwash plains ELTPs on dry ice-contact andsand hills

ELTPs on mesic ice-contactand sand hills ELTPs on herb-poor moraines

WATER TABLEWATER TABLE

WATERWATER

1 10 11 12 20 21 22 24 25

32 34 35 37 40 42 43

Fig. 1.8. Ecological landtype phases (ELTPs) for the upland forests of the Huron–Manistee National Forestsin the lower peninsula of Michigan (Province 211: Mixed Deciduous–Coniferous Forests Province). Site pro-ductivity generally increases with increasing ELTP value. ELTP 1: Northern pin oak/white oak – Deschampsiatype; ELTP 10: Black oak/white oak – Vaccinium type; ELTP 11: Black oak/white oak – Vaccinium type withloamy sand to sandy loam bands in substrata; ELTP 12: Black oak/white oak – Vaccinium type with perchedwater table at 6–15 ft; ELTP 20: Mixed oak/red maple – Trientalis type; ELTP 21: Mixed oak/red maple –Trientalis type with loamy sand to sandy loam bands in substrata; ELTP 22: Mixed oak/red maple – Trientalistype with perched water table at 6–15 ft; ELTP 24: Mixed oak/red maple – Trientalis type with perched watertable at 3.5–6 ft; ELTP 25: Mixed oak/red maple – Trientalis type with coarse loamy substrata; ELTP 32:Northern red oak/red maple – Viburnum type with perched water table at 6–15 ft; ELTP 34: Northern redoak/red maple – Viburnum type with perched water table at 3.5–6 ft; ELTP35: Northern red oak/red maple – Viburnumtype with fine loamy substrata; ELTP 37: Northern red oak/red maple – Desmodium type with sandy loamover fine loamy substrata; ELTP 40: Sugar maple/beech – Maianthemum type; ELTP 42: Sugar maple –Maianthemum type with perched water table at 6–15 ft; ELTP 43: Sugar maple/northern red oak –Maianthemum type with fined texture substrata. (Adapted from Cleland et al., 1993.)

Forest history

The forests of the Northern HardwoodRegion were strongly influenced by theaboriginal people who lived there. Forthousands of years before the arrival ofEuropeans, Native Americans used fire andland clearing to shape the forest to meettheir needs. Accounts of early Europeansettlers indicate that Native Americansburned large portions of the landscapeeach year (Pyne, 1982). Dry fuels in the latespring before ‘greenup’ and again after leaffall during ‘Indian summer’ providedfavourable conditions for burning. Firesoften eliminated forest understorey layers,which in turn encouraged the growth ofedible berries and increased forage thatattracted edible wildlife. A combination offire and tree cutting or girdling also wasused by Native Americans to create andmaintain openings for cultivated crops(Pyne, 1982). Maintaining an open under-storey condition by burning also helpeddefend Indian villages from surprise enemyattacks. Repeated burning helped maintainlarge areas of oak savannas and barrens.Topography greatly influenced the spatialdistribution of fires, and the frequency andintensity of burning were lower on wet ormesic sites and at higher elevations. Thisproduced a landscape mosaic of diversespecies composition.

The land clearing and burning practicesused in New England were carried west-ward by settlers who immigrated to theLake States. Oaks were known as indica-tors of fair to good conditions for agricul-ture. Oak forests therefore were oftengirdled, felled and burned in preparationfor agriculture. Over time, enormous areasin the Northern Hardwood Region werecleared for agriculture. Ultimately, how-ever, it was logging that had the greatestimpact on the region’s forests.

Logging gradually accelerated with theinflux of Europeans to New England in the17th century. Demands for forest productswere initially modest in the developingagrarian society. Local timber harvestingsupplied wood for homes, barns, fences,heating and cooking, and making potashand tannin. Forests were considered more

an impediment to agriculture than a valuedresource. However, this changed withindustrialization during the 19th century(Williams, 1989).

The industrialization of America pro-vided the capacity and economic incentiveto exponentially increase lumber produc-tion from less than 1 to nearly 45 billionboard feet between 1800 and 1900. Thatlumber, which was principally white pineand other softwoods, came primarily fromthe Northern Hardwood Region. In 1839,30% of the nation’s lumber (on a valuebasis) came from New York. Combined,New York, Pennsylvania, New Jersey andNew England produced two-thirds of thenation’s lumber. As supplies of white pinedwindled in the eastern part of the region,timber production moved westward.Although New York reached peak produc-tion in 1849, the northeastern statestogether had by then dropped to half of thenational lumber output. The shift in lum-ber production from the Northeast to theLake States occurred between 1840 and1860. Lake States harvest reached a peak of10 billion board feet annually in 1889.Relatively level terrain, easy access andhigh demand facilitated the rapid rise intimber harvesting across the Lake States.By 1940, Lake States production droppedbelow a billion board feet as the timberindustry moved south (Williams, 1989).

Although white pine was the preferredspecies, oaks and other hardwoods wereutilized locally where they were abundant.Oaks were in less demand by the loggingindustry. Because of their high density, oaklogs could not be floated down rivers aseasily as white pine and required overlandtransportation to avoid losses (Williams,1989). Over time, repeated timber harvest-ing removed the trees of greatest economicvalue leaving behind stands of inferiorquality and composition. Farmers emigrat-ing westward subsequently completed theland clearing.

As a preferred fuelwood, the oak’s uti-lization for that purpose significantlyaffected the region’s forests during theearly agricultural period. A colonial familyused 20–60 cords of wood annually for

Oak-dominated Ecosystems 23

heating and cooking. Although the percapita volume of wood used for fueldecreased over time, the total volumeincreased because of a growing population.In 1880, more than half of America’senergy needs were still met with fuelwood(Whitney, 1994).

Iron furnaces in the region were firedwith hardwood charcoal. Less than 1% ofthe fuelwood burned from 1800 to 1930 wasused to produce charcoal for iron smelting,but large areas of forest surrounding ironfurnaces were greatly affected (Whitney,1994). A typical 18th-century smelting oper-ation consumed 100 acres of forest annuallyto produce charcoal. Because forestregrowth could be repeatedly harvested forthis purpose every 25 years, about 2500acres of forest were required to sustain pro-duction of the ironworks. By the late 19thcentury, charcoal production for large iron-works required annual harvests of 1000–2000 acres, and some large companiesowned 100,000 acres of forest adjacent totheir smelters for that purpose (Whitney,1994). The large clearcuts surrounding theironworks radically changed the age struc-ture of the forests and also influenced theirspecies composition.

Harvesting hardwoods for fuel favouredthe development of hardwood sprouts andincreased the relative proportion of hard-wood trees in areas that were not convertedto agricultural land (Whitney, 1994). Duringthe last half of the 19th century, manycleared acres marginally suited to agricul-ture were abandoned and subsequentlyreverted to forest. During the latter half ofthe 20th century, the combination of aban-doned agricultural lands and natural regen-eration of cutover lands resulted in largeincreases in timber volumes throughout theregion. In New England, forest volumesincreased by 16% between 1970 and 1982(cubic foot basis) (Seymour, 1995). Theincrease was predominantly in hardwoodsthat as a group increased in volume by 24%.The increase in oak volume (16%) was lowrelative to other hardwoods. In 1992, netannual growth in the Northern HardwoodRegion remained at more than twice theannual harvest (Powell et al., 1994).

Oaks as components of the region’s forests

The forests of the Northern HardwoodRegion today are dominated by more thanhalf a dozen recognized northern hard-wood forest types comprising various com-binations of sugar maple, red maple, beech,paper birch, yellow birch and eastern hem-lock. Although northern red oak typicallyoccurs as a minor component within thesetypes, it sometimes forms pure or nearlypure stands (Fig. 1.9A). Wherever it occurs,it is a valuable and desirable species fortimber, acorn production and speciesdiversity. White, black, northern red andchestnut oaks also occur in the southernportions of the region.

Oaks are thus a relatively small compo-nent of northern hardwood forests. Theyare most abundant and attain their bestdevelopment in the southern parts of theregion including New York, Massachusetts,northern Pennsylvania, and central andsouthern Minnesota, Wisconsin andMichigan. There the oak and mixed-hard-wood forests grade into the oak–hickoryforests of the Central Hardwood Region. Inboth New England and the Lake States,about 11% of the forestland is classified asoak–hickory or oak–pine. These foresttypes include 17 billion cubic feet of grow-ing stock (inclusive of oaks and associatedspecies). Throughout the NorthernHardwood Region, sugar maple, red mapleand aspen are the most abundant hard-woods. Conifer forests (red and easternwhite pines, spruce and balsam fir) alsoexceed oak in acreage and volume (Powellet al., 1994).

Oaks often reach their greatest densityon sites that have been repeatedly dis-turbed by fire, timber harvesting and otherevents. After burning or timber harvesting,oaks originating from vigorous seedlingsprouts and stump sprouts often dominatestands. However, in the absence of distur-bance the oak forests of the NorthernHardwood Region are usually successionalto other hardwoods on all but the poorestsites. On the poorer sites, oaks are oftenrelatively permanent members of the forest.

