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 unalyzetheir 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 measuremcnt becomes 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 Spies 1994). Flat projec- by the density and distribution of canopyfoliage tions of tree crowns used to model canopy cf- (Anderson1963, Hom 1971,Reif.snyder et al. ltcts on understolylight may work well in low- 1971).The percentagcof full sunlightthat reaches latitudctropical tbrests or shofi stature,broadleaf thc forcstfloorisa broadmeaswe offi)liage denslty. temperatetirrests (e.g., Runkle 1992). but they Thc most comnronnlcthods of measuringlight do poorl), in the Pacitlc Northwest. penetration.however, integrate the incominglight old-growth fbrests, over the whole sky (Reifsnyderet al. 1971. For Pacific Northwest location and crown dimen Hutchinsonet al. 1980.Oberbauer et al. 1988. measurementof the sionsofindividual treescan be integratedto pro- Chazdonand Pearcy I 99l) providi ng liLtleinfor- vide a stand scale description of canopy stluc- mationregarding thc distributjonof light as a ture. Ground-basedlleasurenrent is a practical t'unctionofcanopy sfucture. Fisheyephotographs compromisebetween direct measurements of a can help isolatethe directionof incorninglight few branchesand remote. large-scalesarnpling (Evansand Coombe 1959, Anderson, 1963, Horn ofa tbrest'souter canopy cover. Widl ground-based 1971,Canham et al. 1990);however. each phQto measurcs.an obseryercan quickly and directly rcpresentsonly a singlelocation, making stand- measurethe individual tree parametersthat col- scalcanalysis diflicult. A larger scalemeasure of Iectivelydescribe the density and distribution of calopy structurethat doesnot use]ight transmrs- a stand'scanopy foliage. Treating each tree crown sion is leaf areaindex (LAI). a measureof the as a simple geometricshape is possible,thus numberoflbliage laycrs over a given areaof for- making canopiesof complex, old-growth stands floor, provides a stand-levelindex of tbliage est relatively easyto model in threc dimenslons. quantities(Waring et al. I 982).A11 of thesetech- solids may be niques.howevel, provide little detailedinfbrma- Although trcating crowns as such simplification tion on canopy structLlrebecause they do not inappropriateat fine scales, provide map of general measurehow tbliageis spatiallydistributcd (Oker can a three-dimensional izeJrrcc erouns thirl ct'llcctively rpprorimutc Blom andKellornrki 1982,Chason et al. l99l). standscale structure. For exrmple,this level of structurcmodeling rnight help assessfbraging CanopyStructure in PacificNorthwest Old- conditionstbr an owl which flies betwecnrather GrowthForests than through crowns. Although this model is a environment, Pacilic Northwest old-growth lbrests have tall. simpJificationof a complexcanopy it is a steptoward the three-dimensionalanalysis multilayeredcanopies, which arediftlcult to ac necessaryk) understandthe influenceof canopy cessand stmcturallycomplex. Much of thc old structureon arboreal processesand speciesdi growthbelow 1000m in clevationis donrinaLed versity. by Douglas-fir(Pseldolsrga rrrr:n:iesii)and wcst- (Tsrigalrcldrupl\Jla). Theselbrests em hemlock MeasuringCanopy Structure: Crown m tall, with lbliagedistdbuted can be up to 80 ModelTechnique throughout(Franklin and Waring19130), so that the amounl of understor)-light provides little in- Data for model devclopnent were collccted in a lbrmation aboutfoliage distribution in the canopy. Douglas-fir / westernhemlock old growth fbrest The trces may be tall but are generally not very at theWind RiverExperimental Forest in thesouth- wide.Branch lengths seldom exceed 8 m andthe ernWashington Cascade Range. The siteis a rela- uppercanopy tends to havedistinct crowns be- tively flat. moderatelyproductive tbrest thatorigi causeof branchabrasion. nated,100-500years ago (Franklinand DeBell

