MEASUREMENTS OF WOODY PLANT ATTRIBUTES FROM LARGE-SCALE AERIAL PHOTOGRAPHS

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1 53 MEASUREMENTS OF WOODY PLANT ATTRIBUTES FROM LARGE-SCALE AERIAL PHOTOGRAPHS DOUGLAS G. PITT Cndin Forest Servie, P.O. Box 490, Sult Ste. Mrie, Ontrio, Cnd P6A 5M7. nd GLENN R. GLOVER Shool of Forestry, Auurn University, Alm, United Sttes (Reeived for pulition 31 Mrh 1995; revision 24 Jnury 1996) ABSTRACT The use of low-ost, lrge-sle eril photogrphs to mesure woody plnt ttriutes ws ssessed for vriety of erly suessionl forest eosystems. Two onventionl 35- mm mers were mounted on oom nd suspended from lloon or tripod pltform to otin nominlly vertil stereo photogrphs (ontt sles rnging from 1:250 to 1:1000). Photo mesurements of individul plnt totl height were generlly unised nd preise, with regression stndrd errors rnging from 3.4 to 10.6 m for plnts up to 4 m tll. Stndrd errors for individul rown dimeter mesurements rnged from 7.1 to 18.4 m for rowns up to 3.1 m in width. Diret photo estimtes of rown re were unised nd onsistent (stndrd error=0.107 m 2, plnts up to 6 m 2 ). With mesurements eing onfined to the inner 70% of the overlp portion of eh photo pir, relief displement hd no disernile effet on the ury of rown estimtes. For ll mesurement vriles exmined, the reltionships etween ground- nd photo-mesured ftors were generlly unffeted y speies ut did shift in response to high levels of rown losure nd/or disrepnies in ground- nd phojto-mesurement protools. The most preise photo estimtes originted from the lrger photo sles tested. Results suggest tht evlutions mde from lrge-sle eril photogrphs my e used to ugment field evlutions in surveys of erly suessionl woody plnt ommunities situted on level terrin. Keywords: eril photogrphy; lloon pltform; vegettion evlution; ommunity struture; ompetition. INTRODUCTION Vegettion surveys re onduted on newly regenerting forest sites for vriety of reserh nd opertionl purposes. For exmple, reserhers ollet informtion on the dimensionl hnges of trget nd rop plnts to evlute nd ompre vegettion mngement tretments. Foresters ollet informtion on stnd struture nd omposition for silviulturl deision mking (i.e., to determine whether or not young plnttion or nturl regenertion New Zelnd Journl of Forestry Siene 26(1/2): (1996)

2 54 New Zelnd Journl of Forestry Siene 26(1/2) re is progressing ording to mngement expettions nd, if not, wht orretive tion to tke). Similr informtion is used y wildlife mngers to ssess qulity nd quntity of wildlife forge nd hitt. Field survey, usully involving diret mesurements of individul plnt dimensions, is urrently the only method used for these purposes. Invrily, suh surveys re lour-intensive nd expensive, hrteristis whih frequently mnifest themselves in defiienies suh s insuffiient smple sizes nd is (Borders & Shiver 1989, Zedker et l. 1993). Inresed smpling effiienies hve een relised in some forestry-relted pplitions of lrge-sle eril photogrphs (LSP) (e.g., Lyons 1967; Needhm & Smith 1987; Hll & Aldred 1992; Alemdg 1986). However, serh of the literture reveled only one study (Smith et l. 1989) where LSP ws used to evlute ompetition levels in young forest plnttions. Though the ury of photogrphi mesurements of the dimensions of mture trees is well doumented (e.g., Allison 1956; Lyons 1966), similr informtion is not ville for lesser vegettion. Pitt & Glover (1993) introdued low-ost eril photo quisition system nd evluted the qulity nd ost of LSP estimtes of ggregte rown re (m 2 /h) on vegettion mngement reserh plots. The ojetives of this pper re to (1) evlute the preision nd ury of diret mesurements'of individul plnt height nd rown dimensions mde from similr low-ost 35-mm LSP, nd (2) identify ftors influening mesurement ury nd preision. Results should e of diret interest to foresters, eologists, nd rnge mngers, ll of whom re onerned with mximising preision, minimising is, nd reduing the mrginl ost of informtion otined on erly suessionl woody ommunities. This pper should lso prove useful to forest mngers who wish to expnd the sope of existing regenertion surveys to inlude evlutions of non-rop woody vegettion. METHODS Field Studies Six forest vegettion mngement studies, representing vriety of erly suessionl vegettion ommunities, were hosen s sujets for this investigtion (Tle 1). Studies1 3 represent omprtive effiy trils, hrterised y woody nd hereous ommunities typilly present 2 or 3 yers fter lerutting. Studies 4-6 represent mehnisti-type growth studies in whih woody speies omposition nd density hve een highly ontrolled nd hereous omponents exluded. Field-mesurements seured s prt of the originl tretment evlutions were used, whereverpossile, s ground-truth dt for this investigtion (methodologil detils re summrised in Appendix 1). Timing nd mesurement proedures differed somewht from one field study to nother (further detils hve een reported y Pitt 1994). Aeril Photogrphy Stereo eril photogrphs were otined y mounting n identil pir of Nikon EM* mers on lightweight luminium oom (Pitt & Glover 1993). The mers ould e * Mention of trde nmes is for informtion only nd does not imply endorsement or dispprovl y the uthors, the Cndin Forest Servie, or Auurn University.