24 Chapter 1

There, oaks frequently invade and succes-sionally replace established pine stands(Seymour, 1995). The loss of the Americanchestnut to chestnut blight fungus in NewEngland oak forests began in the early1900s (Fig. 1.10). This increased the rela-tive importance of oaks because oaks oftencaptured the growing space vacated bydying American chestnuts.

Today, the single-tree selection methodof silviculture is often applied to northernhardwood forests dominated by shade tol-erant species such as sugar maple. Thispractice focuses on maintaining stands ofhigh quality trees while largely relying onthe natural regeneration of shade tolerantspecies to sustain the silvicultural system.Although this system favours the develop-

Oak-dominated Ecosystems 25

Fig. 1.9. (A) A 130-year-old stand of northern red oak in the Northern Hardwood Region of northernWisconsin (Province 211: Mixed Deciduous–Coniferous Forests Province; Southern Superior UplandsSection). The absence of oak reproduction and a sparse sub-canopy of shade tolerant red and sugarmaples are indicators of what is likely to eventually replace the oaks in the absence of disturbance. (B) Xeric northern pin oak–white oak/Deschampsia type (see Fig. 1.8) on deep outwash sand in thenorthern lower peninsula of Michigan (Province 211: Mixed Deciduous–Coniferous Forests Province;Northern Great Lakes Section). This oak stand is mixed with jack pine; oak site index is ≤50 ft. (USDAForest Service, North Central Research Station photographs.)

A

B

ment of high quality oaks in stands whereoaks are already present, regenerating oaksbeneath the relatively closed canopies ofselection forests is usually difficult in thisregion. On the poorer sites, oaks maydevelop beneath a pine overstorey andeventually displace the less shade tolerantpine through natural succession or theexposure of oak reproduction in theunderstorey to full light after timber har-vest. On deep sandy soils of the upperLake States, stands of northern pin, blackand white oaks, often mixed with jack

pine, form relatively stable forest types oflow productivity (Fig. 1.9B).

The Central Hardwood Region

Geographic extent

The Central Hardwood Region includes thepredominantly deciduous broadleaf forestsof Central United States. The region liesentirely within the Humid TemperateDomain. The region includes the two HotContinental Divisions (Divisions 220 andM220), and intergrades with the easternpart of the Forest–Steppes and PrairiesProvince (251) of the Prairie Division (250)(Figs 1.2 and 1.6; Table 1.2). The HotContinental Division is subdivided intotwo provinces: Broadleaf Forests, Oceanic(221a); and Broadleaf Forests, Continental(221b). The Hot Continental MountainsDivision (M220) also is divided into twoprovinces: Deciduous or MixedForest–Coniferous Forest–Meadow (M221),and Broadleaf Forest–Meadow (M222).

The Northern Hardwood Region, theSouthern Pine–Hardwood Region, theForest–Prairie Transition and the westernedge of the Appalachian Mountains boundthe Central Hardwood Region. The CentralHardwood Region extends 1200 miles fromsouthwest to northeast and covers approxi-mately 220 million acres; about half theregion is forested. Approximately three-quarters of the forest area in the CentralHardwood Region is in non-industrial pri-vate ownership. Within that ownership,most holdings are 50 acres or smaller(Birch, 1996). There are seven nationalforests in the region comprising about 4million acres distributed across the south-ern half of the region in Arkansas,Missouri, Illinois, Indiana, Ohio, Kentuckyand Tennessee.

Climate, physiography and soil

The climate in the Central HardwoodRegion is hot continental with warm sum-mers and cold winters. Mean annual tem-perature ranges from 40 to 65°F (4–18°C),with the warmer temperatures in the south.

26 Chapter 1

Fig. 1.10 A standing dead American chestnut(minus bark). Chestnut was a common associateand dominant member of eastern oak foreststhroughout the Appalachians from Maine to Alabamaand westward to Missouri. The chestnut blight,which decimated the species throughout its range,permanently altered the ecology of eastern oakforests. The blight was first identified in New York in1904. Fifty years later it spanned the entire naturalrange of chestnut. Oaks and associated hardwoodsquickly captured the growing space vacated by deadand dying chestnuts. (USDA Forest Service, NorthCentral Research Station photograph.)

Annual precipitation ranges from 20 inchesin the northwest to 65 inches in the south-east and reaches as much as 80 inches onsome Appalachian peaks (Fig. 1.7).Precipitation occurs throughout the year,but tends to be somewhat greater in springand summer. Droughts may occur duringthe summer when evapotranspiration ishigh. Frost-free periods range from 100days in the northern Appalachians to 220days in the southern part of the region(Bailey, 1995).

Topography is diverse in this region.Elevations in the Appalachian Highlands(province M221) range from 300 to 6000 ftwith as much as 3000 ft of local relief.Further west (province 221a), the hills andlow mountains of the dissected anduplifted Appalachian Plateau (includingthe Allegheny and Cumberland Plateau)range from 1000 to 3000 ft in elevation. Inthe western half of the Central Hardwoodregion (province 221b), most of the land isrolling but varies from extensive, nearlylevel areas to areas like the OzarkHighlands where relief reaches 1000 ft.Most of the northern portions of thisprovince were glaciated with the exceptionof the driftless area of southwesternWisconsin and adjacent states. Major soilsare Alfisols, Inceptisols, Mollisols andUltisols (Bailey, 1995).

Forest history

The utilization and exploitation of forestsin the Central Hardwood Region haspassed through various historical phases(Hicks, 1997). Even before the arrival ofEuropeans, humans influenced the natureand extent of the region’s forests (Whitney,1994). The use of fire to control vegetationby Native Americans significantly influ-enced the extent and character of presettle-ment forests (Pyne, 1982; DeVivo, 1991;Olson, 1996). These human-caused alter-ations of the landscape continued for thou-sands of years before the arrival ofEuropeans (Hicks, 1997). After settlementby Europeans, human impacts on the forestexpanded and intensified. Burning, graz-ing, exploitative timber harvesting and

clearing of forests for agriculture occurredon an unprecedented scale. These practicesoccurred about 200 years earlier in theeastern part of the Central HardwoodRegion than in the western part.Historically, different human disturbanceswere further confounded by intrinsic eco-logical differences among oak forestswithin the various ecoregion provinces.Each subregion of the Central HardwoodForest has its own unique combination ofdisturbance history, climate, physiography,soils, species associations and successionalpossibilities. This complicates generalizingthe application of silvicultural methods tooak forests across the region.

As in the Northern Hardwood Region,the loss of American chestnut to chestnutblight increased the relative proportion andimportance of oaks throughout the region.Shortly after 1900, the disease became epi-demic and within 40 years it had invadedthe entire natural range of the chestnut(Kuhlman, 1978). The loss represents oneof the greatest recorded changes in a nat-ural population of plants caused by anintroduced organism (Liebhold et al.,1995). The chestnut comprised 25% of theeastern hardwood forest that covered 200million acres. In the Appalachians, it wasthe most ecologically and economicallyimportant tree species (Kuhlman, 1978).There and in other regions, it grew fasterand taller than associated oaks. Before theblight, chestnut was especially importantin moist upland forests where it sproutedvigorously and often increased in domi-nance after logging. In 1900, half the stand-ing timber in Connecticut was chestnut,which was largely comprised of youngstands of stump sprout (coppice) origin(Smith, 2000). Although American chest-nut provided only about 1% of the nation’shardwood lumber even at the peak of itsimportance, its loss (beginning in the early1900s) had a significant impact on localeconomies in the Appalachians. There itsnuts and bark (for tannin) provided scarcecash income, and its wood was valued for avariety of uses (Youngs, 2000).

The practice of silviculture in theCentral Hardwood Region dates back to the

Oak-dominated Ecosystems 27

genesis of North American forestry in thelate 19th century (Fernow, 1911; Pinchot,1987). From then until the 1960s, the majoremphasis was on uneven-aged silviculture(Roach, 1968). During the 1960s, theemphasis shifted to even-aged silviculture,especially clearcutting, and this emphasispersisted for about 20 years (Roach andGingrich, 1968; Johnson, 1993a). Whereapplied, hardwood silviculture in theregion usually follows the ‘ecologicalmodel’, which relies on the existing forestvegetation and its natural regenerationcapacity. Silvicultural prescriptions areusually focused on controlling stand struc-ture and species composition using cuttingmethods such as those recommended byRoach and Gingrich (1968). This approachcontrasts with the more intensive ‘agro-nomic model’ of silviculture based on arti-ficial regeneration, the introduction ofimproved genotypes, use of herbicides andfertilizer, prescribed burning, and otherintensive cultural methods like those com-monly used in the silviculture of pinemonotypes in the south and elsewhere.

Where applied, silviculture in theCentral Hardwood Region has usuallyfocused on growing high quality sawtim-ber. During the course of stand manage-ment (but before final harvest of even-agedstands), this requires ‘leaving the best’ and‘cutting the worst’ at each harvest. In even-aged silviculture, each timber harvest con-centrates on removing small, sub-canopytrees and poor quality trees in the maincanopy, with concomitant attention tospecies composition. Similarly, in uneven-aged silviculture, timber removals are con-centrated on poor quality trees, but cuttingoccurs across a wide range of diameterclasses in order to create and maintain theuneven-aged stand structure. In both sys-tems the objective is the improvement ofthe quality and the economic value of theresidual stand.