StandScale Crown Model 11 Figurc la. Henisphericril photograph(filhcvc photo) tuken o.35 in a cl(xed portion of an old gro\ith Douglrs-fir/ $estern hemlock forcs(. Thc centerof the image B rs difectlr above lhc calncla. and the uhite circle Totalopenspace - 2.1% fepresents'15degrecsbclo$vcnical.Notethrtsig- 0.3 nificant light penetrationoccur at anglesgreatef thrn.15degrees. o.25 .9 I .q 1988).The tbrest stand is nearthe upper limits of 0.2 the westernhemlock zone (Franklin andDvmess 1973),with an understorytree laycr dominated c by Pacific silver fir (Abies amab il is),P acillc yew o 0.15 (Tarus brevifulia) P and westem hemlock. No de- o tectablemajor disturbancc has occurred since stand initiation.resulting in a multilayeredtbrest. The 0.1 largcsttrees are Douglas firs up to 60 m tall and irom 275-500 ycars old (Franklin and Waring 0.05 l9lJ0),and the westemhemlocks occupy all strata from seedlingsto old trees(Figure 2). All trees tallel than 50 cm *ere mappedfor 0 use in thc spatiallyexplicit model.This height 0153045607590 waschosen because sma11 seedlings can be ephem- Zenithangle eral; population numbers do not stabilize until Figure I b. The distributionofopen spacefof the image in la. undcrstory trees become well established(Van Maximum lighl penetrationis at 25 degrccs.

18 Van Pelt and North 40

30 E

E '{, T 20

10

Douglas-fir western Pacific Pacific hemlock silverfir yew Figure 2. 50 r 20 n cro\s !.clion of thc srudy !ilc at llre wind River BxpenmentalForest. Note rhe diversity of tree heighis and ftc lcnical distribution of thc foliagc.

Pelt,unpublished). Each cro$,n was characterized diameterat breastheight (dbh) were mappedin a by measuringthc trcc'sheight and heightto the 20 m wide bandcentered on the transectline. Each crown base, mapping the crown projection or of theseffees was identified to speciesand also measuringlbur crou n rrdii.und .rs.if ni np u,. rrru n measuredtbr total tree height, height to crown shape(Figure 3). If a dctailcdvicw of $e lower base,and crown radii. Treestaller than 50 cm crown is required,height to clown basecould be were sampledin a 2 m wide transectcentered on replacedby a valucfor the heightto pafiial crown, the sametransect line. Although time permitted the height to lirll crown. and the compassdirec- treeheight measurements fbronly ll5% ofthetrees, tionsof the partialcrown lNorth 1993). regressionsof dbh by specieson the 1429 mea- Two 400 n transectswere placed in interior sured treeswere used to estimatethe remainder portions ()f the frrrest.Trccs grcatcrthan 5 cm in (Table1).

Stand-ScaleCrown Model 19 Cones are perhapsthe most commonly uscd shapefor modelingconif'ers because they are easy to descdbemathematically and are conformable to manycrown shapes(Figure,l).In addition,most conilers and many broadleaftrecs have conical shapeslbr thefirst severaldecades of thcirlives. and indeedmany retainthis shapefbl sevcral hundredyears (Horn 1971). Cylinders arc anothergroup ofgeornetric shapes thlt couldbe u.ed to sinrularecenain lree er,'u ns. Tall.closed canopy fbrest tees oftendevelop deep, narow crowns bestrepresented by cylindrical shapes.Although young conifers are generally not llat toppcd. emergentold-growth trees can de velop flat tops when they have stoppedgrowing in height.The calculationsfbrcylinders are simplc and this shapeshould not be overlookcd for ap propriatesituations. Paraboloidsare versatile shapes that areadapt able to a wide variety of crowns. These shapes Figure 1. l'hc lariable\ required for |hc cro$n modeli a. the can be used for trces that have either the deep lrcc height: b, height ro lull crown base:c. hcight crowns of main canopy trees,or the shor1.wide ruprn i.rl crorr r: J t, r r.r.,u rr:rdrr crownsof suppressedunderstory trees. ln cither case,the widest paft ofthe crown is at or near.the I)ata werealso collectcd in a 0.2 ha circular botton of the crown, which makes pamboloids gap in the ExperimentalForest. This gap was cut an appropriatechoice. as pafi of a larger study looking at the effects of Ellipsoids (including prolateand oblatesphe experlmentalcanopy removal on the fbrest un- roids) or truncatedellipsoids are similar b pa- derstory(Van Pelt et al. 1992).Alltrecs taller than rrboloid.in tharlhe) al50appr,)\imale \ome 50 cm were napped in a 0.4 ha areacentered on old-grewth tree crowns. A half-ellipsoid looks the gap.and mcasurements for eachtree tbllowed similar to a paraboloidofequal height and width, the protocol usedfor the transectsamplcs. yet the paraboloid is narrower at all heights ex- ceptfor the bottom. Old-growth Douglas-firsof- Crown Shape ten haveellipsoidal crowns. with the widest part Formodel construction.the crown ofeach sample of the crown low but not at fhe crown base. treewas classed as oneof severalsoiid geome! Geomctricshapes for eachcrown wereselected ric shapes.No singlcshape can be appliedto aLl in the lleld after viewing crown shapefrom sev- treesbecause old-growth crowns are unique and eral angles.All Douglas-tirtree crowns in this irregular.Several simple lbrms. however,can be study were modeledas truncatedellipsoids, and appliedto mosttree crowns. all other trees are modeled as paraboloids.To