3 TABLE 1 Summry of forest vegettion mngement field studies used for the evlution of LSP tehniques (for speifi evlution methods nd times, see Appendix 1). Field study Lotion Frederiton, New Brunswik, Cnd 2 Auurn, Alm, USA 3 North Florid, nd south Georgi, USA 4 Shorter, Alm, USA Shorter, Alm, USA Shorter, Alm, USA Vegettion onditions t time of ssessment Study type Evlution time(s) Vrying densities of tolernt nd intolernt hrdwood oppie growth (<4 m tll). Crop trees were plnted lk sprue (<2 m tll). Light to moderte hereous over in the understorey. Ftoril omintions of three vegettion omplexes onsisting of plnted lololly pine (<2 m tll), volunteer southestern hrdwood spp. (<4 m tll), nd vriety of hereous spp. South-estern non-roresent speies (<1.5 m tll) nd moderte to hevy hereous over. Plnted slsh or lololly pine (<2.5 m tll). plnted t densities of 0, 1, 2, nd 4 plnts/m 2 (<3 m tll) round fol lololly pine (<1.6 m tll). nd sweetgum plnted in restrited rndom ptterns t rtios of 1:1, 1:2, 2:1, nd 2:2 plnts/m 2 (<3:5 m tll). Pure nd equl mixes of lololly pine nd sweetgum t densities rnging from 0.27 plnts/m 2 to plnts/m 2 (<3.5 m tll). Chemil nd mnul site preprtion nd onifer relese. Eologil: elow-ground eosystem proesses. Chemil nd mehnil site preprtion. Mehnisti ompetition study: Neighourhood pproh. Mehnisti ompetition study: Addition Series design. Mehnisti ompetition study: Nelder I design (Nelder 1962). July '91 Sept '92 Sept '93 Sept'91 My'91 June'91 Sept '91 Mr '92 My '92 Jn '92 Jn '92

4 56 New Zelnd Journl of Forestry Siene 26(1/2) djusted inwrd or outwrd long the oom to hieve desired irse (B). A ule level, pled on the mer frme during instlltion, ws used to orient eh mer in vertil ttitude reltive to the nturl position of the oom. Guy lines, running from the four orners of the oom to the point of tthment, funtioned to level nd stilise the oom nd eliminted the need for giml mehnisms on eh mer. When entred over trget re, the pprtus provided nominlly vertil photogrphs. Two servos, operted y the sme rdio frequeny, were used to depress the shutter relese les simultneously. A 25-m 3 Rven helium-filled lloon provided stle pltform for the 5-kg mer system in wind speeds up to 8 km/h. The lloon ws tethered to single wter-filled 20- ontiner nd moved from one photo sttion to the next y moving the ontiner. A steering line ws tthed to one end of the oom nd used to orient the mer xes reltive to the plots eing photogrphed. Field studies 1, 2, 5, nd 6 were photogrphed with this system (Tle 2). While the lloon proved to e suitle mer pltform for studies with lrge numers of plots in lose proximity nd/or studies requiring lrge re overge, simple luminium tripod ws found to e more effiient for use on smller plots (studies 3 nd 4). With three 3-m setions of tuing per leg, the tripod provided mer height of 7.2 m, weighed only 10.5 kg, nd ould e esily ereted nd moved out forest utover y three persons (Fig. 1). A plywood housing ws used to fix the legs of the tripod together t the vertex nd permit eh leg to swing inwrd nd outwrd, s needed, for deployment in the field. The mer system desried ove ws rised nd lowered using ord nd pulley system. FIG. 1 Tripod used for photogrphing smll plots (less thn 3 x 3 m). Consisting of three setions of luminium tuing per leg, the tripod provided mximum mer height of 7.2 m nd weighed 10.5 kg.