Today, only a small fraction of theforests of the Central Hardwood Regionreceive systematic silvicultural treatment.This is largely due to the pattern of forestownership, which is characterized bynumerous small tracts owned by private

individuals. Many forest owners are unin-terested in silviculture or lack informationon its benefits (Bliss et al., 1994, 1997;English et al., 1997). Consequently, the sys-tematic application of silviculture haslargely been limited to industrial forestsand public lands.

The predominant methods of timberharvesting on private lands are probablycommercial clearcutting and other forms ofhigh-grading. Not to be confused with silvi-culturally prescribed methods, these meth-ods consist of harvesting all trees withcommercial value without regard to regen-eration needs and future stand condition.Such malpractice typically leaves stands ofhighly variable residual stocking com-prised of trees of poor vigour, low qualityand undesirable species composition.These practices persist and continue toimpact negatively on the quality of theregion’s forests. Nevertheless, annual forestgrowth for the region exceeds annual har-vest, and total standing volume of timberhas increased steadily since the 1950s(Powell et al., 1994).

Oaks as components of the region’s forests

The predominant oaks are black, white,scarlet, chestnut, post, northern red, south-ern red and bur oak (Fig. 1.11). Thesespecies typically occur in various combina-tions with hickories, sassafras, floweringdogwood, blackgum, black cherry, redmaple, and other upland oaks and decidu-ous tree species. The Ozark HighlandsSection of the region, which covers south-ern Missouri, and parts of northeasternOklahoma, northern Arkansas and south-western Illinois, comprises one of thelargest contiguous areas dominated by theoak–hickory association in the CentralHardwood Region. Many oak–hickoryforests of today may have originated fromextensive fire-maintained oak savannas ofthe presettlement period; these formedclosed canopy forests when fires were sup-pressed (Johnson, 1993a; Olson, 1996).

The oak cover types of the CentralHardwood Region include various combi-nations of oaks, hickories and other tree

28 Chapter 1

species that vary geographically (Appendix2). Although hickories are common andpersistent members of this forest type, theyseldom represent more than a small pro-portion of trees in the main canopy of amature forest (Braun, 1972). Oak–hickoryforests develop on relatively dry siteswhere oaks persist as dominant membersof the forest through successive distur-bance events. This persistence is facilitatedby the oaks’ drought tolerance and by lightintensities in dry ecosystems that are suffi-cient for the regeneration of the relativelyshade-intolerant oaks (Bourdeau, 1954;Carvell and Tryon, 1961; Abrams, 1990).

Oaks and hickories are found togetheron the drier sites throughout the region andcomprise a commonly occurring speciesassociation. These forests dominate thelandscape in the western part of the region.Elsewhere, oaks and hickories as a groupcommonly occur on dry ridges and south-facing slopes. On the more mesic sites,oaks are often interspersed with otherhardwoods. Slope position and aspectstrongly influence the spatial distributionof these forests and are thus useful indefining ELTPs in much of the region (Figs1.12 and 1.13).

From southern Illinois eastward and innorthern Arkansas, the more mesophyticforests of the Central Hardwood Regiongenerally include more complex speciesmixtures than found in drier oak forests(Fig. 1.14). Although oaks commonly sharedominance with non-oaks on these sites, inthe absence of recurrent fire and grazingthe oaks are often successionally displacedby more moisture-demanding and moreshade tolerant non-oaks (Jokela andSawtelle, 1985; Lorimer, 1985, 1989;Nowacki et al., 1990; Abrams, 1992).Understoreys of these stands are typicallylacking in oak reproduction, especiallylarge oak seedling sprouts. Over time, thedominance of oaks decreases while theproportion of non-oaks increases. The lat-ter include various combinations ofmaples, American beech, black cherry,white ash, American basswood and yellow-poplar. Timber harvesting may acceleratethe successional replacement of the oaks(Abrams and Nowacki, 1992).

Diverse mixtures of hardwoods are com-mon throughout the Ohio Valley, theCumberland Plateau and Highland Rimareas of Tennessee and Kentucky, theAppalachian and Allegheny Plateau of

Oak-dominated Ecosystems 29

Fig. 1.11 A mature black–northern red–white oak stand on a good site in the Central Hardwood Regionof southeastern Ohio (Province 211a: Broadleaved Forests, Oceanic Province). (USDA Forest Service,North Central Research Station photograph.)

30C

hapter 1

3.1, 3.2

2.4 UG Alficcrypt reef bench

7.1 to 7.3 Cherty, non-chertyand glade, variable depth to dolomite

3.1 Exposed, rockyRO/UG Ultic backslope(some 3.2 Alfic)

2.1, 2.2

1.1, 1.21.3

sinkholes 2.1 Ultic, rockyRO/UG Shoulder

(some 2.2)

1.1 Ultic rocky,1.2 Silty

RO Summits

4.1 Protected RO/UGUltic backslope(some 4.2 Alfic)

8.1 Cherty protected,variable depth to dolomite

2.3 UG Ultic crypt reefbench

4.1, 4.2

10 Footslope10 Footslope

Exposed aspects 135–315˚ Protected aspects 315–135˚

Roubidoux (RO)

Upper Gasconade (UG)

Cryptozoan Reef

Upper Gasconade (UG)

Fig. 1.12. Ecological landtype phases (ELTPs) for the Ozark Highlands of Missouri (Province 221b: Broadleaved Forest, Continental Province; Ozark HighlandsSection, upland ELTPs in the Current, Eleven Point and Black River Landtype Associations). Aspect, landform and bedrock geology are factors in the classificationsystem. ELTP 1.1, 1.2, 1.3, 2.1, 2.3 and 3.1: pine–oak/Vaccinium dry ultic (chert) woodland; ELTP 2.2 and 3.2: mixed oak–pine/Desmodium, Vaccinium dry-mesicalfic (chert) woodland; ELTP 4.1: mixed oak–hickory/dogwood/Desmodium dry-mesic ultic (chert) forest; ELTP 4.2: mixed oak (white, red) dogwood dry-mesic alfic(chert) forest; ELTP 7.1: post oak (blackjack oak, pine) bluestem xeric chert woodland; ELTP 7.2: red cedar–hardwood/redbud dry dolomite woodland; ELTP 7.3:bluestem, Missouri coneflower dolomite glade; ELTP 8.1: mixed oak–sugar maple/redbud dry-mesic dolomite forest; ELTP 10: mixed oak (white)/dogwood dry-mesic alfic (chert) footslope forest. (From Nigh et al., 2000, used with permission.)

Oak-dominated Ecosystems 31

Brown County Hills Subsection

Crawford Upland and Escarpment Subsections

Fig. 1.13. Ecological landtype phases (ELTPs) for the forests of the Brown County Hills, CrawfordUpland and Escarpment Subsections of southern Indiana (Province 221b: Broadleaved Forests,Continental Province; Interior Low Plateau, Shawnee Hills Section). Oaks and pines typically dominatethe exposed (hotter) aspects whereas sugar maple, American beech, yellow-poplar and other shade tol-erant hardwoods dominate the protected (cooler) aspects. (Adapted from Van Kley et al., undated.)

western West Virginia and westernPennsylvania, the southern Lake States,and other parts of the region. Specific com-binations of canopy dominants often formdistinct geographic species groupings.Examples include the beech–maple forestsof central Indiana and eastern Ohio, themaple–basswood–northern red oak forestsof the driftless area of southwesternWisconsin, and the black cherry–ash–yellow-poplar forests of the AlleghenyPlateau of Pennsylvania. Toward the east-ern end of the region, eastern white pineand eastern hemlock may be locally impor-tant members of mixed hardwood forests.Mixtures of oak and mesophytic speciesalso occur in northern Arkansas in theBroadleaf Forest–Meadow Province (M222)

(Fig. 1.2; Table 1.2) (Braun, 1972). However,unlike the mixed mesophytic forests fur-ther to the east, yellow-poplar is absent.These are the most mesophytic forests inthe western end of the region. Some ofthese combinations are formally recog-nized as cover types (Eyre, 1980); othersform mixtures that are only locally distin-guished silviculturally.

It is within these mesic, mixed hard-wood stands that northern red oak, one ofthe most commercially valuable treespecies of the region, reaches its bestdevelopment. It is also within these foreststhat the oaks are also the most difficult toregenerate silviculturally (Carvell andTryon, 1961; Arend and Scholz, 1969;Trimble, 1973; Loftis, 1988; Johnson,1993b, 1994a,b). Mesophytic mixed hard-wood forests generally occur where oaksite index (chapter 4) is ≥ 65 ft at an indexage of 50 years.

