By W. Wallace Covington, Peter Z. Fule, Margaret M. Moore, Stephen C. Hart,Thomas E. Kolb,Joy N. Mast,Stephen S. Sackett, and Michael R.Wagner

Restoring Ecosystem Health inPonderosa Pine Forests of the Southwest

major stumblingA block to ecosys-tem management is thelack of healthy ecosys-tems to serve as pointsof comparison. AldoLeopold (1934) recog-nized this problem andsuggested using ecolog-ical restoration tech-niques to reconstructreasonable approxima-tions of naturally func-tioning ecosystems(Callicott and Flader1992). We have initi-ated an integrated setof research projects de-signed to accomplishthis goal and in theprocess determine eco-system health attrib-utes for a southwest-ern ponderosa pine ecosystem.

Previous research has established thatforests of ponderosapine (Pinusponderosa)in the Southwest weremuch more open be-fore Euro-American

settlement (Cooper 1960; Covingtonand Moore 1994b). Until the 1870s,light surface fires every two to fiveyears, along with grass competitionand regular drought, maintained an open and parklike landscape domi-nated by grasses, forbs, and shrubswith scattered groups of ponderosapine trees. After Euro-American settle-

A controlled burn was set by researchersin thinned stands of southwesternponderosa pine to reconstruct the open, parklike landscape that once characterized the ecosystem.

ment, heavy livestock grazing, fire sup-pression, logging disturbances, and cli-matic events favored dense ponderosa pine regeneration, and the open park-lands closed.

Various authors (Covington andMoore 1994a, 1994b; Kolb et al.1994) have described unhealthy char-acteristics of the postsettlement pon-derosa pine ecosystems, including in-creases in tree density, forest floor depth, and fuel loading, with the fol-lowing consequences: (1) decreases insoil moisture and nutrient availability,(2) decreases in growth and diversityofboth herbaceous and woody plants, (3)increases in mortality in the oldest ageclass of trees, (4) decreases in streamand spring flows, and (5) increases infire severity and size.

Our research was guided by the fol-lowing general hypotheses:

that both restoration of ecosystemstructure and reintroduction of fire are necessary for restoring rates of decom-position, nutrient cycling, and net pri-mary production to natural, presettle-ment levels; and

that the rates of these processes will be higher in an ecosystem that ap-proximates the natural structure anddisturbance regime.

Specifically, we hypothesized thatreestablishing presettlement standstructure alone (thinning a majority ofthe postsettlement trees) would resultin lower rates of decomposition, nutri-ent cycling, and net primary produc-tion compared with thinning, forestfloor fuel manipulation, and pre-scribed burning in combination; but

Reprinted from the Journal of Forestry,Vol. 95, No. 4, April 1997. Not for further reproduction. 23

that both treatments would result inhigher rates of these processes com-pared with controls. We further hy-pothesized that without periodic burn-ing to hold them in check, pineseedling irruptions would recur on thethin-only treatment area. We addressedthree specific questions:

1. How have ecosystem structure(by biomass component) and nutrient

storage changed over the past centuryof fire exclusion in a ponderosa pine-bunchgrass ecosystem?

2. What are the implications ofthese changes for net primary pro-duction, decomposition, nutrient cy-cling, and other important ecosystemcharacteristics?

3. Does partial restoration (restor-ing tree structure alone by thinning

Figure 1. Canopy maps of the study area within theFortValley Experimental Forest,near Flagstaff, Arizona. Scatteredgroups of southwesternponderosapinepredominated in the late 19th century (top),before settlement suppressed natural wildfires.Thissuppression causeda far greater profusionofvegetation in dense“doghair” thickets of pinesaplings(middle). Researchersare now attempting, throughprescribedburnsand manualthinning,to restore theecosystemto i t s original condition (bottom).

postsettlement trees)differ from completerestoration (the samethinning, plus forestfloor removal, loadingwith herbaceous fuels, and prescribed burn-ing) in its effects onecosystem structureand function?