5 Field study Photo set d d /* (mm) H (m) B (m) B\H 1:11.5 1:11.8 1:12.5 1:12.8 1:12.9 1:12.9 1:12.0 1:12.0 1:12.0 1:12.0 1:12.0 1:11.8 1:23.5 1:19.2 1:19.2 1:46.7 TABLE 2-Summry of photo speifitions, y field study Slef 1:94.8 1: :78.0 1:78.9 1:79.9 1:79.9 1:54.6 1:54.6 1:54.6 1:54.6 1:54.6 1:61.6 1:61.6 1: : :218.5 Film type Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Gold 200 Kodk Gold 200 Kodk Gold 400 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Kodk Ektr 125 Pltform Blloon Blloon Blloon Blloon Blloon Blloon Tripod Tripod Tripod Tripod Tripod Blloon Blloon Blloon Blloon Blloon Folige sttus Full lef Full lef Full lef Full lef Full lef Lef off Full lef Full lef Prtil lef Lef off Full lef Prtil lef Lef off Prtil lef Lef off Lef off * Atul verified fol length: nominl vlues re provided in text. H = height. B = irse. f Print sle t ground level fter times enlrgement. { Numer of unique plots photogrphed. "Repeted" indites tht ll initil plots were rephotogrphed. SU = smple unit EU = experimentl unit Dte July'91 July '91 Sept'92 Sept '92 Sept'93 Mr '93 Sept'91 My'91 Sept'91 De'91 My '92 Sept '91 Jn '92 Sept'91 Jn '92 Jn '92 Trget size (m) V 4 EU -9x7 EU 5.0 x 5.0 SU 2.1 x2.1 SU 3.0 x 3.0 EU 4.8 x 4.0 EU 13.8 x 13.8 NumerJ photogrphed Repeted Repeted Repeted 7 32 Repeted Repeted Repeted 23 Repeted 4 Repeted 1

6 58 New Zelnd Journl of Forestry Siene 26(1/2) Kodk 125, 200, nd 400 ASA films were used to mintin minimum shutter speed of 1/250 of seond in diffuse light onditions (hzy or loudy onditions). In the fll of 1991, the originl mers were upgrded to two Nikon FM2 models, equipped with MD-12 motor drives. These mers permitted the use of fixed shutter speed (i.e., 1/250 of seond) s well s perfetly synhronised eletroni shutter tivtion. This remedied prolems enountered with the mehnil tivtion system, whih ws prone to mldjustment nd mis-fires. Identil pirs of Nikon Series E 28 or 50 mm, or Nikkor mm lenses were used throughout this investigtion. For eh pplition, tul fol lengths (f) were verified y photogrphing trget of known size (S) nd distne (//) from the perspetive entre nd relting the ojet imge size (s) on the negtive y (ll units in millimetres): f=hxs/s [1] In ll tests, pirs of lenses devited in similr mnner nd the verge vlue ws used in susequent lultions (Tle 2). In ll photo sessions,// //omintions were hosen to llow trget res to fll within the inner 50 to 70% of the stereosopi portion of eh photo pir (inner 50 to 70% of the overlp in eh of the x nd j; diretions). This ws done to () ensure tht mesurements were not mde long the edges of the photogrphs where relief displement nd lens distortion re gretest, nd () llow mrgin of error for pturing the trget re on film. Smple unit (SU) entres were mrked on the ground y 30 x 20-m numered rds (lk lettering on white kground) nd white 20-m dimeter diss. A 2.4-m mesuring stik ws pled on the ground in visile lotion ner eh SU entre to provide hek on photo sle (otherwise determined y// H). Photo Evlutions Prior to print development, the geometri entre of eh negtive ws pin-priked so the position of the prinipl point would e visile on pper print. Imges were then enlrged y ftor of 4.576, to otin 10 x 15-m prints. Conjugte prinipl points were trnsferred stereosopilly nd susequently used to se-line the photos for stereo viewing (Pine 1981, p.56). All photo evlutions were onduted y single interpreter. For height nd rown mesurements, stereo pirs were positioned on 30 x 43-m Kurt digitising tlet nd viewed with n Old Delft snning stereosope (under 4.5x mgnifition). Individul plnt heights (//r p, "p" denoting photo-derived) were mesured using Wild prllx r (for desription of the prllx r nd its use, see Pine 1981, p. 145). Crown res were estimted y digitising the pln (top) view of eh rown, s seen in the three-dimensionl stereo model. The re-y-oordintes method (Brinker & Wolf 1994, p.272) ws used to ompute rown re (CA p ) from the strem of x,y o-ordintes generted y eh tre. Crown dimeter mesurements were onduted y digitising the endpoints of line segment through the widest point of the rown (1; Fig. 2) nd those of similr segment, t 90 to the first, through the entre of the rown mss (2). The formul for the distne etween two points in o-ordinte plne (Anton 1988, p. 26) ws used to ompute the dimeters (l p nd 2 p ) of eh rown. To simplify mesurements, oth the digitising tlet nd prllx r were onneted to single 286 PC where BASIC progrm red inoming mesurement dt nd performed the neessry omputtions. Speifi formule used y the progrm to ompute HT p nd CA p