Oak–pine mixtures occur most fre-quently in the southern and eastern partsof the region and are closely correlatedwith fire and succession in old fields, heav-ily disturbed hardwood stands, and pineplantations. Oak–pine mixtures representan early- to mid-stage in the successiontoward oak–hickory or mixed hardwoodforests. In the absence of fire or other dis-turbances, oak–pine forests may changesuccessionally from predominantly short-leaf pine, pitch pine or Virginia pine tohardwoods as the more shade toleranthardwoods replace the intolerant pines(Cunningham and Hauser, 1989; Sheffieldet al., 1989; Smith et al., 1989; Orwig andAbrams, 1994). The oak–pine mixtures areimportant for maintaining biodiversity aswell as economic timber production(Phillips and Abercrombie, 1987; Cooper,1989; Kerpez and Stauffer, 1989; Leopoldet al., 1989). Consequently, there is increas-ing interest in methods to create and main-tain oak–pine forests (Waldrop, 1989).Specific combinations of oaks and pinevary with subregion and site quality.Because the pines tend to be associatedwith the driest (xeric) sites, the associatedoaks often include species such as post oakand blackjack oak. On somewhat less xeric

32 Chapter 1

Fig. 1.14. A large white oak (47 inches dbh) inDysart Woods in southeastern Ohio (Province221a: Broadleaved Forests, Oceanic Province).This 55-acre old-growth oak forest is dominated bywhite and northern red oaks and is the largestknown remnant of the original mixed mesophyticforest of the Central Hardwood Region in south-eastern Ohio. (USDA Forest Service, NorthCentral Research Station photograph.)

sites, pines are commonly associated withblack, white, scarlet, southern red or chest-nut oaks. In the extreme northwestern partof the region in Minnesota and Wisconsin,jack pine and northern pin oak commonlyoccur together.

Stands of eastern redcedar are closelyaffiliated with the oak–pine mixtures.Eastern redcedar is a common invader ofold fields and glades (Lawson, 1990). Itmay eventually form dense pure stands ifsuccession is allowed to progress unim-peded by disturbance. However, suchstands are short-lived. As the redcedarmatures and forms canopy gaps conduciveto hardwood or pine regeneration, standsmay succeed to oak–pine and oak–hickorymixtures.

The Southern Pine–Hardwood Region

Geographic extent

The Southern Pine–Hardwood Regionincludes broadleaved forests, coniferforests and various hardwood–pine mix-tures. The region includes the twoSubtropical Divisions (230 and M230) ofthe Humid Temperate Domain (Figs 1.2and 1.6; Table 1.2). The region coversapproximately 270 million acres of which60% are forested. The extent of theSouthern Pine–Hardwood Region is bestillustrated by the joint ranges of theoak–pine and oak–gum cypress forest types(Fig. 1.5B and C). The region extends 1300miles from eastern Texas to Virginia andoccurs in a band extending 200–400 milesinland from the coast. At its northernboundary, the Southern Pine–HardwoodRegion meets the Central HardwoodRegion.

Nearly 90% of the forest area in thisregion is privately owned. Four millionnon-industrial private forest owners con-trol about 60% of all timberland. About45% of this ownership is comprised oftracts smaller than 100 acres (Birch, 1996).The 25 national forests in the region com-prise 9 million acres of timberland (Powellet al., 1994).

Climate, physiography and soil

Annual precipitation in the region rangesfrom about 40 to 60 inches and is well dis-tributed throughout the year. Mean annualtemperature ranges from 60 to 70°F (16 to21°C) and the growing season from 200 to 300 days (Bailey, 1995) (Fig. 1.7).

The Southern Pine–Hardwood Regionincludes four major physiographic regions:the Piedmont (Province 232), the CoastalPlain (Province 231), the InteriorHighlands (Province M231) and the lowerMississippi Valley (Riverine IntrazonalProvince (R)) (Fig. 1.2). Gentle slopes char-acterize 50–80% of the area. Elevationsrange from sea level to 600 ft in the CoastalPlain, 300–1000 ft in the Piedmont, and upto 2600 ft in the Ouachita Mountains of theInterior Highlands. Numerous low-gradientstreams, lakes, swamps and marshes char-acterize the flat Coastal Plain. The wethabitats along the Coastal Plain, theMississippi Valley and other major riverssupport bottomland forest types that arelargely absent from the Piedmont andInterior Highlands.

The principal soil groups are Ultisols,Spodosols, Vertisols and Entisols, all ofwhich tend to be low in fertility.Exceptions are the Inceptisols, which occurin the alluvial bottoms of the MississippiRiver (Bailey, 1995).

Forest history

Here, as in other regions, fire greatlyimpacted the early forests. Fire was anessential tool for maintaining agriculturalopenings, eliminating brush and hardwoodreproduction from pine forests, and increas-ing forage for grazing. Native Americansregularly burned the forests where theylived. Increased burning associated withEuropean settlement increased the propor-tion of pine in the region relative to earlierperiods (Pyne, 1982; Skeen et al., 1993).Fire was combined with land clearing toopen the hardwood forests of the south foragriculture. Even today, forest burning is aprominent practice throughout the region.

Oak-dominated Ecosystems 33

In the Piedmont and alluvial river bottoms,vast areas were cleared for agriculturebefore industrial logging peaked in theregion (Sargent, 1884; Hodges, 1995).

Logging and the production of navalstores began on a small scale in the 1600s.But by 1880, forests accessible by water andclose to population centres were heavilycut over. Charles Mohr noted the rapidityat which the cypress swamps were beinglogged in some localities and the apparentlack of forest regeneration. He observedthat ‘the large number of logs harvestedshows clearly with what activity thedestruction of these treasures of the forestis being pushed; and the reports, as ofheavy thunder, caused by the fall of themighty trees, resounding at short intervalsfrom near and far, speak of its rapidprogress’ (Sargent, 1884). However, Mohralso noted that immense areas of pine for-est remained unaffected by logging andthat many former hardwood forests thatwere earlier cleared for agriculture hadreverted to pine after their abandonment.

The South did not become the centre ofthe US logging industry until shortly afterlogging peaked in the Lake States in 1890.By 1900, lumber production in the Southexceed that of the Lake States and by 1910the South produced half of all US lumber.The movement of large lumber companiesto the Southern Pine–Hardwood Regioncoincided with technological advances thatincreased the speed with which logs couldbe removed from the woods and trans-ported to the mills (Williams, 1989). Steampowered stationary skidders and loaderswere mounted on boats and railcars. As raillines were extended into the southern for-est, logging trains followed and systemati-cally removed virtually all timber withinthe long reach of a cable skidder mountedon a rail car. The joint enterprise of railconstruction and logging greatly acceler-ated the harvest of southern pines(Williams, 1989).

By 1925 southern lumber productionbegan to decline and western productionincreased. Following the Great Depression,timber production in the South neverreturned to the levels of 1910–1930, and

the bulk of US timber production movedwest. The subsequent establishment ofsouthern paper mills coupled with success-ful fire prevention and a reduction in open-range grazing accelerated the reforestationof one million cutover acres. This gave riseto the South’s ‘third forest’ which todayagain produces a greater volume of woodthan any other region of the United States.

Oaks as a component of the region’s forests

The oak forests in this region can bedivided into upland and lowland types.Were it not for the complex spatial inter-mingling of upland and lowland forests,they could be treated as two ecologicallydistinct regions. The upland and bottom-land oak forests of the region differ substantially in species composition,ecology and the application of silvicul-tural practices.

The Southern Pine–Hardwood Regiontoday includes about 172 million acres oftimberland. Of the broadly defined oakforests recognized in national inventories,the oak–hickory group occurs on 55 mil-lion acres or one-third of the region’s tim-berland. Oak–gum–cypress and oak–pineeach occur on an additional 16% of thetimberland. Thus, oak forests collectivelycover more than 60% of the region.Loblolly-shortleaf pine and longleaf-slashpine make up most of the remaining forestacreage. A more detailed cover type classi-fication (Eyre, 1980) recognizes 63 covertypes that occur within the region (Walker,1995) – 15 of those include oaks as primaryspecies, and several others include oaks asimportant associated species (Appendix 2).

Southern silviculture has largelyfocused on pine, especially on industrialforestlands. There, intensive silviculture iscommonly practised to maximize timberand wood fibre yields through site prepara-tion, planting genetically improvedseedlings, frequent thinning, prescribedburning, and the use of fertilizers, herbi-cides and pesticides. However, annual soft-wood removals are nearly equal to annualgrowth and may soon exceed annualgrowth (Walker, 1995).

34 Chapter 1

The importance of pine in the Piedmontis related to the region’s history – the his-torical sequence of lumbering, land clear-ing and farming deforested large areas thatwere abandoned before 1930 and burnedfrequently. This disturbance favoured theestablishment of pine forests, which greatlyincreased in acreage relative to otherspecies. Oaks and other hardwoods occurin most natural southern pine stands, andon these sites they increase in importancethrough succession. Fire suppression, silvi-cultural thinnings and partial harvestsoften accelerate this trend (Skeen et al.,1993).

The large oak–hickory acreage in theregion, the increasing hardwood volumesin pine–hardwood mixtures, and the nearlyfull utilization of the annual pine growthin the region has recently shifted the uti-lization of the region’s forests towards thehardwoods. Much of this change hasresulted from the utilization of hardwoodchips for paper production and compositeproducts. Chips can be made from lowquality, small diameter (>4 inches) hard-woods. This technology created new mar-kets for the abundant low-quality trees thatpreviously had been considered a silvicul-tural liability. However, this utilizationcapability has also raised concerns aboutthe potential for overutilization of hard-woods, especially well formed, small hard-wood trees that comprise the futurehardwood growing stock for solid hard-wood products. The region’s oak–hickoryforests attain their best development alongthe border separating the SouthernPine–Hardwood Region and the CentralHardwood Region.