TABLE 1. Regressioncquations used to prcdict heighrffo m dbh tor the treesused in thc crown model; 1,{29lrees were usedro develop the equaiions.

Species Prudi!rir! equ.rton 1\ aot r_

Tsuguhetet0phllla lir = 50 863*(l-e':r'oBri':i) 693 o.94',7 Ht= +:.se+1t-e rlrrl r1 ,168 0.958 Pse dotsuganln.iesii Ht = 136.189+(l-e 149 0.746 Hr=17.5:12l(l 119 0.60.1

20 Vau Pelt ud North figurc,l. Polcndrl crown shrp.s usedir fte nodel. lioln leli to right: cone, cyhrdef, ellip$id. pafaboloid (wider lhan tall). and arother paruboloid (taller lhan $ide). qualitativelyassess the tjt betweenactual crown m transects.This estinate is presentcdonly to shapeand selectedgeomeffic shapes, photographs illustratethe model'scapabilities because we do of l3 treesu'ere digitally scanned.Two-dimen- not havean exactmeasure ofthe real fbliage dis- sional silhouettesof the crowns were compared tribution.The sccondexample is a spatiallyex with the geometricshape selected in the field plicit applicationthat estinates understorylight (Figure5). The geometric shapcs cxcludc irregu- surroundingthe experinental canopygap. In this lar, lower branchesand long branch tips. and filI application,a subsetof the model's resultsfron in crown gapswhere branches are missing. specific locations are comparedto fisheye pho- The selectedgeometric shape, crown length. tographstaken at the samelocations. and crown radii were used to nodel the crown shapeand height of eachtree at its specificX-Y App icatlon :Vertica Distributionol location along the transect.The resuliing stand Fo age ntodelclo-el1 lpprorimrtes cron n locllionsu. The crowns for all treesalong the transectwerc ing specificheight and positionmeasurements. modeledand then horizontally sliced at 5 m height but it sinplifies crown shapeas a gcomctricsolid. intervals.Crown cross-sectionalarea and volume u'ere calculated for each tree at cach interval. Applications ot the Crown Model Frustumvolumes were summedby speciesfor The following two eramples illustrate how thc each 5 m layer to estimatethe vefiical distribu- model can be usedto describecanopy conditions. tion of crown foliage. These data are presented Thefirst estimatesthe verticaldistribution oftb- in Figure 6 in two ways: graphA showsthe total liage basedon measurementsfrom thc 20 x 400 crown volume/ha at eachheight forcach species;

Stirnd-ScaleCroun Model 2l Douglas-fir -80

o c) 0)

40 Western o hemlock qt o

20

Figure 5. Top: Six photoSraphicsilbouettes of old-growlh Douglas-fir trees with a truDcatedellipsoid overlaid to lhow the nrodel s approximationof the actual crown shape.Bottom: Sevenpholographic silhouettes of old growrh wcslcrn hcmlock lrees wiih parabolic shapesoverlaid to sho* the versatility of paraboloids 1c'rmodcling a divcrsity ofcroqn structurcs,

Van Pelt and North is open.Shade-intolerant Douglas-fir extends and Weslernhemlock dominatesthe upper canopy environment.pro- DouglasJI vidingmost ofthe crownvolume abovc 43 m. In total volume,however, the standis dominatedby Pacificsilver fk u estefnhemlock and in the lower canopymost of the spaceis occupiedby hcmlock crowns. In this stand.although large Douglas-fir may punc- tuatethe outer canopy,most of the canopystruc- ture is determinedby the densityand disftibution of subdominant,shade-tolerant specics.