7 Pitt & Glover Mesurements of woody plnts from eril photogrphs 59 pln (top) view of rown losed trverse round mjor points of hnge in the rown perimeter true ross-setionl re of the rown FIG. 2 Shemti digrm of the proedures used to mesure rown dimensions. L,UJ nu 2 re rown dimeters tken through the rown t its widest point nd t 90, through the pproximte entre of the rown mss. The solid ellipse depits the rown re estimtedfromthe two rown dimeters (CA gd, otinedfromeqution 3). The roken line depits the losed trverse used to estimte true rown re on the ground (C4 g ; Appendix 1). were dpted from Lyons (1966) nd re provided in Appendix 2. The pproximte height of the rown t its widest point (HT p (mid)) nd the rdil distne from the entre of the rown mss to the entre of the photogrph were lso mesured for eh rown to ompute relief displement (d): r x HTJmid) d P - [2] where: r H HT p (mid) H rdil distne from the entre of the photogrph to the entre of the rown mss (sme units s d), mer height ove the ground (m), nd the pproximte height of the rown t its widest point (m), determined y prllx mesurement. Finlly, eh rown ws lloted to one of three visiility lsses ording to its degree of openness on the photo. A lss "O" rown ws lerly defined nd not overlpped y other vegettion. At lest 50% of the mrgin of lss "P" rown ws visile, the remining portion eing osured y other vegettion. Finlly, less thn hlf of the mrgin of lss "F" rown ws visile, using the opertor to tively "interpret" muh of its perimeter. This lssifition permitted the study of reltionships etween ground nd photo mesurements of individul plnts under vrying degrees of interpreter onfidene.

8 60 New Zelnd Journl of Forestry Siene 26(1/2) Dt Anlysis Following the onvention used to rrive t orretion urves in prolems of pplition, ground mesurements were regressed on photo mesurements (Allison 1956). The ury nd preision of photo mesurements were evluted y studying the hrteristis of these regression reltionships. Speifilly, informtion on is in photo mesurements is ontined in the estimted regression interept (expeted to e 0 in the sene of is) nd slope (expeted to e 1 in the sene of is). Informtion on the preision of photo estimtes is ontined in the stndrd error of the regression (SE, or the squre root of the residul men squre). Under lrge smple sizes, this vlue pprohes the error with whih response vrile of interest my e predited from given photo determintion. When differenes etween lss vriles were of interest (e.g., speies), simple regression reltionships etween ground nd photo determintions were expnded to inlude inditor vriles for these ftors (Drper & Smith 1981). If n F-test suggested tht the expnded model ounted for greter vrition thn the simple model, ttempts were mde to group the lss vriles on the sis of meningful priori riteri (e.g., rown form), suh tht the expnded model ould e presented in its simplest form. RESULTS AND DISCUSSION Oserved Aury nd Preision Levels The reltionship etween ground- nd photo-mesured individul woody plnt height ws strong nd generlly onsistent ross the six field studies (Tle 3; Fig. 3). All reltionships were liner nd, with few exeptions, did not differ signifintly (p < 0.05) from the expeted line of equlity (0 interept nd unit slope). Coeffiients of vrition (CV) were onsistently elow 10% nd frequently elow 5%, suggesting high degree of preision. As in height, the reltionship etween ground- nd photo-mesured mximum rown dimeter (1) ws resonly strong nd onsistent ross the five field studies where suh determintions were mde (Tle 4; Fig. 3). All reltionships were liner nd suggested slight photo-mesurement is in some instnes. Stndrd errors nd oeffiients of vrition tended to e lrger thn those oserved for height, rnging etween 7.1 nd 18.4 m, nd 5 nd 22%, respetively. Detiled rown re estimtes mde y losed trverse on 91 hrdwood rowns in field study 1 (C4 g, "g" denoting ground-sed mesurement) (Fig. 2; Appendix 1) provided sound evidene in support of the hypothesis tht CA p provides true representtion of tul CA (Fig. 3). The reltionship etween CA % nd CA V did not differ signifintly from the expeted line of equlity (0 interept;? = nd unit slope;? = 0.450) nd exhiited ppreile preision (SE = m 2, CV = 6.9%). Differenes etween photo sets I (n = 67) nd I (n = 24) ould not e deteted (F 2i87 = ,p = 4). Also, the reltionship refleted the full rnge of rown sizes enountered in ll six field studies where CA determintions were mde. This result is of prtiulr importne, onsidering the diffiulty nd expense involved in the quisition of urte, diret rown re mesurements in the field. Indiret estimtions of rown re, sed on two mesurements of rown dimeter, re ommonly relied on in prtie (Fig. 2):

