Diameter Caps for ThinningSouthwestern Ponderosa PineForests: Viewpoints, Effects, andTradeoffs

Scott R. Abella, Peter Z. Fule, and W. Wallace Covington

Upper size limits of trees allowed to be cut, termed diameter caps, have resulted in polarization,litigation, and delays and alterations to thinning projects in many western forests. Using southwesternponderosa pine forests as an example, we summarize viewpoints on caps, simulate effects of caps onthinning prescriptions, and provide examples of ecosystem-level tradeoffs of leaving extra trees duringthinning projects. The importance placed on trees versus other ecosystem components primarilydifferentiates those who support caps and those who do not. We conclude that diameter caps mayenhance some ecosystem components, such as densities of large trees, but they negatively impact manynontree components.

Keywords: ecological restoration, treatment prescription, fuel reduction, ecosystem management

T ree densities have increased sharplyin many western frequent-fire for-ests since the late 1800s because of

fire exclusion and other factors (Lynch et al.2000, Allen et al. 2002). In southwesternponderosa pine (Pinus ponderosa) forests, forexample, there is widespread agreement thatmechanical tree thinning is necessary to re-store ecosystem health, reduce crown firehazards, and make human habitations safer(Covington et al. 1997, Allen et al. 2002,Nowicki and George 2004). There also isagreement that, regardless of their size, oldtrees that were established before Euro-American settlement (“presettlement”) inthe late 1800s generally should be con-served. Consensus disintegrates, however,when attention turns to thinning young,postsettlement trees of relatively large diam-eter (often defined as �16 in.). Some indi-viduals believe that thinning these large,

young trees is sometimes necessary to en-hance ecosystem health (Coughlan 2003),whereas others argue that large trees, regard-less of age, generally should be retained(Allen et al. 2002, Nowicki and George2004). To retain all large trees, some peoplesupport a policy of diameter caps, defined asupper size limits of trees allowed to be cut.Sizes of caps in ponderosa pine forests haveranged from as small as 5 in. to greater than18 in. (Coughlan 2003, Fule et al. 2006).Caps are imposed frequently by the US For-est Service, National Park Service, and otherorganizations, partly to show that fuel re-duction and restoration thinning projectsare not simply logging operations in an at-tempt to gain support for these projects(Larson and Mirth 2001, Coughlan 2003).Nevertheless, dissent about diameter capshas resulted in delays and modifications ofnumerous fuel reduction and ecological res-

toration projects in western forests(Coughlan 2003).

Peer-reviewed scientific publications ondiameter caps are rare (Larson and Mirth2001, Coughlan 2003). We suggest that dis-agreements about caps are frequently basedon personal opinion and may be concen-trated on trees at the expense of other impor-tant ecosystem components. In this article,using southwestern ponderosa pine forests asan example, we first provide an overview ofthe viewpoints on diameter caps. Next, weuse stem maps from a field study in northernArizona to assess effects of caps on pine den-sities and patterns in a simulated thinningprescription. Finally, we summarize antici-pated tradeoffs of diameter caps for severalecosystem components and economicsbased on peer-reviewed literature. Our focusis on 16-in. caps, because this cap size hasbeen widely proposed in the Southwest, al-though similar analyses could be applied tocaps of any size. We hope that these analyseswill broaden thinking on diameter caps tomore fully include ecosystem considerationsbeyond the trees themselves. In addition,these analyses may be useful to stakeholdersand policy makers concerned with imple-menting urgently needed thinning projects.

ViewpointsViewpoints on 16-in.-diameter caps

generally fall into three categories: support-ive, neutral, or opposed (Table 1). Depend-

Journal of Forestry • December 2006 407

AB

ST

RA

CT

policy

ing on the individual, advocates of theseviewpoints cite one or several reasons to sup-port their views. Supporters believe thatlarge trees have decreased in density becauseof past logging, so all extant large trees re-gardless of age should be retained during for-est thinning (Nowicki and George 2004).Supporters also believe that these large treesrepresent the next cohort of old trees, areimportant habitat features, will result insnag formation important for wildlife, anddo not need to be removed to reduce crownfire hazards. In addition, underlying supportfor caps is the concern that without caps,forest thinning under veils of restoration orfuel reduction might be motivated by eco-nomics rather than ecology. This concernlikely emanates from distrust of public land-management agencies (Coughlan 2003).