The StudyOur study area is a

virgin stand of pon-derosa pine within theFort Valley Experi-mental Forest approxi-mately seven miles northwest of Flagstaff, Arizona. Stand struc-ture is an uneven-agedforest with trees insmall, uneven-agedgroups (fig.1), typicalof ponderosa pine inthe Southwest (Cooper1960; Schubert 1974; White 1985). Pole-sized trees, approxi-mately 4 to 14.5 inches dbh, are the predomi-nant size class, withscattered groups of pre-settlement trees 14.5 to41 inches dbh anddense “doghair” thick-ets of saplings up to 3.9 inches dbh. The treat-ment plots are in an eight-acreportion of theG.A. Pearson NaturalArea, which was decom-missioned in 1987. In-dividual tree growth andmortality records for alltrees greater than six

The time has come for science to busy itselfwith the earth itself.The first step is toreconstruct a sampleof what we had tobegin with.

--Aldo Leopold, 1934

inches dbh are available at five-year in-tervals from 1920 to the present (Averyet al. 1976; and unpublished data).

The area is gentle in topography(slopes less than 5 percent) with asouthwest aspect. Elevation is 7,200 to 7,400 feet. The climate is cool andsubhumid with early summer drought.Mean annual precipitation is 22.3inches and mean annual temperature is45.5°F. Approximately half the precip-itation falls as snow. The average frost-free growing season is 94 days. Soils atthe study site are derived from basaltand volcanic cinders and are a fine montmorillonitic complex of frigidTypic Argiborolls and Argiboralfs.

The understory consists primarilyof Arizona fescue (Festuca arizonica),mountain muhly (Muhlenbergia mon-tana), mutton bluegrass (Poa fendleri-ana), pine dropseed (Blepharoneurontricholepis),bottlebrush squirreltail (Si-tanion hysterix),and a variety of forbs.Buckbrush (Ceanothus fendleri) is the only common shrub.

The site has never been harvested;the only major disturbances have beenlivestock grazing between 1876 and1910 and fire exclusion since 1876.Dieterich (1980) determined that thelast natural fire in the vicinity occurred in 1876, before that, fires occurred atan average interval of about two years.

Rationale forTreatmentsA fundamental issue is what treat-

ment or combination of treatments will rapidly achieve some facsimile of ahealthy ponderosa pine ecosystem.Thetwo leading candidates for ecological

24 April 1997

restoration of ponderosa pine ecosys-tems are prescribed burning and thin-ning from below (Covington andMoore 1994b; Arno et al. 1995).

Previous research near our studyarea had shown that although pre-scribed burning alone (without thin-ning or manual fuel removal) couldreduce surface fuel loads, stimulate ni-trogen availability,and increase herba-ceous productivity, it caused highmortality of the presettlement trees(60 percent mortality over a 20-yearperiod) and lethal soil temperaturesunder presettlement tree canopies(Covington and Sackett 1984, 1990, 1992; Harrington and Sackett 1990;Sackett et al. 1993). Although somethinning of postsettlement ponderosa pine trees was accomplished by pre-scribed burning (Harrington andSackett 1990), results were localized,unpredictable, and difficult to con-trol. Furthermore, even under veryconservative prescriptions (low airtemperatures and low windspeed), re-burning can produce dangerous fires(Covington and Sackett, pers. observ.,1984-95) because the continuing high density of postsettlement treesprovides a continuous fuel ladder andthus a high crown-fire potential.Clearly, research shows that prescribedburning in today’s unnatural structure will not restore natural conditions inponderosa pine-bunchgrass ecosys-tems. Thus, some combination ofthinning, manual fuel removal, andprescribed burning will be necessaryto quickly restore these systems to nat-ural conditions.