9 TABLE 3-Summry of sttistis for the regression of ground-mesured tree height on photo-mesured tree height, y field study (differenes etween rown visiility lsses not signifint (p > 0.05) exept in study 6 see text). Field study Photo set d f* (mm) H (m) B (m) Speies omplex North-estern hrdwoods North-estern hrdwoods South-estern hrdwoods & lololly pine Slsh pine # ^ l ^j ^j ^j l * Atul verified fol length; nominl vlues re provided in text. H = height. B = irse. t Men pired differene etween photo nd ground estimtes. J (se) = stndrd error of prmeter estimte. Mx. height (m) Men diff.f (m) Interept (se)t (0.014) (4) (0.010) (0.009) (0.009) (2) (0.032) (0.012) (0.056) (0.008) (0.041) (0.030) (2) (8) (0.010) (0.046) () () (0.036) (0.016) (0.075) (0.071) (0.115) (0.064) (0.531) (0.273) Slope (se) (0.011) (0.017) (0.009) (0.008) (0.005) (0.016) (0.019) (0.015) (0.143) (0.006) (0.061) (8) (0.016) (0.042) (0.006) (0.044) (0.006) (0.008 (0.018) (0.009) (0.036) (0.037) (0.060) (0.032) (0.193) (0.108) SE = stndrd error of regression (root men squre residul) # Trees with visile terminls only H Crown visiility lsses O nd P only n SE (m) v (%) r Theoretil preision (m) < CO B g_ O O 3- T3 I w O B P 2 O ^ 3 T3 C/3

10 TABLE 4 Summry of sttistis for the regression of ground-mesured mximum rown dimeter on photo-mesured mximum rown dimeter, y field study (see text for definition of rown lsses) Field study Photo set & Speies omplex North-estern hrdwoods South-estern hrdwoods & lololly pine Lololly & slsh pine & lololly pine Crown lss 0 P F 0 P F 0 P 0 P F 0 P F O P F 0 0 O Mximum Men dimeter diff.* (m) (m) * Men pired differene etween photo nd ground estimtes t (se) = stndrd error of prmeter estimte j SE = stndrd error of regression (root men squre residul) Interept (se)t () (0.04) (0.04) (0.11) (0.11) (0.09) (0.05) (0.06) (0.05) (0.04) (0.06) (0.05) (0.11) (0.08) (0.08) (0.07) (0.10) (0.18) Slope (se) () (0.05) (0.09) (0.15) (0.07) (0.04) (0.06) (0.04) (0.04) (0.05) (0.04) (0.07) (0.06) (0.06) (0.05) (0.07) (0.14) n SEJ (m) v (%) r Theoretil preision (m)

11 Pitt & Glover Mesurements of woody plnts from eril photogrphs 63 E D (/) E D ) Mximum rown dimeter (m) Photo mesurements E CU E C Photo mesurements FIG. 3-Exmples ofthe reltionships etween ground- nd photo-mesured woody plnt () height, () rown dimeter, nd () rown re for field study 1. Points represent pired oservtions; solid lines represent the ordinry lest squres fit to the oservtions. CA gd l 0 x2 Q xn For the 91 rowns mesured y losed trverse, CA gd over-estimted true CA y widely vrying mounts (men differene etween CA gd nd CA g = 0.23 m 2,p < 0.001). In light of these findings, it would generlly e undesirle to orret CA p on the sis of CA gd ; orretion urves for omission is my need to e sed on ground-truth mesurements otined y more urte methods (e.g., vegettion hrting desried y Bonhm 1989). Further, C4 p, whih etter pproximtes true CA, my provide more sound index of plnt ompetitive sttus thn C4 gd. The omprtively poor representtion of true rown re y CA gd my lso provide possile explntion for its lklustre performne reltive to other ompetitive indies in Knowe's (1991) study. Effets of Photo Chrteristis The interreltionships etween fol length, mer height, nd irse influene ftors suh s photo sle nd relief displement nd therefore hve diret influene on the preision nd ury of height nd rown determintions mde from LSP. Pitt (1994) [3]