App cation llr Using the Crown l\,4odeto EstimateLight Beach ng the Forest Floor Hemispherical(fisheye) photography can be used A to estimatethe amount of light peneftatingacanopy. Through the usc of the crown model and a com- puter animation techniquecalled ray tmcing. a conrputerizedequivalent of fisheye photos can be generated.From a selectedpoint. thc visible hemisphere'stree crowns are drawn by using the model, and thc aninratiootechnique presents an or erherdr ieu fr,'m t1.r.trornO. tnis r iru- .imi- Iar to a fisheye photo, can be analyzedand the EJV amountofvisible opensky canbe calculated.Thc ; ray tracingprogram (POV-RAY 2.0. 1993)rnod .9 els the light through the canopy by mathemati- -20 T cally placing a camera,light source,and objects Weslernhemlock in three dimensionalspace usingASCII text dcti- x nitions. A window of a given resolution is de- DouqlasJir E- lined and the programthen calculatesthe path of Pacificsilver fir Jightrays from eachpixel back to the light sourcc tI basedon the reflective naturc of the objects.Al Pacificyew thoughray tracingis commonly usedfor detailed photorealisticimages, its accuraterendefing of 0 20 40 60 80 100 objectsin three-dimensionalspace makes i1 use- Spaceoccupied by crowns (%) 1ul for scientific illustration. figure 6. \tnical folirge distribution for thc iorest .rr Wind The canopy's effect on understorylight was River: A, lbr eachspecics indilidurlly m cubic explorcdusing the ray tracingtechniqucs andcrown neters per heclarer B. (individual and. b), spccics models to simulatethe shades).or tdal stand (onter bou.dar!) as x per canopy.To estimatclight centoftot.rl .rv.rilable open space. distribution throughout the 0.21ha plot contain- ing the experimentalgap. ray-traced images were generirledlbr 204 slalion. plirceJ4 m apart in graph B is an areagraph showing thc total three- cachgap. A radial grid. like thoseused in fisheye dimensionalspace occupied by each speciesby photo analysis(Rich 1989),was imposedon each height interval. ray-tracedimage. By using the grid. the tQtalper- The modelhelps visualize foliage distributions centageof open sky in eachimage was calcu- that cannotbc measuredwith above canopy re- lated. Each cell in thc grid is analyzedfor thc mote inrageryor understorylight conditions.Slic- pcrcentageof the cell blocked by tree crowns. ing the canopy clearly showed that the highest and cell percentagesare summed over the im densityof foliage was between20 and ,12m. Yet age. The image tbr the gap center is shown in evenin this verticalzone..107c ofthc canopyspace Figure7A. This prccessis repeatedfor all images

Stand ScaleCrown Model 23 FigufeT. A, A sinlulatedilsheye image trken liom lhe centerol a 0.2 ha openingin thc Wind Rivcr old growth lbrcsl(120 degree aperture):and B. the same image bul with transparenttextures added to nore closely approximateactual conditions. 'liansparent textufeswefe calibratedwith a subsetofactual henlisphericalphotos.