Closely related to the oak–hickoryforests are mixtures of oak and pine (Fig.1.15). These mixed forests are increasinglyrecognized for their importance in main-taining forest biodiversity and their histor-ical importance in the region. Theoak–pine type, which occurs on 16% ofthe timberland of the region, rates high inaesthetic appeal and species richness com-pared to even-aged pine stands. However,relatively little is known about the long-term management and productivity of

oak–pine mixtures for lumber, fibre orother values. In the absence of distur-bance, the oaks tend to successionally dis-place the pines and harvesting the pineoften accelerates the process.

Southern bottomland hardwoods com-monly include 11 species of oaks (cherry-bark, Delta post, laurel, Nuttall, overcup,pin, Shumard, swamp chestnut, water,white and willow oaks) (Hodges, 1995).These oaks occur in mixture with otherbottomland species along the major riversof the Coastal Plain as well as the lowerreaches of the Mississippi, Arkansas,Missouri, Ohio and Wabash Rivers (Fig.1.16). In total, southern bottomland hard-woods cover more than 27 million acres(16% of the region’s timberland) and arephysiographically and ecologically distinctfrom surrounding upland oak–hickory,oak–pine and pine forests.

Oak-dominated Ecosystems 35

Fig. 1.15. A black oak–white oak–shortleaf pinestand in the Ozark Highlands of Missouri (Province221b: Broadleaved Forests, Continental Province;Ozark Highlands Section). (USDA Forest Service,North Central Research Station photograph.)

Although bottomland forests are rela-tively flat, elevational differences of onlya few feet alter soil formation processes,soil moisture regimes and species compo-sition. Thus, changes in species composi-tion are often associated with relativelyminor differences in physiography (Fig.1.17). Moreover, floodplain physiographycan quickly and frequently change as aresult of scouring and deposition of sedi-ments. These factors, coupled with thehigh tree species diversity of bottomlandforests, complicate classifying forest typesand developing silvicultural prescriptionsappropriate to each. Up to 70 tree speciesoccur in southern bottomland forests(Putnam et al., 1960), and species mix-tures often change over short distanceswithin stands. Consequently, species asso-ciations are difficult to classify meaning-fully into more than a few broad types.Although Eyre (1980) listed 14 bottom-land cover types (six named for oaks),

Hodges (1995) reduced the number tothree types (cottonwood–willow, baldcypress–tupelo, mixed bottomlandhardwoods), and the USDA ForestService usually reports only two types (oak–gum–cypress, elm–ash–cot-tonwood) in regional summaries.

The Forest–Prairie Transition Region

Geographic extent

Within the United States, theForest–Prairie Transition Region extendsfrom southern Texas northward toMinnesota and North Dakota (Fig. 1.6).The region coincides with two ecoregionprovinces: Forest–Steppes and Prairies(251) and Prairies and Savannas (252)within the Prairie Division (250) (Fig. 1.2,Table 1.2). The region includes Braun’s(1972) Grassland or Prairie Region,Forest–Prairie Transition and PrairiePeninsula Sections, which fall within herOak–Hickory Forest Region.

As its name implies, the Forest–PrairieTransition Region is transitional betweenthe eastern forests and the prairies anddry woodlands of the Dry Domain to thewest. On its eastern border, the regionadjoins the Northern Hardwood Region,the Central Hardwood Region and theSouthern Pine–Hardwood Region. TheForest–Prairie Transition Region spans1400 miles in latitude and varies in widthfrom as little as 100 miles along theCanadian border to 600 miles betweeneastern Nebraska and western Indiana.The region includes approximately 191million acres, about 7% of which areforested. Between Canada and Oklahoma,forests cover 5% of the landscape withmost of the remainder devoted to tilledcropland or pasture. The forest coverincreases to 13% in parts of Oklahomaand Texas. Most of the forestland in theForest–Prairie Transition Region is pri-vately owned. Although there are threenational grasslands within the region,only 15,000 acres of national forest land(in central Missouri) are included.

36 Chapter 1

Fig. 1.16. A cherrybark oak–sweetgum bottom-land stand near the Tombigbee River in Alabama(Province 232: Coniferous-Broadleaved Semi-evergreen Forests Province). (USDA ForestService, Southern Research Station photograph.)

Climate, physiography and soil

Precipitation within this vast region variesfrom less than 20 inches per year in thenorth to 55 inches along the gulf coast ofTexas. One-half to two-thirds of the precip-itation typically falls during the growingseason and snowfall is common north ofTexas. From north to south, mean annualtemperature ranges from 36° to 70°F (2 to21°C) with corresponding growing seasonsranging from 111 to 320 days (McNab andAvers, 1994) (Fig. 1.7). Throughout theregion precipitation is largely offset byevapotranspiration, creating soil moistureconditions in many localities that are mar-ginal for tree growth.

Most of the region comprises gentlyrolling plains, although high rounded hillsoccur and steep bluffs border some rivervalleys. Elevations range from sea level to2000 ft. Local relief is less than 165 ftthroughout most of the region, but it reaches500 ft in the Flint Hills of Kansas (McNaband Avers, 1994). Soils are predominantlyMollisols although Vertisols occur on theprairies, and Alfisols occur on savannas andwithin the Mississippi Valley (Bailey, 1995).

Forest history

Native Americans who were largelynomadic inhabited the region for at least10,000 years. Crops were cultivated as

Oak-dominated Ecosystems 37

Major Bottom

Minor Bottom

Riverbaselevel

Riverbaselevel

Oak

s

BARLE

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FLAT

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TERRACE

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aks,

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amp

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ow o

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lar

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olly

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e

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SWAM

P RIDGE

RIDGE

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ow

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tonw

ood

Elm

, syc

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ecan

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arbe

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all o

ak, g

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le

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owO

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ater

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um, w

ater

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er h

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ut o

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m, b

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Fig. 1.17. The topographic distribution of southern bottomland oaks and associated species in majorand minor stream valleys of the Southern Pine–Hardwood Region (Province 232: Coniferous-Broadleaved Semi-Evergreen Forests Province). (Reprinted from Hodges and Switzer, 1979, by permission of Society of American Foresters, Bethesda, Maryland. Not for further reproduction.)

early as 1000 years ago. A few largeNative American communities developedin the major river valleys. One of thesewas Cahokia (near present-day St Louis),which flourished between AD 1000 and1400 with an estimated population of25,000. The forests in the region were animportant resource for both nomadic peo-ple and larger permanent communities.The demise of Cahokia may have beencaused by the exhaustion of the surround-ing forests that were used for fuel and forthe construction and maintenance of a 2-mile-long perimeter wall around the city(Lord, 1999).

Frequent fires were essential to themaintenance of prairie and savanna vegeta-tion in many parts of the Forest–PrairieTransition Region, and Native Americansburned the grasslands and woodlandswhere they lived. Grazing and trampling byherds of bison and other ungulates alsowere important in maintaining prairies andpreventing the encroachment of forests andother woody vegetation. By mid-19th cen-tury, European settlers began farming theprairies and draining prairie wetlands. Thelatter produced some of the nation’s mostproductive agricultural lands.

Trees were largely confined to ripariancorridors, steep slopes and scatteredsavannas. With the exclusion of fire andthe elimination of free-ranging ungulates,forests frequently encroach upon aban-doned fields and pastures. In 1884, Sargent(1884) stated that, ‘Dakota, with the excep-tion of its riverlands and the small territorybetween the north and south forks of theCheyenne River, is practically destitute oftimber. The bottoms of the principalstreams contain extensive groves of hard-wood.’ In Iowa he observed that ‘since thefirst settlement of the state the forest areahas increased by the natural spread of treesover ground protected by fire, and by con-siderable plantations of cottonwood,maples, and other trees of rapid growthmade by farmers to supply fuel and shel-ter’. Further south, in Texas, Charles Mohrnoted, ‘The timber growth immediatelywest of the Brazos is stunted and scanty;large areas of grass land intervene between

the scrubby woods until all at once lig-neous growth disappears and the seem-ingly boundless prairie, in gentlyundulating swells expands before the viewon all sides’ (Sargent, 1884).

Since that time, farms have been estab-lished on virtually all the lands suitable forrow crops or forage production (McNaband Avers, 1994). Depending on the farmeconomy, the forested acreage hasdecreased or increased as forests andwoodlands were cleared to create morefarm land, or as marginal farm landreverted to forest through tree planting orabandonment.

Oaks as components of the region’s forests

The best forest development in this regionoccurs on its eastern border where it abutsthe Northern Hardwood Region, theCentral Hardwood Region and theSouthern Pine–Hardwood Region. Of the7% of the Forest–Prairie Transition Regionthat is forested, three-quarters is classifiedas oak–hickory or oak–pine. Few of thesavannas that formerly occupied the tran-sition zone between forest and prairieexist today.