Design and SamplingThe study area was divided into 15

plots, five each in the complete restora-tion (3.5acres), partial restoration (3.3acres), and control (3.8 acres) treat-ments. Permanently marked photopoints were established. All plots werecompletely stem-mapped in 1992, in-cluding all living and dead trees andlarge downed logs. Species, conditionclass, and diameter at breast height(4.5 feet) were recorded for all trees,and all retained and control plot treeswere permanently tagged. All presettle-

ment and potentially presettlementtrees-those with diameter at breastheight >14.8 inches or yellowed bark(White 1985)-were cored and cross-dated (Stokes and Smiley 1968) to de-termine age. Cross-sections or incre-ment cores were taken from all pon-derosa pine snags, stumps, anddowned trees that were potentially ofpresettlement age. Samples were cross-dated and, when possible, death date, pith date, and diameter in 1876 were

determined. A random 5 percent sub-sample of postsettlement trees was alsocored and aged.

Herbaceous and fuel variables were measured on 55 replicated circular subplots (16.4 feet in diameter) locatedwithin four overstory strata: (1) preset-tlement patches, (2) postsettlementpatches to be thinned with some treesretained, (3) postsettlement patches tobe converted to openings, and (4) rem-nant natural grassy openings. Herba-

Journal of Forestry 25

ceous species composition, basal cover, and production were measured on two10.7 ft2 quadrats within each subplot.Forest floor fuels were averaged fromtwo 16.4-foot planar transects on each subplot (Sackett 1980).

Both partial and complete restora-tion areas were thinned in fall 1993and the stumps cut to ground level.All presettlement trees were retained,as were all trees above 16 inches dbh.In addition, smaller-diameter postset-tlement trees were retained nearstumps, snags, and down presettle-ment logs in an attempt to recreatethe clumped pattern of the presettle-ment forest. Realizing that many ofthe spindly postsettlement trees wouldnot survive, we left approximatelythree postsettlement trees for everydead presettlement tree.

Slash was removed in spring 1994and an eight-foot electrified elk fence was constructed around the study areaperimeter. O n the complete restora-tion treatment plots, the litter layer(Oi) was raked aside, accumulatedduff (Oa and Oe) was removed, andlitter was scattered back across the for-est floor. Coarse woody fuels of pre-settlement origin, such as the limbsand boles of fallen presettlement trees,were retained, but postsettlementwoody fuels were removed. Averageduff depth in the presettlement stra-tum dropped from 3.3 to 0.4 incheseven though litter depth was nearlyunchanged (0.5 to 0.6 inches). Be-cause the presettlement fuel structurewas composed primarily of grasses,approximately 600 pounds per acre ofsupplemental native grass fuels (har-vested from a nearby natural meadow in September 1994) was scatteredacross the complete restoration plots.Soil and tree cambial thermocoupleswere installed in representative loca-tions in each complete treatment plot(Sackett and Haase 1992). The plotsslated for complete restoration werethen burned on October 12, 1994, when the temperature was 58o to60oF, winds were five to 10 mph fromthe east-southeast, and relative hu-midity was around 30 percent.

In addition to tree density, herba-

Figure 2. Mean volumetric soil moisturecontent of mineral soil (0- to 12-inchdepth) for control,partial, and complete restorationtreatments duringthe I995growingseason.Soil moisture intreated areas i s appreciably higherduringthenormalearly summer drought,creatingmore favorable conditionsfor the survivalofyoungtrees and other vegetation.

Figure 3. Mean mineralsoil temperature (3-inch depth) in control, partial, andcomplete restoration treatments duringsummer and fall I995.The higher mineralsoil temperaturesin treated areas are presumedto be the result of increased insolationat the soil surface and decreasedinsulationat the forest floor.

ceous vegetation cover, and fuel load-ing, we are monitoring a broad rangeof ecological attributes related to eco-system health.