Neutral viewpoints reflect the perspec-tive that because many sites contain fewyoung, large trees, retaining these trees haslittle immediate ecological or economic ef-fect (Table 1). Holders of this viewpoint also

Table 1. Summary of viewpoints aboutdiameter caps and commonly usedarguments to support the viewpoints.a

SupportiveLarge trees are rare so they should be retained

regardless of age or locationLarge trees are ecologically valuable, such as for

snag formationLarge trees are fire resistant so their removal is not

needed for fuel reductionSelling large trees should not pay restoration

project costsEconomics should not drive restoration and fuel

reduction projectsNeutral

Because often there are few young, large trees,they may as well be retained

Leaving a few extra large trees probably has littleecological effect

Diameter caps may allow projects to avoidlitigation

OpposedRemoving young, large trees is sometimes

necessary to restore openingsRetaining too many excess trees compromises

other ecosystem componentsTrees are inflexibly retained even if the stands that

result fail to achieve objectivesLeaving excess trees necessitates future heavy

thinnings and multiple entriesResidual trees grow rapidly after thinning so large

trees accrue quicklySelling young, large trees offsets project costs and

funds follow-up managementSelling young, large trees may allow contracts and

projects to proceed

a Viewpoints and reasons cited in support of the viewpoints arebased primarily on Allen et al. (2002), Coughlan (2003), Fie-dler et al. (1999), Fule et al. (2006), Larson and Mirth (2001),Lynch et al. (2000), and Nowicki and George (2004).

Figure 1. The large tree in the bottom center of the top photo is 23 in. in diameter but onlyapproximately 95 years old. The nearest presettlement evidence is 34 ft away from thispostsettlement tree, indicating that the tree likely invaded a historical meadow opening.This young tree would be retained in thinning projects with 16-in.-diameter caps. Thebottom photo shows presettlement evidence (gray stump) surrounded by small postsettle-ment trees. These photos illustrate a tradeoff of diameter caps: retaining the large post-settlement tree in the top photo provides immediate large-tree structure, yet precludesrestoration of a historical opening. Retaining the small trees around the presettlement stumpin the photo at the bottom would maintain historical locations of trees and openings, but thesmall trees will take longer to develop large-tree characteristics. It should be recognized,however, that on this site with high densities of postsettlement >16-in. trees (42/ac), largeopenings probably cannot be reestablished under any thinning prescription with a 16-in.cap (Figure 2a). (Photos by S.R. Abella, Jan. 17, 2006, Coconino National Forest, northernArizona [35�15�50� N, 111�40�40� W]).

408 Journal of Forestry • December 2006

believe that caps are beneficial if they pre-vent thinning projects from being delayedby litigation.

However, opponents of diameter capsalso have several reasons they believe thatcaps should not be imposed (Table 1). Forexample, they believe that large, young treessometimes need to be removed to restorecanopy openings (Figure 1). Large, youngtrees also may need to be cut if these treessharply exceed presettlement densities, re-sulting in unsustainable stands still suscepti-ble to crown fire after thinning. In addition,opponents believe that residual trees growrapidly after restoration thinning, which issupported by published research (Skov et al.2005), so that caps may not greatly benefitthe development of large trees. Another ar-gument used against caps is that harvestingsome large, young trees can offset the finan-cial costs of restoration and fuel reductionprojects, allowing these projects to proceedmore quickly and encompass greater area.

Simulating EffectsTo understand effects of 16-in.-diame-

ter caps on a thinning prescription, we stem-mapped a 2.47-ac (1 ha), 328 � 328 ft plotin 2004 on each of eight sites studied as partof previous research (Abella and Covington2006). Plots were located in ponderosa pineforests in the Coconino National Forest andNorthern Arizona University CentennialForest near the City of Flagstaff in northernArizona. These plots were not intended toencompass a complete array of stand condi-tions in this region, but plots did cover arepresentative range of stand densities (12–412 live pine/ac) and structures (Abella andCovington 2006). Using tapes laid out tocreate 100, 0.025-ac (33 � 33 ft) cells oneach plot, we mapped all live trees and evi-dence of presettlement tree locations(stumps, stump holes, logs, and snags) to thenearest 0.3 ft in an x,y-coordinate system.We identified presettlement evidence fol-lowing methods in Fule et al. (1997). Wechose the year 1880 to represent settlementand fire-regime disruption on these plots,based on previous research in the study areathat has recorded disruption dates rangingfrom the mid-1870s to the mid-1880s (Cov-ington et al. 1997, Fule et al. 1997).