24 Van Pelt and Nofih generatedfor eachlocation and kriging is used difluse light conditions in the understory,which to spatially intelpolate the light distribution be- in our exampleoccurs at the centerof the epen- twccn the 20:1stations. ing (Figure 8A). A moreaccuratc simulation ofthe light distri- To investigatethe influenceof latitude on un- bution canbe madeby using texturingtechniques derstorylight distribution.sun-paths throughout within the ray tracingprogram. Tcxturing lets the the growing seasonwere modeled.This measure treecrowns transmit light in a more rcalisticman- providesan idealizedvalue (i.e. no cloudcover) ner.A subsetof tisheyephotographs taken from of directlight conditionsin the gap (Figure8b). the Expedmental Forest were used to calibrate The maximum direct light possibleover the eo- trce transparencyin the model. Figure 78 shows tire year (Direct Site Factor,DSF) is 0.325 and is wherethis wasdone frlr the samelocation as Figue shifted 17 m north of thc gap center.The 0.325 7A. valuc meansthat 32.5 percentof the possibledi- To evaluatehow well this application of the rect light reachesthat spot, given no clouds or ero$n Inrdei.irnulrlc. lighl lranrmirsi0n. com- haze.The shifi indicatesthat light filtering through parisonsofray tracedimages and fisheyephoto- tall Pacific Northwest forests intluencesunder- graphstiom the samelocation were madefor l0 story condl ns at an oblique angleto the tbrest stationsin two cxperinentalgaps at Wind River floor Simplevertical or hodzontalmeasures \\"ill The flt between actual and predicted values of not give an accurateassessment of tree reponse open sky were significant when sumned either to canopygaps, because the changesmay be com- by zenith anglesor by azimuth direction (Table pletely shifted out ofthe gap.This result is sini- 2). lar to thatpresented by Canhamet al. (1990),who modeledgaps as cylinders at the samelatitude.

TABLE 2. Regre\srcn strtistics for comparisoinsof ac$al fisheyephotos with simulatedfi sheyephotos. For Discussion = the zenith angle regressiors,N 20;ford:tnrth The measureofan ecologicalmodel is its ability direction. \ = il. All valuesare significant. to accuratelyilluminate complexprocesses at the Zenifi angle Azinuth direction expenseof some simplification. Powerful com- Plor LoLrtiol r p laluc r' p lalue putersand new statisticaltechniques have made three dimensionalmodeling ofthe fbrest canopy I lTmN 0.988 <0.001 0.576 0.0r8 possible.Yet thesemodels will always bc built I l.lnN 0.992 <0.001 0.799 0.002 on somesimplified measurmentsofcanopy stluc I Center 0.997 0.001 0.789 0.00,1 ture determinedby the selectionof scaleand ac- I l,lns 0..171 0.001 0.u97 <0.001 cessmethod. I lSmS 0.962 <0.001 0.928 <0.001 I 25nN 0.999 <0.001 0.786 0.001 Becausethe model is a simplificationofa com- ] 1]M N 0.998 <0.001 0.966 <0.00t plex structure,the model is meantfor stand-scale 2 Centel 0.989 <0.001 0.911 <0.001 research,not tbr detailedanalyses. Crown shapes 2 llms 0.919 <0.0{11 0.817 0.001 are treated as solids in this model, yet fbliage I 25mS 0.982 <0.001 0.851 0.001 densitywithin a singlecrown is actually distrib- utedas a foliageshell ofvariable thickress (Jensen and Long I982). For young treesand shade-tol- Onccthc cntirearray of locationshas an im- erantold trees,the inside ofthe shellis an areaof age.surface maps of the distribution of open sky high shadeand is thus largely photosyntheticaJly by percentagethroughout the gapwere generated inactive.Old trees,howerer. have unique branching using a geographicinformation system (ARC/ pattems and can develop very open crowns. At INFO,v. 6.1.2.l993). This can bc combinedwith the standscale this variability is minimized. but a nrodei of the amount of diftuse radiation dis more dctailed models will be neededif a closer tributedacross the sky to estimatethe total amount examinationof forest structureis required. For of dift'useradiation that is penetratingthe canopy. old-growth treecrowns, trttaining the next higher The ratio of this to the total amount of diffuse level of detail is a major step that will require radiationin an unobscuredsky is known as Indi modeling tree crowns through their individual rcct Sitc Factor(lSF). The ISF is a measureof branches.Such a model would requirenumerous

Stand-ScaleCro\ n Model 25 I 0.6 I I E 0.45 ffi o.4 W 0.35 ffiir*ld 10 Wffiifffi0.3 t:r-.;:;i meters l;:)r;r,:l0.25 o.2 - E 0.15

FiSure 8. A. Resulls ol the difTuselighl model. Indirccr Sire F.rctor for a 0..1ha arca sunoundins the 0.2 ha gap. Thc outer boundarl is thc linil ofthe pld and the inegular lvlile line repfesentsthe crown projccrionsofthe remaining canopr- oee\. 1he gap ccnlcr has the highestISF. B. A direct light model sholvingrhe perccnug. oi pos\ible direcr sun (Direcl Site Factor. DSF) o\cr lhe couf\e of a gro{lng scason.Tlis llnal}sis makes no predictions fbr clouds or hazc. The marimu lrlue is shifiecl l7 nl norih of saDccnrer.