Results and DiscussionDendrochronological analysis re-

vealed that forest structure had changed substantially in our study areasince fire regime disruption (table 1).Particularly striking is the populationgrowth of ponderosa pine from 22.8trees per acre in 1876 to 1,253.5 treesper acre in 1992. Although small-di-ameter trees (<16 inches dbh) in-creased from 3.4 per acre in 1876 to

1,240.7 in 1992, larger trees decreased from 19.4 per acre in 1876 to 12.8 in1992, despite protection from logging. The irruption of smaller-diameter pine trees also created a continuous treecanopy cover (fig. 1) at the expense ofherbaceous vegetation. In 1876 only19 percent of the surface area wasunder pine canopy, with the balance(81 percent) representing grassy open-ings, whereas in 1992pine canopy cov-ered 93 percent of the area, with only7 percent left as grassy openings.

The spatial pattern of presettlementtrees in the Pearson Natural Area isstriking. These trees grew in distinct

26 April 1997

0.05- to 0.7-acre groups of two to 40 trees (White 1985; Fule et al. 1993).Although canopy cover within a groupcan be quite high, when averaged withthe interspaces that have no treecanopy cover, the result is a low canopycover overall.

Thinning resulted in the removal of901 trees per acre. A total of 5,500board feet per acre (3,700 board feetper acre of 9- to 16-inch dbh trees plus1,800) board feet per acre of 5- to 8.9-inch dbh trees) was removed. Most ofthe smaller-diameter trees (629 treesper acre in the 1- to 4.9-inch dbhclass) were used as latillas for adobehome construction, but the 37 tonsper acre of thinning slash posed amajor problem. Because there was nomarket for this material, it was hauledto a borrow pit and burned; the re-moval required about 75 loads in 18- wheel dump trucks.

Before the forest floor was raked,fuel loads averaged 28.3 tons per acre.Approximately 21.3 tons per acre ofduff was removed by raking, some ofwhich became garden mulch. Additionof mown grass (0.3 tons per acre) onthe complete restoration treatment in-creased forest floor fuel loading to 7.5tons per acre; burning consumed 3.2tons per acre, leaving 4.3 tons.

For the complete restoration treatment, fire intensity was low:flame lengths averaged approxi-mately six inches with occasionalflareups of one to two feet. Severallarge dead and down presettlementlogs, however, burned intensely andwere completely consumed. Cambialheating was also low or insignificant,showing minor heat pulses (mean in-crease = 44°F, n = 23) during the burn. Individual thermocouples ontwo trees registered higher tempera-tures (maximum increases 82° and147°F), but only on one of the sixprobes per tree. Surface soil temper-atures peaked in excess of 400°Fduring burning, but subsurface tem-peratures (one to four inches) ex-ceeded 150°F on only one of fivesoil temperature sampling sites.Temperatures under dead presettle-ment-era logs remained high for the

longest period, with surface and soiltemperatures exceeding 200° F (peaktemperature >400°F) for 24 to 48hours. Fuels were reduced by theburn to an average 4.3 tons peracre-a total reduction of 85 percentfrom the pretreatment fuel loading.

During the 1995 growing season,soil moisture and temperature wereconsistently higher in treated areasthan in the control (figs. 2 and 3). Soil moisture in treated areas is higher probably because the canopyand forest floor no longer intercept asmuch precipitation, and there is alsoless transpiration by postsettlementtrees. Higher mineral soil tempera-tures are presumably the combinedresult of increased insolation at thesoil surface and decreased insulationat the forest floor.

We hypothesize that those micro-environmental differences betweentreated and control areas will resultin higher rates of such soil processes as fine root production, litter de-composition, and nitrogen mineral-ization in treated stands. Further-more, we hypothesize that the changes in the soil process rates will in turn increase herbaceous produc-tion, tree growth, and resistance toinsect attack. In fact, resin flow of

presettlement trees was higher on thethinned area (4.4 ml per 24 hours)than on the control area (3 ml per 24hours), as was foliar toughness (76 gof tension on thinned trees com-pared with 69.8 g of tension on con-trol trees), suggesting increased resis-tance to bark beetles and foliage-feeding insects. No changes in popu-lations of turpentine beetles (Den-droctonus valens) were observed.