We used the maps to simulate a resto-ration thinning prescription with and with-out a 16-in.-diameter cap. This prescriptionis designed to reduce tree densities to withinan approximate range of historical variabil-ity, while reestablishing presettlement tree

patterns (Fule et al. 2001). In our simula-tions, 1.5 postsettlement trees �16 in. wereretained within 30 ft of each dead presettle-ment tree, or three postsettlement trees �16in. were retained if larger trees were notavailable. This replacement ratio is intendedto account for possible tree mortality fromtreatment operations (Covington et al.1997, Fule et al. 2001). All living presettle-ment trees were retained.

We found that diameter caps affectedpostthinning stand conditions the most onplots containing many �16-in. postsettle-ment trees and where these trees were notlocated close enough to presettlement evi-dence to be used as replacements. Thinningsimulation results are illustrated in Figure 2for two plots exemplifying different levels ofcap influences. On a plot containing a highdensity (42/ac) of postsettlement trees �16-in., for example, the cap resulted in the re-tention of an extra 28 trees/ac �16 in. com-pared with cap-free thinning. Because thecap-free thinning grouped retained treesaround presettlement evidence, inevitablythe cap led to the retention of additionaltrees within groups or the retention of treesoccurring in openings away from presettle-ment evidence. These extra trees limited thereestablishment of historical openings or anyopenings (Figure 2a). In addition to higherdensities, residual basal area also was sharplyhigher in the cap-constrained prescription(87 ft2/ac) than in the cap-free prescription(39 ft2/ac). Another plot provided an exam-ple where 16-in. caps had little effect becauseonly nine postsettlement �16-in. trees/acreoccurred on this plot. All but one of thesetrees could be used to replace dead presettle-ment trees (Figures 2b and 3).

The restoration prescription based onpresettlement tree locations illustrated inthis article (Figure 2) is only one of manypossible thinning prescriptions that could beused, with or without diameter caps. For ex-ample, Allen et al. (2002) suggested usingexisting structure, rather than presettlementtree locations, to guide thinning and rapidlyreestablish groups of large trees. The pre-scription based on presettlement tree loca-tions differs from this structural approachonly by emphasizing the retention of treesnear presettlement evidence and the reestab-lishment of historical locations of canopyopenings (Figure 1). However, neither ofthese approaches, when constrained by a di-ameter cap, could establish canopy openingson sites containing high densities of �16-in.trees (Figure 2). Caps may similarly affect

other fuel reduction and thinning prescrip-tions, such as those based on meeting targetbasal areas. In cap-constrained basal areacutting, the basal area of �16-in. trees setsthe minimum level of basal area remainingafter thinning, whether or not that levelmeets management objectives.

Our simulations illustrate that influ-ences of caps likely change across the land-scape with variations in abiotic site factors,disturbance history, and past managementthat affect tree size, density, and patternwithin stands to be thinned. The next sec-tion provides examples of long-termtradeoffs for multiple ecosystem compo-nents of imposing or not imposing diametercaps on southwestern ponderosa pine thin-ning projects.

Ecosystem and EconomicTradeoffs

Canopy Openings. Canopy openingsand meadows are key habitat for many or-ganisms in ponderosa pine forests, but thesehabitats have been severely reduced duringthe 1900s by invasion of postsettlementtrees (Moore and Huffman 2004). Caps af-fect canopy openings by precluding their re-establishment altogether or by causing thespatial locations of openings to shift awayfrom their historical locations (Table 2).There is some evidence that openings con-tain soils that differ from those below trees,reflecting long-term vegetation influenceson soil development (White 1985, Kerns etal. 2003). Switching the locations of thesepatches in current forests may have unfore-seen influences on herbaceous productivityor other variables.

Understory Vegetation. Much of thespecies diversity in ponderosa pine forests iscontained in understory vegetation, whichalso provides forage for herbivores (Moore etal. 1999). Understory biomass can be �10times higher in remnant and restored open-ings than even under sparse ponderosa pinecanopy cover (Clary 1975). These overstory-understory relationships illustrate a tradeoff:retaining extra trees because of diameter capsdecreases understory productivity and diver-sity (Table 2).