12 64 New Zelnd Journl of Forestry Siene 26( 112) outlined simple mens of omputing the theoretil preision (TP) of tree height mesurements from given set of photogrphs tken under idel onditions (i.e., perfetly vertil lens positions, eh photo of the stereo pir hving extly the sme sle, no imge motion or lens distortion, et.). These vlues re provided in Tle 3. Though oserved regression SEs were slightly higher thn TP vlues (euse of vrition ttriutle to smpling), there ws strong orreltion etween oserved nd expeted vlues (r = 0.89). From these, it is pprent tht s the B\H rtio is deresed, preision is srified. To lesser extent, inresing the f/h omintion, while keeping sle nd B://reltively onstnt, lso redues preision. These omprisons suggest tht TP my e used s guide for plnning LSP pplitions. Sine HT p mesurement preision will e gretest with lrge prllx differenes, TP my e mximised y hoosing n irse tht will produe lrge ut omfortle levels of vertil exggertion for the size of the vegettion eing studied. To illustrte, in Fig. 4 TP vlues re plotted over B, for f/h omintions leding to ontt sle of 1:250 (trget size = 3 x 3 m, vegettion height = 2 m). Eh of the urves termintes when AP (the differene in solute prllx etween the top nd the ottom of the plnt) is pproximtely Theoretil preision (m) Airse (m) FIG. 4 Theoretil photo height mesurement, preision plotted over irse, for four different fol length / mer-height omintions leding to ontt photo sle of 1:250 t ground level (similr to tht used in field studies 3 nd 4).

13 Pitt & Glover Mesurements of woody plnts from eril photogrphs mm (the upper limit of prllx for omfortle viewing). The TP level of m tht ws ttined with/= 28.8 mm, H- 7.2 m, nd B = 0.6 m ould lso hve een ttined, in theory, with/= 35 mm,h = 8.75 m, nd B = 0.9 m. Smll srifies in preision would e required if lrger f/h omintions were used; the omintion of hoie would depend on limittions of the mer pltform eing used nd ville equipment. A simple lgorithm for lning eonomy nd preision in LSP pplitions hs een provided y Pitt (1994, p.232). Just s the limit of prllx pereption defines the si level of preision for height mesurements on pir of photogrphs, the resolution of the digitiser, reltive to the photo sle, defines the si level of preision for l p nd CA p determintions. The prtiulr tlet used throughout this study hd resolution of ± 0.38 mm. This resolution n esily e trnslted into rown-dimeter mesurement units (m) for the vrious photo sles used in this investigtion. For exmple, preision for the lrgest print sle used (1:54.6) ws ± mt ground level (0.38 mm x m/mm); for the smllest print sle used (1:218.5) it ws ±0.08 m (Tle 4). These vlues represent mximum errors of pproximtely ±3.7% nd ±14.8%, respetively, for rown re equl to i m 2. Photo hrteristis ply n importnt role in mesurement ury s well (Appendix 2). Even smll errors in the speifition of/ H, B, or EF will led to ised results. For exmple, if the nominl fol length of 70 mm were used in the lultion of plnt heights on study 2 (insted of the tul 67.3), photo height determintions would e inflted y n verge of 7 m. The mgnitude of is would vry with plnt size nd therefore would e refleted in oth the slope nd interept of the regression etween ground- nd photomesured height. Further, photo sle ffets the ury of photo determintions of mximum rown dimeter (l p ) nd rown re (C4 p ) through the wy in whih it is used in the lultion of these response vriles. With LSP, plnt heights n represent signifint frtion of the overll mer height nd ised results re likely if photo sles re not djusted to reflet the height t whih rown mesurements re mde (i.e., HT p (mid)\ Appendix 2). For exmple, when CA p ws omputed y djusting photo sle for one-hlf of totl plnt height, the regression slope for the 91 rowns in field study 1 fell elow 1 (p = 0.004) nd the stndrd error inresed to m 2 (p = 0.077). Preision ws even further redued when no sle orretions were mde t ll (SE = m 2,p = 0.006). The extrprllx reding required to otin the height of rown t its widest point (HT p (mid)) is worthwhile, prtiulrly if "push-utton" redings n e mde. Relief displement is the shift in photogrphi position of n ojet, used y its elevtion ove or elow seleted dtum (Wolf 1974). Relief displement uses trees to pper to e lening in the photogrphs nd, lthough it is the very phenomenon tht llows one to view stereosopi photos in the third dimension (nd mesure height), it hs the potentil to introdue is in l p nd CA p determintions. Conern is for over-estimtion used y inlusion of portions of the side view in the tre of the pln view of the rown. When relief displement (d) ws omputed for eh rown (Eqution 2) nd relted to rown mesurement error (photo - ground), evidene of is ws not deteted (Fig. 5). Under the null hypothesis tht errors in photo-estimted rown dimensions re independent of d (t lest within the rnge of r employed), dt should e sttered out line with 0 slope. If