The prairie fires that historicallyrestricted the extent of the region’s forestshave been replaced by agricultural prac-tices that now limit most forests to ripar-ian areas or to slopes unsuitable forforage or other crops (Fig. 1.18). Beforethe mid-19th century, fire was the pri-mary regulator of the distribution of treespecies in the region. Narrow bands offorest along streams and ravines, some-times called gallery forests, providedrefuges for trees from the frequent firesthat burned across the prairie. Oaks dom-inated many of these forests. With theadvent of farming in this region, the fre-quency of wildfires was greatly reduced.This allowed the gallery forests to expandinto untilled areas that were formerlycovered by native grasses (Abrams andGibson, 1991). However, the invadingwoody species were generally speciessuch as American elm, hackberry andeastern redcedar rather than oaks. The

38 Chapter 1

reduction in wildfires also allowed thosespecies to increase in abundance withinexisting forests that were formerly domi-nated by oaks, especially on the moremesic sites. In much of this region, fre-quent fires are required to prevent thedisplacement of the oaks by other species(Penfound, 1968; Abrams, 1988; Abramsand Gibson, 1991).

Oak–hickory forests extend from theCentral Hardwood Region westward acrosseastern Oklahoma and into northern Texas(Fig. 1.5A). From east to west the forestsbecome increasingly scrubby and open. Anexception is the relatively dense oak forestof the Cross Timbers Region. In Texas, theCross Timbers comprise two bands ofscrubby oak woodland extending 175 miles

Oak-dominated Ecosystems 39

Fig. 1.18. (A) Aerial view of the distribution of forests in the Forest–Prairie Transition Region (Province251: Forest-Steppes and Prairies Province) of northwestern Missouri. Throughout much of this ecore-gion, forests are largely restricted to narrow belts occupying steep slopes along rivers and drainagesinterspersed with agricultural lands. (B) Forested bluffs dominated by oaks (background) along theMissouri River in central Missouri fronted by cultivated bottomland fields. Before settlement byEuropeans, these bottomlands were covered by lowland forests dominated by American elm, silvermaple, green ash, eastern cottonwood, bur oak and pin oak. (USDA Forest Service, North CentralResearch Station photographs.)

A

B

southward from the Oklahoma border.These bands are 20–50 miles wide and sep-arated by the Fort Worth Prairie. Forestcover occurs along outcrops of sandy soilsof greater porosity than adjacent prairiesoils (Braun, 1972).

The Cross Timbers were prominentlandmarks for westward travellers whootherwise traversed relatively open land-scapes (Dyksterhuis, 1948). Although theheavier forest cover in the Cross Timbersarea of Texas is somewhat evident fromFig. 1.6, the two distinct strips of wood-land are not distinguishable at the reso-lution shown. Post oak and blackjack oakare the dominant tree species and accountfor 60% and 20% of the trees, respectively.Except in floodplains, these oaks seldomexceed 12 inches in diameter and 30–45 ftin height. At one time, the herbaceousvegetation in the Cross Timbers was prob-ably similar to that of the surroundingprairie, but grazing during the last centuryhas greatly altered the species compositionof the herbaceous layer (Dyksterhuis,1948).

The Cross Timbers vegetation extendsnorthward through Oklahoma and eventu-ally disappears in southern Kansas. Exceptfor the Cross Timbers Region, the uplandwoodlands of eastern Oklahoma were for-merly post–blackjack oak savannas main-tained by frequent fires. Grazing and areduction in burning have since reducedgrass cover and facilitated the establish-ment of dense tree reproduction in manyareas; post and blackjack oaks dominatemost stands. Although the average basalarea of these forests historically has beenrelatively low, in the absence of burning ithas increased from 49 ft2 acre�1 in 1957(Rice and Penfound, 1959) to 80 ft2 acre�1

in 1993 (Rosson, 1994).From Kansas northward there are few

forests, but where they do occur, oaks oftendominate (Figs 1.2 and 1.6). Many of theoak forest types and conditions occurringin the Central Hardwood Region extendwestward through the central portion ofthe Forest–Prairie Region. The central partof the region is capable of supporting forestvegetation and is successional to forest in

areas protected from cultivation. However,because agriculture is the dominant landuse, forests are usually restricted to ripariancorridors, wet areas, steep slopes andhighly erodible lands, or other sitesunsuited to agriculture. Nevertheless, oaksand other hardwoods often develop intocommercially valuable stands in thoseparts of the region lying within Illinois,Iowa, northern Missouri and easternKansas.

Bur oak is the dominant oak species inthe northern reaches of the Forest–PrairieTransition Region. It is the only major oakspecies with a natural range that extendsacross western Minnesota and into theDakotas. Bur oak is well adapted to thisregion because its deep taproot makes itresistant to drought and able to invadeprairie grasslands (Johnson, 1990). Its thickbark makes it highly resistant to fires thateliminate most other woody species. Buroak also thrives on moist alluvial bottomsthat support dense hardwood forests in thenorthern portion of the Forest–PrairieTransition Region. Here, the bur oak typecovers approximately 2% of the land areaand is the principal forest type.Cottonwood, quaking aspen and Americanelm are other abundant hardwoods in thenorthern part of the Forest–PrairieTransition Region.

Western Oak Forests

The Southwestern Desert–Steppe Region

Geographic extent

The Southwestern Desert–Steppe Regionincludes the scattered oak forests ofArizona, New Mexico, southern Utah, westTexas and southwest Oklahoma (Figs 1.2,1.6; Table 1.3). Although the range ofGambel oak extends northward as far assouthern Wyoming, the oaks there are asmall component of the vegetation. Forestsand woodlands cover about 20% of thearea, but only 7% of this is considered pro-ductive forest. Soil moisture deficiencieslimit the distribution of oaks and otherplant life throughout the region. Oaks

40 Chapter 1

occur as scattered trees and in open wood-lands. Their distribution within the regionis often limited to discontinuous eleva-tional zones that provide the requiredregime of precipitation and temperature.

The region lies entirely within the DryDomain and comprises parts of threeDivisions: Tropical/Subtropical Steppe(310), Tropical/Subtropical SteppeMountains (M310) and Tropical/SubtropicalDesert (320). Included are six ecoregionProvinces: Coniferous Open Woodland andSemideserts (311), Steppes and Shrubs(313), Shortgrass Steppes (314), Steppe orSemidesert–Mixed Forest–Alpine meadowor Steppe (M311), Semideserts (321), andDeserts on Sand (323) (Fig. 1.2; Table 1.3).

The Southwestern Desert–SteppeRegion extends 1200 miles from northwest-ern Arizona to the Gulf of Mexico in south-ern Texas. It varies from 300 to 700 milesin width, and encompasses roughly 250million acres including the Mojave Desert,the Sonoran Desert, the Painted Desert, theColorado Plateau, the southern RockyMountains, Texas High Plains and theEdwards Plateau. Within the region, oakforests are widely scattered and cover onlya small fraction of the landscape (Fig. 1.2).The federal government or NativeAmericans own two-thirds of the forestsand woodlands in Arizona and NewMexico, but in Texas and Oklahoma mostare privately owned (Powell et al., 1994).

Climate, physiography and soil

A defining characteristic of this region is arate of surface evaporation that exceedsprecipitation. The climate varies from dryto desert. Annual precipitation ranges fromless than 10 inches to 30 inches (Bailey,1995). Even in areas with greater precipita-tion, high rates of evaporation limit mois-ture availability. Average annualtemperature ranges from 40 to 70°F (4 to21°C). Although temperatures decreasewith increasing elevation, mean monthlytemperatures generally exceed 32°F (0°C)(Fig. 1.19). Elevation ranges from sea levelalong the southern Texas Gulf Coast to7000 ft in the Colorado Plateau; some

mountain peaks are substantially higher.Soils are variable throughout the regionand include Mollisols, Aridisols and dryEntisols (Bailey, 1995).

Forest history

As in other regions of the United States,Native Americans customarily burned theforests and woodlands where they lived.Lightning was also a common cause of com-bustion. These fires maintained an openunderstorey in the extensive ponderosapine forests of higher elevations. In 1880alone, about 75,000 acres burned – whichaccounted for 0.1 to 1% of the woodlandwithin the settled area (Sargent, 1884).

Beginning in the mid-19th century,European settlers were drawn to the regionby opportunities for mining and livestockproduction. Lands were not suitable foragriculture, and the great land clearing thatdecimated the eastern oak forests did notoccur here. However, logging, grazing andchanges in fire regimes changed the speciescomposition of forests and woodlands. Inrecent decades, the suppression of fires hasincreased the amount of tree reproduction,especially conifers, and decreased grassesand forbs growing beneath forest canopies(Long, 1995).

Throughout the region, oaks have histor-ically had little commercial value. In 1884,Sargent (1884) described the forests in andaround New Mexico: ‘The deciduous treesof this entire southwestern region, often ofconsiderable size, are generally hollow,especially the oaks; they are of little valuefor any mechanical purpose, althoughaffording abundant and excellent fuel.’Then, as now, ponderosa pine was theprincipal timber species.