Furthermore, herbaceous produc-tion has responded markedly to thetreatments (fig. 4) with the greatest re-sponse to date in the grassy substra-tum. By 1995 the treated areas were producing almost twice as muchherbaceous vegetation as the control.

Figure 4. Total aboveground herbaceous production (pounds per acre per year) by treatment and patches. In postsettlement retained patches, postsettlement treeswere retained to replace presettlement trees that had died since 1876. Controltreatment has no patches where postsettlement trees were removed. Grassypatchesare natural openings in the overstory.

Journal of Forestry 27

Conclusions and OutlookPreliminary results from our ecosys-

tem restoration work are encouraging. The combination of thinning and burning-the complete restoration treatment-has changed forest struc-ture from fire behavior fuel model 9(Anderson 1982), in which crown firesare common, to fuel model 2, in whichsurface fires occur but crown fires arehighly improbable.

The reduction in tree competitionhas improved moisture availability andhas likely increased insect resistance ofpresetdement trees. Grasses, forbs, andshrubs are responding favorably aswell, indicating a shift away from a net primary productivity dominated bypine trees toward a more diverse bal-ance across a broader variety of plants.

Ecosystem restoration research re-quires a long-term, interdisciplinary commitment. We expect the ecosystemattributes we are measuring to be intransition for the next 10 to 20 yearsbefore they stabilize around some long-

term mean. We plan to continue thisproject for the next 24 years with eightburnings at three-year intervals, rimed to coincide with the natural spring andsummer burning season. Data on otherattributes will be used to increase un-derstanding of the restoration treat-ments on a range of ecosystem charac-teristics. For example, we hypothesizethat the complete restoration treat-ment will increase soil moisture andmineralization and uptake of nitrogen,leading to increased photosynthesis,growth efficiency, and resin produc-tion by presettlement trees, and in-creased cover and production byshrubs and herbaceous vegetation. With favorable climatic events, we ex-pect pine seedlings to become estab-lished on the partial restoration rreat-ment, repeating the pine irruptionsthat occurred after fire regime disrup-tion in the 1870s.

Realizing that the intensive re-search-oriented treatments we haveused in this research project are im-practical on an operational scale, wehave joined with the Arizona Strip Dis-trict of the Bureau of Land Manage-ment to test practical ecosystem res-toration treatments on a larger scale inthe Mount Trumbull Resource Con-servation Area, north of the Grand Canyon. There we are using an adap-tive ecosystem management approach(Walters and Holling 1990) to restoremore than 3,000 acres of ponderosapine to conditions approximating those that existed before Euro-Ameri-can settlement. We are using morepractical approaches, such as rakingthe forest floor away from the base ofthe presettlement trees (instead of re-moving it completely) and monitoringa subset of our basic ecosystem healthattributes.

Because the treatment areas atMount Trumbull are large, we are now able to measure some variables that op-erate on a larger scale, such as passerinebird populations, community structureof selected insect guilds (butterflies),and other indicators of landscape-scaleecosystem health. Our hope is thatthrough such a set of integrated, adap-tive ecosystem restoration projects, we

can alleviate many of the symptoms ofecosystem pathology while simultane-ously increasing our understanding ofecosystem structure and function.

We argue that healthy conditions should be based on an understandingof the environmental conditions thatshaped the evolution of native forestecosystems, and we urge the use ofmultidisciplinary measures of ecosys-tem condition for understanding eco-system health.

28 April 1997

Literature CitedANDERSON, H.E. 1982. Aids to determining fuel

models for estimating fire behavior. General Tech- nical Report INT-122. Ogden, UT: USDA For-est Service.

ARNO, S.F., M.G. HARRINGTON, C.E. FIEDLER, and C.E. CARLSON. 1995. Restoring fire-dependent ponderosa pine forests in western Montana.Restorationand Management Notes 13(1):32-36.

