RobertVan Pelt, and Malcolm P North,Co egeoi ForestResources. Box 352100 U n versltyof Washngton Seatte Washngton 98195 AnalyzingCanopy Structure in PacificNorthwest Old-Growth Forests with a Stand-ScaleCrown Model Abstract In fofe\ts. the cmopt is the locxle of critical ccoslslcrn procc\sessuch as photos)nthesi\ xnd evapotranspifation,rnd it prolides essentialhabitat ibr . highl] dilcrsc ana! of animal!. plants, and other organisms.Despite it\ impofirnce. the structureof the canopy as a $hole has had lildc quantiratirc nudy becauselimited acce\s makesquantificatl1r| difficuh and irtegfaiion of de- t.riled nersures of nary individual trcc cro$ns is nccsssar\'.A inodel is presentedfof simulaiing the cro\ln architectufe of individual tree\ in the Prcific Nofihwest to unalyze their collcclilc nilucncc on canopv sirucrureat the ltand scale. Ihe model ules ground-basedmer\ufements of tree height. height to crown basc. lnd crown radii. Crown shapcsare modeled as \olid gcometric shapesbased on these measures.Tur exanples.rc usedto illuslrare thc nodcl s applicalionsi an cslimadon ol thc venical disribution of foliage in an old-grorv{hDouglas-lir^vestem hennock ibr.sl: and m cstrmarionol undcrsloryljgh! condi tions in an exferimental gap, made by appLyingfty trtrcirg rechnologyro a modcl ol rhc o!!'rstor! lrcc cro$ns. Bolh examples illurrate ho\r the N)del can increasedel|il in the descipdm ol canopy slr'ucturc.Furlher application and luture testing of the nmdel$ i11helD reliDe its Drecision. lntroduction Spies 1992,Wu and Strahler1994). Detailed measurcsof branchpattems and leaf odentation The forest canopy has beenhailed as one of the can be usedto asscssshade tolerance or branch Iastbiological frontiers and the -greatcstsource architecture(Hutchinson et al. 1986,Canhan (Er$'in of speciesdiversity in forests 19E2, 1988),but thesemeasures are specific to the struc Schowalterl9ll8, Lesicaand Antibus 199l, tureofindividual treesorthe canopyprocessunder Terbough1985. 1992. Wilson 1992).Canopies study (Norman and Campbell 1989).Aerial pho arecdtical elements ofthe tbrestecosystem liom tographyand satelliteimagery areused to exam numerousvicwpoints. They modify both meso- inechrngc. in crnop)(o\er u ith sucee.rionor lnd rricro-climrtc.lffect rcgionale\upor liun. disturbance(Cohen et al. 1990,Nel et al. 1994, cloudcondensation, and temperature fl uctuations Nilsonand Peterson 199;[). but theydo not pro- (McNaughton1989. Hatr 1982,Chen l99l). The vide detailedinfirrmation on thc intcrnrl struc- canopyis the exchangesite fi)r atmosphcdcgases, ture of thc canopy. preeipit:rtion.and prrticulate interception Measuresof canopy structureat intermediate (Hutchinsonet al. 1986.Ctrmpbcll and Norman spatjalscales are not well developedbecause stand- 1989,Gao and Li 1993).hr old growthlbrests of scaleanalysis requires dircct measuement ofmany the Pacific Northwest,the complexcanopy struc- tall. complex hee crowns.In manue and old-growth as essentiallbr nlany ture has been recognized forests,accessing and measuringa sufllcient (sev speciesofv ildlife, anhropods,and epiphytes sampleof treecrowns to describea staDd'scanopy eral articles.this issue).Research in forest cano- stmctureis dillicult. In spiteofthese difflculties, pies has been limited. however,by both the lo- a three-dimensionalstand-scale measure of canopy gisticalproblems ofaccessing the tall trcc crowns. stmctureis neededtbr at leastthree reasons:this and the heterogeneousstructure of thc canopy \calei\ necc.saqto linl..proce.se. operating across environment.Despite numerous studics of canopy fine (individual treearchitecture) and coane (re processesand speciesdiversity. thc structureof motcmetrsures oflarge-scale canopy cover) scales the canopy remains poorly understoodand has of:lru(lure:mrnS arborerl u ildlile.pecie.mc5 rarely bccn quantitatively analyzed. respondto canopystructure at this scale;and land Othcr sLudiesof canopy structurehave exam- managementnonitoring and planning often oc ined the fine scale of individual tree crown ar- cur at the standscale. chitecture(Massman 1982) or large-scalecanopy A directmethod ofmodcling thc canopystruc- corer for a watershedor landscape(Cohen and ture of old growth forest stards in the Pacific NorthwestScience. VrJ.70. Special Issue. 