14 66 New Zelnd Journl of Forestry Siene 26(1/2) Crown dimeter (m): T3 C 3 O o JZ QL i_ 13 C/) 03 U Crown re (m 2 ): ^ g t% o - - o o Relief displement (m) FIG. 5-Crown mesurement error (photo - ground) v. relief displement for trees in study 1. Points represent pired oservtions for rown visiility lsses O (open) nd P (solid); solid lines provide referene t 0 error. Oservtions for rown re represent the 91 rootstoks for whih detiled rown re determintions were mde. d is signifint enough to use rowns to e mesured more from the side thn from ove, oth 7 p nd CA p vlues should generlly e inflted nd the trend in the errors should slope upwrd to the right with inresing d. Regression sttistis for the lines in Fig. 5 suggest the presene of wek trends (slope = 0.362,/? = 0.038, for 1\ slope = 0.197,/? = 0.041, for CA). However, it ppers tht the proportion of vrition in mesurement error ssoited with relief displement is smll enough to e of little prtil signifine in either se. Stter plots for the other studies displyed similr ptterns. Further sustntition of this onlusion omes from dt olleted for 168 plnts in field study 4 tht ourred on two or more photo pirs tken during the sme session. Pirs of repet oservtions (sme plnts, different photo pirs) were differened, so tht the oservtion with the smller r ws sutrted from tht with the lrger r. The men pired differene for r ws m (t l61 = ,/? < 0.001) nd for G4 p, m 2 (t l67 = , p = 0.479). Hd d influened G4 p through misinterprettion of pln views, the pired differenes in CA p ought to hve refleted this. Thus, the generl prtie of onfining mesurements to the inner 70% of the stereo portion of the photogrphs ppers to provide suffiient protetion ginst is due to relief displement (t lest t the photo sles nd plnt heights exmined).

15 Pitt & Glover Mesurements of woody plnts from eril photogrphs 67 Effets of Speies Chrteristis In generl, the reltionship etween ground- nd photo-mesured plnt height did not vry with speies (p > 0.122). However, ertin rown strutures, hrterised y rnhless terminl shoots ontining smll leves (< 2 m), used is nd redued preision of height mesurements due to inonsistent visiility on the LSP. These inluded suh speies s sprues (Pie spp.) nd firs (Aies spp.), nd some non-roresent speies like Vinium. In study 1, for exmple, the height of some lk sprue (Pie mrin (Mill.) B.S.P) rop trees ould e urtely determined, while the vst mjority were under-estimted on the photos (Fig. 6). If suh speies re to e evluted on LSP, (1) speies-speifi heightorretion equtions my need to e developed, (2) photo sles should e s lrge s logistilly fesile (i.e., > 1:225 ontt), nd (3) inresed ground-truthing should e plnned. If the speies in question is entrl to the investigtion, evlutions my e more effetively onduted on the ground. 0, ^W- / / / o, ' n 5? et/ ~ r o,~ n0 ' o ~j5 s / o / s / / S! &-: u?poy- i! 3*, 1 J 1 ; ^!, Photo mesurements (m) FIG. 6-Ground- v. photo-mesured lk sprue height for field study 1. Points represent pired oservtions; roken line shows referene slope of 1 nd n interept of 0 (these dt re not inluded in the regression sttistis provided in Tle 3),. Similrly, speies differenes were generlly not signifint in the reltionships etween l g nd l p (e.g., field study 1, p > 0.383). Speies differenes in the reltionship etween CA g nd CA p ould not e thoroughly explored euse of insuffiient degrees of freedom for individul speies (field study 1, 91-tree suset). Effets of Crown Closure As rown losure inreses, prllx redings t the ground (se of the plnt) re mde with deresing onfidene. This is used diretly y folige ostruting the view of the ground nd (or) indiretly y shdows osuring the ground surfe. In studies 1-5, differenes in the reltionship etween ground- nd photo-mesured height due to rown visiility (lsses O, P, nd F) ould not e deteted (p > 0.05; tests pplied to individul speies groups when differenes etween speies were deteted). This result ws not