Oaks as components of the region’s forests

Forests and woodlands cover only a smallportion of the total area in the region, andthe oaks comprise only a small percentageof that. In Arizona and New Mexico, only15% of the land base is forested. Only 3%of the area of those two states can producemore than 20 ft3 of timber per acre per year,

Oak-dominated Ecosystems 41

42 Chapter 1

Astoria, OR 51˚F 76 in. Abilene, TX 65˚F 25 in.

Pasadena, CA 62˚F 19 in. Brawley, CA 72˚F 2 in.

Tahoe, CA 42˚F 31 in. Colorado Springs, CO 48˚F 14 in.

Fig 1.19. Representative climates for selected ecoregion Divisions in the western United States. Meanmonthly precipitation is shown by the solid lines (right axis) and temperature by dashed lines (left axis).Mean annual values are given above each graph. Periods of drought are indicated where the precipita-tion line falls below the temperature line (e.g. as in Division 260). Division boundaries are shown in Figs1.2 and 1.5. (Ecoregion and climatic data from Bailey, 1995.)

and virtually none of that is oak forest.Commercial forests include ponderosa pine(75%), Douglas-fir (13%), spruce–fir (9%)and aspen (3%). The only recognized oakcover type here is western live oak(Appendix 3) (Eyre, 1980). It occurs at ele-vations from 4000 to 6000 ft in the foothillsand lower mountain slopes of Arizona andNew Mexico. At higher elevations, thewestern live oak cover type gives way toponderosa pine and pinyon–juniper, withoak–conifer mixtures occurring in thetransition. At lower elevations the westernlive oak type yields to an open growth ofshrubby evergreen oaks. Mesquite anddesert vegetation typically occurs belowthat. Characteristic species of the westernlive oak type include Emory, Arizonawhite, Mexican blue and silverleaf oaks(Eyre, 1980) (Fig. 1.20). Ajo oak, Dunn oak,grey oak and Havard oak also occur inArizona and New Mexico.

At the eastern end of the SouthwesternDesert–Steppe Region towards the HighPlains and Edwards Plateau of west-centralTexas, precipitation increases and oaksbecome more prominent. The Mohr (shin)oak forest type covers more than 8 millionacres in Texas where it develops best under

20–25 inches of precipitation annually(Eyre, 1980). However, that amount of pre-cipitation represents the upper end of therange for the region (e.g. see Fig. 1.19,Division 310). Other oaks that occur inwest-central Texas include Arizona white,blackjack, bur (marginally), chinkapin,Durand, Emory, Havard, Lacey, live, sand-paper, Texas and Texas live oaks.

The Pacific Mediterranean–Marine Region

Geographic extent

The Pacific Mediterranean–Marine Regionincludes the oak forests and woodlands ofCalifornia, Oregon and Washington (Fig.1.6). The region lies within the western por-tion of the Humid Temperate Domain andincludes the Mixed Forest–ConiferousForest–Alpine Meadow Province (M261)and the Mediterranean Woodland orShrub–Mixed Coniferous Forest–Steppe orMeadow Province (M262) within theMediterranean Mountains Division (M260)(Fig. 1.2). Oaks also occur within the CoastRanges of California, which includes theMediterranean Hardleaved Evergreen Forest,

Oak-dominated Ecosystems 43

Fig. 1.20. Emory oak woodland in the Peloncillo Mountains of southwestern New Mexico, CoronadoNational Forest, New Mexico (Province M321: Semideserts Province). (USDA Forest Service, RockyMountain Research Station photograph.)

Open Woodlands and Shrubs Province (262)and the Redwood Province (263). At itsnorthern extent, the Pacific Mediterranean–Marine Region also reaches the Mixed ForestProvince (241) of the Marine Division (240)in Oregon and Washington. The Pacific–Mediterranean–Marine Region also includesCalifornia’s Central Valley (province 261)and the mountainous zones of Washingtonand northern Oregon (M261) where oaks arenot abundant.

The Pacific Mediterranean–MarineRegion extends nearly 900 miles fromWashington to southern California but lessthan 200 miles from the Pacific Ocean tothe eastern slopes of the Sierra NevadaMountains. Although the region coversabout 75 million acres, the oaks are limitedto relatively narrow elevational zones.

In California, Oregon and Washington,slightly less than half the timberland is pub-licly owned (Powell et al., 1994). In contrastwith the eastern United States, most of theprivately owned timberland in this region isheld by corporations rather than by non-industrial private owners (Birch, 1996).However, the ownership of oak forests andwoodlands does not follow this trend; aboutthree-quarters of that acreage is in non-indus-trial private ownership (Thomas, 1997).

Climate, physiography and soil

Climate is strongly influenced by thePacific Ocean and by the Coast and SierraNevada Ranges, which dominate the phys-iography of the region. Elevations rangefrom sea level to more than 14,000 ft. In themountain ranges, increasing elevation isassociated with decreasing temperaturesand variation in precipitation. For a givenelevation, precipitation is generally greateron western slopes than on eastern slopes.Latitude also influences climate so that agiven climatic zone occurs, from north tosouth, at progressively higher elevations.However, mountainous topography createsclimatic irregularities and discontinuities,and the distribution of oaks and associatedtree species varies accordingly.

Most of the precipitation occurs duringthe autumn, winter and spring. Annual pre-

cipitation generally ranges from 10 to morethan 60 inches in the ecological provinceswhere oaks occur. Temperature extremesand moisture stress are reduced near thecoast where fog supplements precipitationand the ocean reduces fluctuations in tem-perature. Elsewhere the region’sMediterranean climate is characterized by 2to 4 months of drought during the summer(Table 1.3, Fig. 1.19). Low precipitationgenerally occurs at lower elevations and onthe east faces of mountain ranges. Soilsinclude Ultisols, Alfisols, Mollisols,Entisols and Inceptisols (Bailey, 1995).

Forest history

The historical importance of oaks isrecorded in ancient bedrock mortars thatwere used by Native Americans to grindacorns into flour. Acorns were a staple foodof Native Americans in this region, andBiswell (1989) suggests that oaks were soimportant to their diet that they burned oakwoodlands to both encourage oak repro-duction and to facilitate acorn gathering.Although human-caused fires have beenhistorically associated with the oaks of theregion for thousands of years, there isuncertainty about what proportion of thelandscape was regularly affected byhumans. During the post-settlement periodof 1850–1950, the mean interval betweenfires in the oak–pine forests of the foothillsof central California was 8 years (Stephens,1997).

Commercial logging in the region haslargely focused on the conifers. In 1884,Sargent (1884) stated:

The forests of California, unlike those of theAtlantic States, contain no great store ofhardwoods. The oaks of the Pacific forests, oflittle value for general mechanical purposes,are unfit for cooperage stock. No hickory,gum, elm, or ash of large size is found inthese forests, California produces no treefrom which a good wine cask or wagon wheelcan be made. The cooperage business of thestate, rapidly increasing with thedevelopment of grape culture, is entirelydependent upon the forests of the Atlanticregion for its supply of oak.

44 Chapter 1

Sargent further noted that large quantitiesof chestnut oak (sic tanoak), once commonin the northern Coast Range of California,are ‘now becoming scarce and in danger ofspeedy extermination’ due to utilizationby the tanning industry. Sargent’s refer-ence to the oaks of Oregon andWashington is slightly less disparaging. Inthe Willamette Valley, he noted thatOregon white oak woodlands were becom-ing re-established after reductions in firefrequency. Along the Yakima River inWashington, he noted that Oregon whiteoaks were limited to 15 ft in height and 6inches in diameter.

The logging industry on the PacificCoast was established in the 18th centuryunder Hispanic influence. Through themiddle of the 19th century the relativelysmall industry served markets in SouthAmerica, Australia and the Pacific Rim(Williams, 1989). The gold rush of 1849and the completion of the transcontinen-tal railroad opened additional markets,but the increase in lumber production inthis region occurred gradually, beginningabout 1900 when the large timber compa-nies and railroads moved west afterexhausting the ready supply of timber inthe Lake States. Increases in timber pro-duction in the region continued into theGreat Depression, but output eventuallydropped by 75%. By 1950, however,annual timber production in the Westexceeded 16 billion board feet annually,which was greater than that produced inother regions of the United States. Todaylumber production in this region lagssignificantly behind that of the south.Harvest of hardwood growing stock hasremained nearly constant since 1976, butthe volume of hardwoods harvestedannually is only about 5% of the region’stotal.

Historically, oak forests were littleaffected by commercial logging, but locallythey were widely utilized for firewood andfence posts. Ranchers and farmers had thegreatest influence on the oak woodlands ofthe foothills and lower slopes as a conse-quence of clearing them for agriculture andgrazing. Sargent (1884) noted:

The permanence of the mountain forests ofCalifornia is severely endangered, moreover,by the immense herd of sheep, cattle, andhorses driven to the mountains every year, atthe commencement of the dry season, tograze. From the foothills to the highest alpinemeadows, every blade of herbage and everyseedling shrub and tree is devoured.