26 Van Pelt ard North lo.szs Io.s lo.zts po.zs ffio.zzs ffio.z Wo.tts fu;T,#$lo.rs ittttut ^ . ^- i:-::lu'rzc l lo r Io.ozs [ 1o.os

StandScale Crlru n Motlel 27 measurementswithin individual crowns. rathcr ture which could further a more analyticalexplo- than a f'ev easily oleasuredcrown dlmenslons. rationof thecanopy cnvironment. The applicationspresented describe tu'o ofthe manypotential uses of this modelto stand-scale Conclusion research.Two applicationsillustrate how crou'n Studiesof forest structurehave of'tcnfocused on volumesare vertically distributed and how crowns trce stems.and the function and composition of affect light penetration.The next logical steptbr theimmediate environment tear theglound. In a this applicationis to combincthe verticaldistri- plant community where the vegetationis 30 to bution with horizontal spacingof trees to come 80 m tall.hor.rerer. nran5 of thc dlnrmii ec,. up with a threc-dimensionalanalysis of crown logicalprocesses and biodiversitywill occurin distribution. A corollary to this approachis an the arboreal environment about which little is analysis in three dimcnsions of the open space known. Betbre the ecologicalinteractions of spe- within the tbrestcanopy. For arborealspecies such ciesand processes can be linkedwith their as the northcrn tlying squirrel (.Glaucomys envi- sobrinus) or the northern spottcd owl (Stri,r ronment! a measurcof the aggregatecrown or oc.ident.llis cutrinu), the amount and distribu- stand-scalestructure is needed.This measure would tion of open spacemay be importantto move haveat leastthree benefits: until most of thc spe- mentor tbragingsuccess (Carey l985, Rosenberg cies and complex interactionsof an ecosystem and AnLhony 1992.1. are well studied,measures of structurcmust of'- ten serve as a surogate measureof lunction or The computer-generatcdhcmispherical pho- biotic diversity;for experimentaldesign and sam- tosalso have wide applicability.Notonly canlarge pling, researchersneed to know how the cano areasbc analyzedence the standparameters have pies ofdit'ferent standsare distinct and what fac- been mapped, but analysis of canopy openness torsmake them so; iiLnd mostmonitoring, planning, at variousvertical locations is easilyaccomplished. and managementofforcsts areat the standscale. aD aspect that has limitcd canopy phokrgraphy A measureof canopystructure that usesground- from the ground. Simulations of proposedhar based methods would provide foresters with a vestingactivities is yet anotherapplicalion ofthc meansof comparingcanopy environments between light model.Selective logging, green-ffee reten ditlerent stands. tion..tripcutting. rnJ,,ther li,re:lr) prJ( lice\ crn be simulatedunder current and futurc conditions. ln this paper,we presenta method1br model- At presentthe accuracyof the model is ditfi- in-qold-growth fbrest canopiesat the standscalc cult to assessbecause there is no absolutemea usingground-based measures. This techniqueis surcof canopystrucnue. Unlike ground'levelforest basedon easilymeasured crown dimensions and shucture,the canopy cnvironment does not readily can aid in three-dimensional analysis ol forcst imply a standardunit ofmeasure.A stand'sbasal processes.Two applicationsof the model were areais easilysampled because tree boles are dis usedto examinethe vertical distribution of fbli- crete stmcturcs. Trec crowns, however. can be ageand understory light levels in a Pacilic North considereda fractal shape(Zeide 1993)becausc west old-growth tirrest.The model indicatesthat their dimensionswill changedepending on the muchofthe tirliagemass in the old growthsample size of the sampleunit. Any model's representa- standsis donrinatedby shadetolerant,subcanopv tion of thccanopy is relativeto thescale it works \pccic\.xnd thrr lighrcondition. lor a poinron at and the size of its smallest sanple unit. The thc firrestUoor are more inlluenced by thecanopy's cro$'n model presentedis lbr stand scaleassess structureat an oblique anglethan thc treecrowns ment of canopl,structure using individualtree directly overhead.We belicvethe modelcan help crnwnsuhich lre lrealeda' :imple gc,'mclrie researchcrsanalyze the complex structureof the solids.Given theseparameters we believethe ovcrsk)ryenvironment as they explore the ecol- model canhelp visualizepattens in canopystruc- ogy of the canopvliontier.