1996 15 Northwestis prcscntedin this paperThe model ofthe foliagedistribution in individualold growth is usedto quantitativelydcscribc thc verticaldis- Douglaslir treesat theH.J. Andreu's Experimental tributionol cro$n fbliagewithin a standand as Forest in central Oregon showedthat the crown sociatedeffects on the amountof light in the un- shapeofeach treervas uniquc and irregular(Pike dcrstory.Estinrrtes of individualcrown volumes et al. 1977.Massnran 1982). are made tiom ground based measuresof trcc height.height to thelive crown.cro*n shape,and Scale and Access crown diamctcr.Although these methods were developedin coriferous old-growth forcsts.the Canopies are studied at many dil'tlrent scales techniquesshould be applicableto otherforest depcndingon the arborealspecies orprocess. From types.to successionalstages $,hcrc canopy struc- landscapesto standstoindividual trees tobranches. turc may be lesscomplex. or both.As lesealch the scalcofinterest determinesthe detail at which activities in forest caDopiesincreasc, standard- canopystructwe shouldbe measured (Kruijt 1989). izetlrrreth,'J. lur qurntitrli\ el) rnilly/ingcanop) Researchersinterested in tine-scaleprocesses are structurearc nccdcd.This modelis presentedas notconcemed with theunique shape ofeach old a preliminary approachtoward mcasuringstand- grou'th tree crown. When canopy-dwelling scalecanopv strxcture \\hile the model'saccu- arlhropodsare being studied,lbr example, foli- racy is testedfuflhcr. agedistribution on a singlc branchcan be exam incd for habitatuse and availability (Winchcster. Approachesto MeasuringCanopy thisissue). For studiesofphotosynthesis and par- Structure tieulatcintt rlepti,'n. ci'n,'p) .tmcturc rna\ requilc a map of foliage or branch distribution within a CanopyStructure and Tree Age single crown. When working at thcsc scales.di- Iect accessto the canopyis possibleusing tow- Canopystmcture relers to thc spatialarrangenrgnt ers. lifts, or rope techniquesdeveloped by urban of stems. within tree branchesand lbliage a for- arborists.Athough these systemsare time con- est.As forest standsdcvclop, the vertical and suming to establish. they aflbrd precise horizontaldistribution ofthe ciinopybecomes more nleasurmcntsof crown components. hetercgeneous,and its measuremcntbecomes more difticult. In young stands of trees, particularly Landscapeecologists and forestersintcrested conif'crs.canopy structure is fairly homogeneous in canopl' structureon a watershedor rcgional and growth pattems e prcdictablc.Models of scalerely on remoteimagery from airplanesand individual treescan predict annualbranching satellites(Woodcock et al. 1990.Wu and Strahlcr pattems. branch angles, and lcader and branch 199.1).Renote mcthodseliminate access prob- extensionlengths with someaccuracv for young lems and inc|easethe canopysample size. but trees(Hamilton 1969.Horn 1971.Hatch et al. alsodecrease detail. Digital satelliteimagcry has 1975.Halle et al. 1978).Growth and yield mod- becnused te classitylbrest stands by ageclass cls.such as ORECANON (Hannet al. 1992),can ircn\.\ lrnd\\'ufe\.b1 usingrhe reflecranc.. :ig- provide good estimatcsof a tree'scrown growth. nal of canopies,(Morison 1989).Digital imag- includingresponses to tlinning mortality. ery or videographytiom low-f1ying aircraft can distinguishindividual tree crowns. shadows and As treesage. however, they becometaller and the opennessof a stand(Cohen et al. 1990). their crownslonger. more complcx.and more idiosyncratic.Trees in an eld-gr-owthtbrest are Remoteinagery providcsvaluable large-scale productsof hundrcdsof yearsol individualre- inages of changesin canopy structurewith age spotlsesto disturbance.compelition. injury/. light or disturbance,but it providcs little intbrmation rr.rihhilitlrrnd quulit5. und other stocha.tic erent. aboutthe intedor canopycnvironment. In lbrests For eachlree. the sumof thesecollective difler- with multiplecrown layers,high foliagedensi encescreates aunique, complex. crown shapethat ties, or irregular crown cover. measuresof onJy is difficult to predictor moclcl(Canham 1988. the outcr c.rnopysurtlice provide little informa- Vrung andHubbell 19911.In coniferforests. the tion rboul canop):lru.lure. A.\er\inF canop) canopymay begin as a fairlv unifbrm band of \rructureirl lhc sland.crlercqurre. integrution cone-shapedcrowns. and developinto a rnulti- of nunerous measuresof individual tree archt layeredarray ofpolymorphous crorvn shapes. Maps tecture.Measurements must include more than a 16 Vrn Peltrnd North sampleof a few treecrowns, but must provide To contbundmatters. the amountoflight reach greaterdetail than can be obtrincd from a rcmote ing the torest floor may not reflcct the canopy image of the canopy's upper surlbce.This com- structuredirectly overhcad.ln the Pacific North- promisebetwcen sample size and detail,along west(at latitudesfrom,+4 to 58), sunangles are u ith prohlemrliccs.in! mrturc furerlcanofie.. obliqueduring most of thegrowing season. Analy has often led to the use of indirect measuresof sesoftisheye photos reveal that significant amounts (anop)\trUcture. ruch r. lightlrrn:nlis\ion. of light filter through t Pacific Nofih$'est forest canopyat'+5 degrees or more offvertical (Figure The amount of light in a fbrcst is intluenced 1aand lb) (Easterand