AVERY, C.C., F.R. LARSON, and G.H. SCHUBERT. 1976. Fifty-year records of virgin stand develop- ment in southwestern ponderosa pine. GeneralTechnical Report RM-22. Fort Collins, CO:USDA Forest Service.

BASKETVILLE, G.L. 1965. Estimation of dry weight of tree components and total standing crop inconifer stands. Ecology 46:867-69.

BEARE, M.H., C.L. NEELY, D.C. COLEMAN, and W.L. HARGROVE. 1991. Characterization of asubstrate-induced respiration method for mea-suring fungal, bacterial and total microbialmasson plant residues. Agriculture, Ecosystems andEnvironment 34:65-73.

BINKLEY, D., and S.C. HART. 1989. The compo-nents of nitrogen availability assessmentsin for-est soils. Advances in Soil Science 10:57-112.

BROWN, J.K. 1974. Handbook for inventorying downed woody material. General Technical Re-port INT-16. Ogden, UT: USDA Forest Service.

CALDWELL, M.M., and R.A. VIRGINIA. 1989. Root systems. In Pland physiological ecology: Fieldmethods and instrumcntation, 367-98. NewYork Chapman and Hall.

CALLICOTT, J.B., and S.L. FLADER, eds. 1992. TheRiver of the Mother of God and other essays by Aldo Leopold. Madison: University of Wisconsin. Press.

COOK, C.W., and J. STUBBENDICK, eds. 1986.Range research: Basic problems and techniques. Denver: Society for Range Management.

COOPER, C.F. 1960. Changes in vegetation, struc-ture, and growth of southwestern pine forestsince white settlement. Ecological Monographs30(2):129-64.

COVINGTON, W.W., and M.M. MOORE. 1994a.Changes in multiresource conditions in pon-derosa pine forests since Euro-American settle-ment. Journal of Forestry 92(1):39-47.

--------, 1994b. Post-settlement changes in natural fire regimes and forest structure: ecologicalrestoration of old-growth ponderosa pine for-ests.Journal of Sustainable Forestry 2(2):153-81.

COVINGTON, W.W., and S.S. SACKETT. 1984. Theeffect of a prescribed burn in southwestern pon-derosa pine on organic matter and nutrients inwoody debris and forest floor. Forest Science

-------, 1990. Fire effects on ponderosa pine soils and their management implications. In Effects of fire management of southwestern natural resources, 105-11. General Technical Report RM-191.Fort Collins, CO: USDA Forest Service.

------, 1992. Spatial variation in soil mineral ni-trogen following prescribed burning in pon-derosa pine. Forest Ecology and Management

EDWARDS, N.T. 1982. The use of soda-lime formeasuring respiration rates in terrestrial ecosys-tems. Pedobiologia 28:321-30.

FRITTS, H.C., and T.W. SWETNAM. 1989. Den-droecology: A tool for evaluating variations inpast and present forest environments Advancesin Ecological Research 19:111-88.

FULE, P.Z., W.W. COVINGTON, and M.M. MOORE. 1993. Scaling sample plots to estimate patchcharacteristics in southwestern ponderosa pine.Paper presented at the Eighth Annual US Land-scape Ecology Symposium, March 24-27,1993, Oak Ridge, TN.

HARRINGTON, M.G., and S.S. SACKETT. 1990.Using fire as a management tool in southwest- ern ponderosa pine. In Effects of fire management of southwestern natural resources. General Techni-cal Report RM-191. Fort Collins, CO: USDA Forest Service.

1992. Decomposition and nutrient dynamics inponderosa pine needles in a Mediterranean-typeclimate. Canadian Journal of Forest Research22:306-14.

KOLB, T.E., M.R. WAGNET, and W.W. COVING- TON. 1994. Concepts of forest health. Journal of

KOZLOWSKI, T.T., P.J. KRAMER, and S.G. PAL- LARDY. 1991. The physiological ecology of woody plants. New York: Academic Press.