28 Van Pclt and North LiteratureCited C.ro,w.. and B.l-.I-i. 1991. nh\,e let an.rl)-sis of coherentslruc rurcslr ficarmosphereforest intedace. J. Appl. Nlelerol. ,\nderson.N{. C. 1963.Studic\ ol thc $oodlandlight clim.rte 32:lTll 1725. lr thc photogrrphiccomputrtion ol lighlcondrrions. Hulle.f.. R. A. A. Oldcrnan.and P B. T(nrlilxon. I978.Tropicul J.Ecol. 52: 17 11. Trec\ and Foren\: An Afchitectural ,A.n.rl!sis. ARCI/INFO.Ver\n 6.l.l. 1993.En\ nirrmental Si stem\Re- Springer'\tflag. \carchInstitute. Inc.. Redlands. Calilomra. ll|rrl nr. C.l. l(16(rThe Jep( i.l r!<"r \Jl rn. ,.r. rcnl C.rmphell,G. S., andJ. M. Norman.1989. The cle\criprion of indi!idurl treeson furnirunce. cr0wn dimcnsions. '2: lnd easuremeniof tl.lnt crnop,r'fslruclure, tr C. -rd i^Irn.t tr"r.f .r<..r) ll I ll Russell.B. V.rshall. rnd P Ci. Jar\is (eds.).Phnt Hann.D. \\'.. C. L. Olsen.and A. S.Hester 1992.ORGANNON: CxnoprcsrTheir Groxth. Fonn.LndFunction. Caln User'sNlal1unl. Edition,l.0. 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(tech. eds.).[colog]' and managementof the spolled Eftects of sk) brighurcs\ drstributior upon penetfr- (r\\'lin lhe Prcilic No h\!est. USDA For.Ser\'. PN$'- ti(nl ofdiiluse diation lhrough canop! gapsrn r de- GTR 11i5.Pac. North$. For. Rrnge Exp. S|r.. Porl ciduous1bcn. Agric.Meleorol. 22: ll7-1,17. land.Oregon. Pp. 100 | l.l. Hutchinson.B. A.. D. R. Nlart.li. T. NtcNfillen.L. J. Gross, Chrson.J.w., l). D. B.rldocchi.and M. A. Hu\lon. 1991.r\ S. J. Tajchlnal1.and J. Ni. \oflnan. 1986.The rchi- comprri\on ol dirccl rnd indirect nrethodsfbr esti rectufe of a deciduousIorest canop) in easiefnTen- naling torcs! canop! lerf .L|ea.Agfic. For. Nlctcorol. 57:107-128. nessee.U.S.A. J. Ecol.7,1: 635 6,16. ot a Ch zdon.R. L.. ard R. w. Pearc).l99l. Tbe inporuncc ot Jcnsen.E. C.. rnd J. N. Long. 1982.Cro$n siructure sunflecksfbr lifest undenror)plants. uioscience.ll: codominantDoug las-fir. Car. J. foi Rcs l3r 26.1269. 760 t66. Kruij!, B. 1989.Estimating canop) slructurcol an oak ibrest !i\ertr (\ '64 -e-. Chen.J. 1991.Edge cllccrsi nicroclimaie prttern and bio :,r \''r fur<.lr\ 62: loCicalfesporses in old gro$lh Douglasiir tb|esr\. Lesica,P. and R. K. Antibu!. I991. Canop,vsorls and epi- Univcrsir,"ofwashingt( . Seattle,USA. Ph.D.Dis phyterichness. Nall. Gcog.Rcs. trxplo. 7: 156-165. N4assmrn.\\i J. 1982.Folialc disrrbution in old gro\rih co- Cohen.W B.. T. A. Spics.and G. A. Br.rdsha$.1990. Using niferoustree canopies. Can. J. For.Rcs. l2i l0 17. \cmi lariogfams ofsefiri \ideograph) ibr analysisof IucNaughlon,K. a;. 1989.Region l inleractionsbcl$ccn cano coniler cllllop,,-sfuct urc. Rem.Se n. Envir . 3.:]:I 67 178. picsand the aimo\phefe. 