In California, oak woodlands were reducedfrom an estimated 10–12 million acres toabout 7 million acres today (Thomas,1997). The oak woodlands are predomi-nantly owned by farmers and ranchers, andbetween 1945 and 1970 the primary loss ofwoodland acreage resulted from conver-sion to rangeland. Invasion of non-nativegrasses and the suppression of fire havecreated problems in maintaining oak wood-lands and savannas (see Chapter 9 fordetails of savanna restoration and manage-ment). More recently, the greatest losses ofoak woodland have resulted from suburbanresidential development (Bolsinger, 1988).This has given rise to concern for propertydamage from the wildfires historicallyassociated with the oak woodlands.

Oaks as components of the region’s forests

Most of the region’s oak forests and wood-lands occur in California where they accountfor approximately one-quarter of the woodedacres. Oaks surround California’s CentralValley in the foothills of the Sierra Nevada,Cascade and Klamath Ranges (Figs 1.5 and1.6). Although oaks were formerly abundantwithin parts of California’s Central Valley(province 261), their distribution has beengreatly reduced there (Griffin, 1977). Oaksalso occur on the western slopes of the CoastRanges in central and southern California.The range of Oregon white oak extends north-ward into central Oregon and Washington inthe Willamette Valley and the Puget lowlandsbetween the Cascade and Coast Ranges.Included are 18 species of oak trees andshrubs plus additional hybrids (Bolsinger,1988; Thomas, 1997). Eight oak species thatreach tree size are abundant: California black,blue, interior live, coast live, canyon live, val-ley, Oregon white and Engelmann oaks(Plumb and McDonald, 1981).

Oak-dominated Ecosystems 45

Western oak forests are often categorizedas either timberland (forests suitable forcommercial wood production and capableof producing at least 20 ft3 acre�1 year�1 ofmerchantable volume), or woodlands (sitesof lower productivity primarily utilized forforage and firewood). In California, onlyabout 1 in 4 acres of hardwood forest quali-fies as timberland. Oak woodlands aresparsely covered with trees compared tooak timberlands. The statewide volume ofoaks in woodlands and in timberlands isnevertheless nearly equal because theacreage of woodlands is approximatelythree times that of timberlands (Table 1.4).Three-quarters of the oak woodlands aregrazed and these account for about one-third of California’s total forage (Thomas,1997). Only about 500,000 board feet ofhardwood lumber was produced inCalifornia in 1992 (Ward, 1995).

The combined effects of temperature andprecipitation (which latitude, elevation,slope and aspect affect) regulate the distrib-ution of oaks. In the PacificMediterranean–Marine Region, many oakforests and woodlands are restricted to ele-vational zones in the transition betweengrassland and chaparral at lower elevationsand coniferous forest at higher elevations.Mean temperatures within the regionincrease with decreasing latitude, and the

oaks occur at higher elevations at lower lati-tudes. Due to the interaction of climate andmountainous topography, the distribution ofoaks in this region is more geographicallyrestricted than in the eastern United States.

Several classification schemes havebeen proposed for the complex vegetationrelationships that occur in the PacificMediterranean–Marine Region (e.g. Griffin,1977; Paysen et al., 1980, 1982; Barbour,1988; Allen, 1990) (Fig. 1.21). Eyre (1980)recognized five oak cover types and twoadditional types where oaks commonlyoccur in mixtures with other species(Appendix 3). The Oregon white oak typeis found in the northern portion of thePacific Mediterranean–Marine Region fromnorthern California to Vancouver Island.This type occurs at lower elevations(0–3900 ft) and primarily in inland valleysor lower slopes between the Coast Rangesand the Cascade or Sierra Nevada Ranges(Eyre, 1980). The type makes its best devel-opment in the vicinity of the WillametteValley where closed-canopy Oregon whiteoak stands developed from former oaksavannas when periodic ground fires wereexcluded (Thilenius, 1968). The speciesalso occurs in mixtures with other hard-woods and conifers including Californiablack oak, canyon live oak, ponderosa pineand Douglas-fir (Appendix 3).

46 Chapter 1

Table 1.4. Standing oak volumes in California timberlands and woodlands. Although oaks make up 63%of the total volume of California’s hardwoods, oak timberlands (commercial forest lands) comprise only8% of the 50 billion cubic feet total volume (softwoods plus hardwoods on California timberlands).a

Volume in Volume in Species total astimberlands woodlands Total a proportion of

Species (million ft3) (million ft3) (million ft3) all oaks (%)

California black oak 2,254 277 2,531 32Canyon live oak 1,302 731 2,033 26Blue oak 1 1,112 1,113 14Coast live oak 126 755 881 11Oregon white oak 211 389 600 8Interior live oak 45 508 553 7California white (valley) oak 34 164 198 3Engelmann oak 0 10 10 0Total oak 3,973 3,946 7,919 100Total hardwoods (all species) 7,661 4,855 12,516 –

a Adapted from Shelly (1997) and Bolsinger (1980, 1988).

California black oak attains a greater vol-ume (Table 1.4) and is distributed across agreater area than the other California oaks(Plumb and McDonald, 1981). TheCalifornia black oak type occurs from cen-tral Oregon to the Mexican border acrosselevations ranging from 200 to 8000 feetwith corresponding annual precipitation of25–85 inches annually. Best development ofthe forest type occurs in the northern half ofCalifornia in the Klamath and CascadeMountains and the Coast and Sierra NevadaRanges. There the forest type is found at ele-vations between 1500 and 3000 ft with cor-responding annual precipitation between 30and 50 inches (Eyre, 1980). After distur-bance, this species maintains itself throughsprouting to form even-aged stands. On sub-optimal sites it is successional to other for-est types. Associated species include otheroaks, ponderosa pine, Douglas-fir andPacific madrone (Appendix 3).

Canyon live oak occurs from theWillamette Valley to the Baja Peninsula andeast into Arizona at elevations from near sealevel in the north to 9000 feet in the south(Eyre, 1980). It comprises about one-quarterof California’s oak volume and is secondonly to California black oak in this regard(Table 1.4). Canyon live oak forms purestands on very steep slopes and dry canyonbottoms. Elsewhere it occurs in mixturewith Douglas-fir, ponderosa pine and otherconifers. The species is shade tolerant whenyoung and often maintains itself in rela-tively stable communities (Eyre, 1980).

The blue oak–digger pine forest typesurrounds California’s Central valley atelevations between 500 and 5000 ft,although blue oak occasionally extends tothe valley floor (Fig. 1.22). This forest typeoccurs between the valley grasslands andthe montane forests above, where it can

Oak-dominated Ecosystems 47

14,500

12,000

10,000

8000

6000

4000

2000

Ele

vatio

n (f

t)

Topographic moisture gradient

Mesic Xeric

Alpine

Subalpine forest

Lodgepole pine forest

Red fir forestJeffreypineforest

Ponderosapine forest

White fir forest

Upland live oakwoodland

Mixed evergreenforest and

black oakwoodland

Chamisechaparral

Blue oak woodland

Mixed

chaparra

l

Lowland

live oak

woodland

Mea

dow

Juni

per

woo

dlan

d

Fig. 1.21. Relation of oak forests to elevation, moisture gradients and other forest types found in thePacific–Mediterranean–Marine Region of California (Ecoregion Provinces 261, 262, 263 and M261). Oakforests and woodlands are usually found above the chaparral zone and below the ponderosa pine zone.(Redrawn from Barbour (1988) and Vankat (1982).) Reprinted with permission of Cambridge UniversityPress.)

endure a meagre 10 inches of annual pre-cipitation (Eyre, 1980). Forest cover rangesfrom 30 to 80% with canopy heightsbetween 15 and 50 ft. Associated speciesinclude California live oak, interior liveoak, valley oak and California black oak(Barbour, 1988). At low elevations blueoak and valley oak mixtures developsavanna communities. Valley oak savannasextend into the Central Valley where theymake their best development on alluvialsoils (Griffin, 1977).

The California coast live oak forest type(sometimes referred to as southern oakwoodland) occurs on the west side of theCoast Range in the southern two-thirds ofCalifornia. It extends inland on north-facingslopes of narrow valleys and other coolsites. This type occurs at elevations of up to3000 ft in the northern part of its range andto 5000 ft in the southern portion. Althoughit can form pure, closed canopy stands, it is

considered a woodland type and commonlyoccurs in savannas comprised of scatteredoaks or in mixture with conifers (Appendix3). California coast live oak is long-lived,moderately shade tolerant, and forms rela-tively permanent woodlands. When treesreach about 8 inches dbh they are alsohighly resistant to fire (Eyre, 1980).

The ecological importance of California’soak woodlands and timberlands is receiv-ing increased attention (Pillsbury et al.,1997). Although their value for commercialproducts is low, their importance towildlife, water quality, aesthetics, soil pro-tection, recreation and fuelwood is widelyacknowledged (Helms and Tappeiner,1996). A principal silvicultural problemrelated to the oak woodlands of the PacificMediterranean–Marine Region is ensuringthat the regeneration of oaks is sufficient forreplacing trees periodically lost to naturalmortality and timber harvesting.

48 Chapter 1

Fig. 1.22. Blue oak woodland in the Sierra Nevada Range (Province M261: Dry Steppe Province).(USDA Forest Service, North Central Research Station photograph.)

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