Large Pine and Snags. Large pine andsnags are important habitat features formany wildlife species (Ganey and Vojta2004). Diameter caps may increase or main-tain densities of large pine and snags, at leastin the short term, after thinning (Table 2).However, consideration also needs to be

Journal of Forestry • December 2006 409

Figure 2. Stem maps showing distributions of ponderosa pine in 2004 (current) and after a simulated 1.5–3 restoration thinningprescription with and without a 16-in.-diameter cap. (a) High densities (42/ac) of >16-in. postsettlement trees limit reestablishment ofmeadows under any thinning prescription with a 16-in. cap. (b) Sixteen-in. caps have little influence on postthinning structure because thesite contains few >16-in. trees. Plots were mapped on the Coconino National Forest, northern Arizona (panel a: 35�15�50� N, 111�40�42�W; panel b: 35�02�30� N, 111�38�33� W).

410 Journal of Forestry • December 2006

given to how limiting snag densities are towildlife populations and what species maybe limited by snag availability. For example,Brawn and Balda (1988) concluded thatonly three of six cavity-nesting bird speciesthey studied in northern Arizona ponderosapine forests were limited by nest-site avail-ability in snags. Food and foraging substrate,which often decrease with increasing pinedensities (Clary 1975, Waltz and Covington2004), may more strongly limit bird densi-ties on some sites.

Wildlife Habitat. Diameter caps mayinfluence wildlife by affecting stand struc-

ture and other variables impacted by thisstructure such as understory vegetation andmast-producing Gambel oak (Quercus gam-belii). Caps probably will promote wildlifespecies benefiting from higher pine densi-ties, while negatively impacting species de-pendent on open forests. For example,Dodd et al. (2003) found that tassel-earedsquirrels (Sciurus aberti) in northern Arizonaponderosa pine forests were more abundantin stands with high densities of interlockingcanopy trees. On the other hand, thesestands constitute poor habitat for species de-pendent on open-forest features, such as

high solar radiation and understory produc-tivity. Maintaining open stands is consistentboth with ecosystem-based (rather than sin-gle species) management and the evolution-ary environments many wildlife species en-countered in historical ponderosa pineforests (Moore et al. 1999).

Pine Autecology. Diameter caps couldhave no effect, negative effects, or positiveeffects on pine variables such as genetics andregeneration (Table 2). Thinning reducestree population sizes, which may decreasegenetic diversity. However, Kolanoski(2002) found that much of the genetic di-versity of ponderosa pine at a northern Ari-zona site was related to differences in treeages and establishment periods. Many post-settlement 16-in. trees are similar age andoriginated under similar conditions shortlyafter Euro-American settlement. It is possi-ble that thinning these 16-in. trees to createregeneration opportunities for new cohortswould increase genetic diversity for estab-lishment in current environmental condi-tions. However, different findings in otherregions suggest that additional research isneeded for clarification (Linhart et al. 1981).Another consideration is whether the epi-sodic timing of natural regeneration of pon-derosa pine can meet desired levels in post-thinning forests. Bailey and Covington(2002) evaluated regeneration rates afterseveral tree thinning and prescribed burningprojects in northern Arizona and concludedthat regeneration rates were sufficient or ex-ceeded those needed to maintain presettle-ment tree densities. It is unclear if regenera-tion might be more or less prominent inother regions. In areas where regeneration isdeficient, caps might be used to increase seedsources, but they also may decrease densitiesof regeneration microsites if litter accumu-lates in the absence of burning. Many addi-tional factors warrant consideration, such aspotential impacts of caps on density-depen-dent tree-damaging agents such as insectoutbreaks.

Timber Production. Although diame-ter caps decrease timber production in theshort term, they probably increase the grow-ing stock in the long term. Caps can result inevenly spaced stands of young, rapidly grow-ing trees (Figure 2a). Skov et al. (2005)found that growth of 80-year-old residualponderosa pines increased within 1 year af-ter thinning in northern Arizona, suggestingthat 16-in. trees left by caps grow quickly.Ironically, a major reason cited to supportcaps is that timber production and eco-

Figure 3. Tree densities corresponding with stem maps in Figure 2. Total trees per acre isshown at the top of each bar. The presettlement category represents tree densities at thetime of fire exclusion after Euro-American settlement in approximately 1880. (a) A 16-in.-diameter cap nearly doubled the tree density remaining after a simulated restorationthinning prescription. (b) In contrast, a cap altered residual density by only one tree per acrebecause the plot contained few >16-in. postsettlement trees (nine per acre). There were noliving presettlement-origin trees in panel a, while panel b contained two live presettlementtrees per acre, which were >16 in. in diameter.