1lr G. Russell,B. Mar\hall. Cohcn. W. U.. and T. A Spies. 1992. E\lnnaling structLrral andP C. Jan is (eds.).Phnt Crnopies:Thcir Crowlh. attiburc\ ol Douglasfir^vestem henlock ibresl s|Inlds Forrn and Function. C.rnlbndgeUDi\. Prc!!. 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Stantl-ScrlcCrou n Model 29 Okcr Bloln.P, andS. Kellonali. 1982.Theoretical conpu Terborgh.J. 19E5.The lenical componenrof plan! spccies tationso. thc role ofcro\!.n \hape in thc absorytionof diversityintemperatc and ropical foren\. Aner Nltur lighth] lbrcsttree\. N{ath. tsiosci. 59: 291-31L 126 160-tt6. Pike.L. H.. R. A Rldell. and\!'. C. I)cnison 1977.A,100 Tcftorgh. J. lg9l. Dilcrsit] and the Tropicrl Rlin Fores!. yearold Douglas-firtree and ils cpiphyres:biomars. Sci. Amer. Lib. New York. Van Pelt. R., T. A. suriacearc!. and tbeif distributions.Crn. J. For Res. Spies. |nd J. Fl Franklin. 1992. Djslur brnce succcssion.lnd specic\ 7: 680 699. inler.tctionsaround canop) grps in o1d-growthDouglas,fiffofests. Nonlw. Reifsnydcr.$'.E., G.N{.Frrnjval. rnd J.L. Horo$irz. 1971. Enliron. J.8: 201 201. Spatial rnd temporal dinriburim of solar radiatioD \Ulson. E.O. I 991.Thc Diversityof Lite. llan ardLrni\'. Press. benea$ lbrest crnopies. Agric. \,leteor. 9: 21 37. Cambridgc.\I.rssrchuset!s. Rich,PM. l9li9 Amanurlfi)r anal,vsisof hciisphericrl canop,v Woodcock.C. E.. V Jakabhary.S. Nlacomber.S. Ryherd A. photogrrph).MrnualLA I 1733M. LosAl.rmorNa H. Str hler andY Wu. 1990.Timber in!cnlory using tlo.al Laboratorv.Lo! Alanros. NNL Landsat Thematic Nlapper imagery and canop) re Ro\enberg.D. K. andR. G. Anthon,"-.1992. Chafacterisrics lleclance models.1,r Proceedings ofrhc 23rd Inrerna ofl]orthern tlring squirrelpopuladons in second and tional S,vmposiumon Remote Sensingof the Envi old growth fbfestsin we{crn Oregon.Can. J. oflool. ronmen!. Bangkok Thdland, April 1990.Vnume 2. 70:161-l66. Environ.Res IDst.Ntich.. AnnAibof Nlichigan.USA Runkle. J. R. I992. aiuidelinesand sampleprolocol lbf \rm- Pp.937-9,18. Wu. Y. andA.H. Strahlcr.199,1. Remote pling lbrcst gaps. USDA For. Scr!. Cen. Tech. Rep. cstimrtion of crou n si7e.standdcnsit). and bioma\s on rheOregon ltanscct. PN\\'GTR-283. Pac. Nonhw. Res. Sir.. Porlland. Flcol.Appl. .1:299 I I2. Orcson. .1.1pp. Young.TP. rnd S.P Hubbell.I 991 Cror n as)-nJnerry.rreefalls. \cl,os Jl Jr.T. D. locc. a anon\ r\ .lru. - hruloJ r,,mrnun and repert disrurbanccof broad-leared lbrest grLps. ture .rnd hefbi|or) in old growth and regencraling Ecolog) 7l: 1.16.1l.l7l lbrest\in \|e\rernOrcgon. Can. J. For.Res. l9: 318 Zcxle, B. 1993.Prinrary unit of the tree cro$ n. Ecologv 7.1: 321. 1598160:.

30 Van Pelt and North