LEOPOLD, A. 1934. The arboretum and the univer-sity. Parks and Recreation 18(2):59-60.

LEVERENZ, J.W., and HALLGREN. 1991. Measur-ing photosynthesis and respiration of foliage. InTechniques and approaches in forest tree ecophysi- ology, eds. J.P Lassoie and T.M. Hinckley,

PARIZEK, R.R., and B.E. LANE. 1970, Soil-watersampling using a pan and deep pressure-vacuumlysimeters.Journal of Hydrology 11:l-21.

RITCHIE, G.A., and T.M. HINCKLEY. 1975.The pm-sure chamber as an instrument for ecological re- search.AdvancesinEcological Research9: 165-254.

RUNDEL, P.W., and W.M. JARRELL. 1989. Water in the environment. In Plant physiological ecology today-- field methods and instrumentation, eds. R.W. Pearcy,J. Ehletinger, H .A. Mooney, and R.W. Rundel,29-56. New York: Chapman and Hall.

SACKETT, S.S. 1980. Reducing natural ponderosapine fuels using prescribed fire: Two case studies.Research Note RM-392. Fort Collins, CO:USDA Forest Service.

SACKETT, S.S., and S. HAASE. 1992. Measuring soiland tree temperatures during prescrbied fires withthermocouple probes. General Technical Report

30(3):183-92.

54(2):175-91.

HART, S.C., E.A. PAUL, and M.K. FIRESTONE.

Forestry 92(7):10-15.

303-28. Boston: CRC Press.

PSW-GTR-131. Albany, CA: USDA ForestService.

SACKETT, S.S., S. HAASE, and M.G. HARRINGTON.1993. Restoration of southwestern ponderosapine ecosystems with fire. In Sustainable ecologi-cal systems: Zmplementhg an ecological approachto land management, eds. W.W. Covington andL.F. DeBano. General Technical Report RM-247. Fort Collins, CO: USDA Forest Service.

SCHUBERT, G.H. 1974. Silviculture of southwesternpondnosa pine: The status-of-our-knowledge. Re-search Paper RM-123. Fort Collins, CO: USDAForest Service.

SMITH, R.H. 1971. Red turpentine beetle. ForestPest Leaflet 55. Washington, DC: USDA ForestService.

STOKES, M.A., and T.L. SMILEY. 1968. An intro-duction to tree-ring dating. Chicago: Universityof Chicago Press.

WAGNER, M.R., and Z.Y. ZHANG. 1993. Hostplant traits associated with resistance of pon-derosa pine to the pine sawfly Neodiprion fulvi- reps. Canadian Journal of Forest Research 23(5):839-45.

WALTERS, C.J., and C.S. HOLLING. 1990. Large-scale management experiments and learning bydoing. Ecology 71(6):2,060-68.

WARING, RH., and G.B. PITMAN. 1985. Modify-ing lodgepole pine stands to change susceptibil-ity to mountain pine beetle attack. Ecology66(2):889-97.

WHITE, A.S. 1985.Presettlement regeneration pat-terns in a southwestern ponderosa pine stand.Ecology 66(2):589-94.

ABOUT THE AUTHORS

W. Wallace Covington is regends' professor, College of Ecoosystem Sciences and Manage- ment, Northern Arizona University,Flagstaff 86011; Peter Z. Fulp is researchspecialist, Northern Arizona University;Margaret M. Moore is associate professor,Northern Arizona University; Stephen C.Hart is assistant professor, Northern Ari-zona University; Thomas E. Kolb is assis-tant professor, Northern Arizona Univer-sity; Joy N. Mast is assistant professor,Northern Arizona University; Stephen S.Sackett is research forester Forest Fire Lab-oratory, Pacific Southwest Research Station,USDA Forest Service, Riverside, Califor-nia; Michael R Wagner is professor, North-ern Arizona University. Funding for thisresearch was provided by the National Sci-ence Foundation (Grant DEB-9322706).

Journal of Forestry 29