Journal of Forestry • December 2006 411

nomic returns from selling large postsettle-ment trees should not drive restoration andfuel reduction projects (Table 1). On somesites, caps may necessitate major sawtimberharvests in the future to again reduce wild-fire risks and improve ecosystem health (e.g.,Figure 2a). However, depending on residualstand density and stem spacing, caps alsocould result in increased competition andslowed growth of both young and old trees(Skov et al. 2005).

Gambel Oak Vigor. As one of only afew deciduous trees in ponderosa pine for-ests, Gambel oak constitutes an importanthabitat feature for many wildlife species byproviding acorns and cavities (Reynolds etal. 1970). Large-diameter oaks have uniquevalues, and pine thinning may increase oakgrowth to speed the recruitment of largeoaks (Onkonburi 1999). However, diametercaps may constrain thinning postsettlementpine around oaks, slowing oak growth andhastening declines of old oaks (Onkonburi1999).

Soil Nutrient Cycling and Microor-ganisms. Kaye and Hart (1998) found thatcanopy openings had annual net N mineral-ization rates twice as high as those below re-tained postsettlement trees 2 years after thin-ning in northern Arizona. Similarly, Boyle etal. (2005) reported that during dry periods,soil respiration rates were 33% greater incanopy openings than below postsettlementtrees. These studies suggest that maintainingstands of high tree density may slow soil nu-trient cycling and microbial activity, possi-bly decreasing long-term soil productivity(Boyle et al. 2005).

Invertebrates. In northern Arizona,Waltz and Covington (2004) found thatbutterfly richness and abundance were two

to three times greater in restoration (thinned� burned) units than in paired control units2 years after treatment. These increases pri-marily resulted from increased sunlight,which can affect butterfly flight durationsand patterns (Waltz and Covington 2004).Retaining additional trees because of diam-eter caps consistently reduces solar radiationreaching the ground. However, habitat re-quirements of invertebrate species varywidely and some species using pine litter orotherwise strongly associated with pine maybenefit from diameter caps. As a result, capsmay have positive, negative, or no effects oninvertebrate species, and it remains unclearhow caps may affect the invertebrate com-munity as a whole.

Water Relations. On the Beaver Creekwatershed in Arizona, Brown et al. (1974)monitored streamflow for 4 years after athinning using group selection removed75% of basal area. Thinning increasedstreamflow on average 22% per year, while agrazing treatment (60% use of perennialgrasses) increased streamflow only 8%. Re-ducing tree densities, therefore, enhancedstreamflow more sharply than reducing her-baceous vegetation. Correspondingly, it isunlikely that diameter caps will increasestreamflow, and they may decrease flow be-cause of increased tree canopy interceptionand water use.

Fire Behavior. We estimated foliarbiomass following Kaye et al. (2005) for themapped plots in Figure 2. Fine-canopy fuels,especially foliage, comprise the fuel that car-ries crown fire. Although the vertical andhorizontal arrangement of these fuels is alsoimportant, foliar biomass is directly linkedto canopy bulk density and crown fire be-havior (Cruz et al. 2003). On a plot contain-

ing a high density of �16-in. trees (Figure2a), only 21% of foliar biomass remainedafter simulated restoration thinning withouta cap, compared with 51% remaining aftercap-constrained thinning. In contrast, resid-ual foliar biomass was only 4% greater with acap than without a cap after simulated thin-ning on a plot containing few �16-in. trees(Figure 2b). Because reducing crown firehazard is currently a part of managementplans for many thinning projects in westernforests, careful consideration should begiven to whether this objective is met with adiameter cap. Focus should not be restrictedto stand conditions immediately after thin-ning and should include the potential forcanopy ingrowth.

Economics. Small-diameter ponderosapine logs have little economic value, andeconomics is one of the major factors limit-ing the implementation of thinning projects(Lynch et al. 2000, Larson and Mirth 2001).Larson and Mirth (2001) modeled the eco-nomic effects of 16-in. caps on a northernArizona thinning project. Caps reduced netprofits of thinning contractors by a pro-jected 22–176%, which resulted in netlosses to contractors in some units. Whilemarkets for �16-in.-diameter logs vary geo-graphically and through time, caps likelynegatively impact contractor availability andthe economics of thinning projects by re-ducing the size of logs produced (Larson andMirth 2001). Furthermore, selling somepostsettlement �16-in. trees might generateforest products and funds that could be usedfor postthinning management, such as ex-otic species control, seeding native species,and prescribed burning (Fiedler et al. 1999).

In summarizing tradeoffs of caps, a cen-tral tenet in ecology is that resources are lim-ited in ecosystems. Retaining additionaltrees inevitably decreases resources availableto many other organisms. Most modernponderosa pine forests are vulnerable to cat-astrophic change precisely because ecosys-tem biomass dramatically shifted towardponderosa pine after tree density irruptionsin the late 1800s (Covington et al. 1997,Allen et al. 2002). Diameter caps appearmost useful for maintaining high densities ofyoung, large trees immediately after thin-ning, creating habitat for specific species(e.g., tassel-eared squirrels) favored by hightree densities and sustaining a large timberbase (Table 2). Based on published research,diameter caps will not improve and willlikely negatively impact canopy openings,understory productivity, Gambel oak vigor,

Table 2. Anticipated effects of diameter caps on several ecosystem components andeconomics in southwestern ponderosa pine forests. Effects can be chiefly positive (�),neutral (0), or negative (�).

Variable Effect Example reference

Canopy openings � Moore and Huffman 2004Understory vegetation � Clary 1975Large pine and snags � or 0 Ganey and Vojta 2004Wildlife habitat �, 0, or � Dodd et al. 2003Pine autecology �, 0, or � Kolanoski 2002Timber production � or 0 Skov et al. 2005Gambel oak vigor � Onkonburi 1999Soil microorganisms � Boyle et al. 2005Nutrient cycling � Kaye and Hart 1998Invertebrates �, 0, or � Waltz and Covington 2004Water relations � Brown et al. 1974Fire behavior 0 or � Fulé et al. 2001Economics � Larson and Mirth 2004

412 Journal of Forestry • December 2006

soil microorganisms, nutrient cycling,streamflow, fire behavior, and the econom-ics of thinning projects.

ConclusionsOur analysis revealed the following six

major considerations about the use of 16-in.-diameter caps in thinning projects. First,prethinning densities of �16-in. trees deter-mine the magnitude of the effects that capshave on postthinning tree structure and eco-system components affected by trees. Sec-ond, caps can result in switching the loca-tions of tree and meadow patches from theirhistorical locations. However, caps may beuseful to take advantage of existing treestructure to rapidly reestablish groups oflarge trees, if maintaining meadow locationsis not an objective (Allen et al. 2002). Onthe other hand, caps may result in slowedtree growth, depending on residual standdensity and stem spacing. Third, caps are aone-size-fits-all policy, which seems at oddswith the diversities of sites and managementobjectives in western forests. This observa-tion underscores that informed policiesabout caps should consider desired futurestand conditions defined by managementobjectives and whether or not these condi-tions are best met with cap-free or cap-con-strained thinning.

Fourth, maintaining high tree densitiesmeans fewer resources are available for eco-system components dependent on openstand structures, which characterized mostevolutionary environments in southwesternponderosa pine forests (Moore et al. 1999).Fifth, evaluations of diameter caps shouldnot be restricted to conditions immediatelyafter thinning. Trees grow, and effects of fu-ture canopy ingrowth should be considered.Sixth, we suggest that a reasonable evalua-tion of the tradeoffs of diameter caps includewhether or not a cap-constrained thinningproject on a specific site establishes canopyopenings and reduces crown fire hazards. Ifthese key objectives are not met, thinningprojects may have limited benefit for ecosys-tem health or for the safety of human settle-ments in fire-prone western forests. Capscan affect the trajectory and the magnitudeof ecosystem change after thinning, with re-sulting ecosystem characteristics sometimesdiffering substantially between thinningwith or without caps (Fule et al. 2006). Al-though our analysis focused on southwest-ern ponderosa pine forests, similar assess-ments of the tradeoffs of diameter caps maybe applicable in other forest types for mak-

ing informed policy and management deci-sions.

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Scott R. Abella (scott.abella@unlv.edu) is as-sistant research professor, Public Lands Insti-tute and Department of Biological Sciences,University of Nevada Las Vegas, Las Vegas,NV 89154-2040. Peter Z. Fule (pete.fule@

nau.edu) is associate director and associate pro-fessor, and W. Wallace Covington (wally.covington@nau.edu) is director and regents’professor of forest ecology, Ecological Restora-tion Institute and School of Forestry, NorthernArizona University, Flagstaff, AZ 86011-5017. The authors thank Charlie Denton,John Bedell, Judy Springer, Diane Vosick, andfour anonymous reviewers for providing help-ful comments on earlier drafts of this work.

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