16 68 New Zelnd Journl of Forestry Siene 26(1/2) unexpeted, sine ll of the field studies were situted on reltively level terrin nd none of the nopies ws so dense tht representtive se reding ould not e otined within 2-m rdius of given plnt. In field study 6, eyond out 1 plnt/m 2 ( plnts/h), ground visiility eme limited nd there ws tendeny to over-estimte plnt height (Pitt 1994). This my e voided, under level-terrin onditions, if plne is generted from three prllx redings tken t representtive lotions in the trget re where the ground is visile. Then, eh plnt's se reding n e interpolted from itsx,y position on the plne (e.g., Aldred & Hll 1975). When this ws done in study 6, is t high rown losure levels ws eliminted (Pitt 1994). In study 2, where/ H, nd B were more representtive of the vlues tht might e used in prtie, the use of plne-interpolted se redings resulted in mrginl loss in preision of individul plnt height estimtes (SE = m v m). The more miro-relief within the trget re, the less desirle this tehnique will e from the perspetive of minimising preision. As rowns ppered interloked nd overlpped, it lso eme inresingly diffiult to distinguish the mrgin of one rown from nother nd the reliility with whih rown dimeter nd rown re mesurements were mde deresed (Tle 4). Individul-plnt evlutions t high densities re extremely ostly (nd often questionle) on the ground. Depending on survey ojetives, dvntges mye gined y shifting fous from individul plnts to groups of similr omposition nd struture. Then, LSP my e used to otin urte estimtes of response vriles suh s verge height, perentge over, nopy re, nd speies omposition for eh suh unit delineted. Effets of Photo Timing Severl photo sets were tken in the deiduous lef-off ondition (Tle 2 2d, 4, 5, 6, nd 6). In prtie, suh timing my e used to dvntge to enhne the visiility of onifers tht would otherwise e osured y tller deiduous or hereous vegettion. However, the extent to whih hrdwood speies n e urtely evluted in lef-off ondition ppers questionle. In study 4, representing the lrgest photo-sle tested for lef-off mesurement, sweetgum (Liquidmr styriflu L.) terminls tht were lerly visile on the photogrphs were oded so tht they ould e isolted for seprte nlysis (40 of 357 plnts mesured). Not surprisingly, the reltionship etween ground- nd photomesured height on these plnts fell on the line of equlity (0 interept, unit slope,/? > 0.568; SE = m; Tle 3) while the heights of remining plnts were under-estimted y vrying mounts (SE = m; Tle 3) (typil of the results illustrted in Fig. 6). Similr trends were oserved for hrdwoods in studies 5, 6, nd 6. Inresed vriility ws lso oserved for lef-off 7 p nd CA p determintions (e.g., study 6 v. 6; Tle 4). Most of the hrdwood rowns on photo set 2d ould not e visully seprted from logging slsh, hereous vegettion, nd other deris on the forest floor nd mesurements were not ttempted. Effets of Ground-truthing Chrteristis When primry response vriles n e mesured on LSP, identil ground- nd photomesurement proedures ovite orretions for "uilt-in" ises. As exemplified y the

17 Pitt & Glover Mesurements of woody plnts from eril photogrphs 69 losed-trverse method used for CA g determintion, this my not lwys e prtil or logistilly fesile. In other situtions, however, ises might esily e voided y reful plnning of ground-truth efforts. For exmple, in field studies involving pine {Pinus spp.) height mesurements (studies 2-6), pprent speies differenes in the regression reltionship etween ground- nd photo-mesured height my e ttriutle to differenes in the wy pines nd hrdwoods were mesured in the field (i.e., pines were mesured to the tip of the terminl ud, while hrdwoods were mesured to the tllest lef). Mesurements on the photos were mde to the top of the tllest visile portion of eh plnt; for pines this is often the tuft of needles 3 to 7 m ove the terminl ud. This disrepny in methods ws refleted sttistilly in field studies 4, 4, 4d, nd 5, with pine regression interepts rnging from-0.03 to-0.07 nd slopes not signifintly different from 1 (p > 0.421; Tle 3). Field studies 2 nd 2 hd interepts not signifintly different from 0 (p > 0.394), ut slopes less thn 1 (0.93 to 0.98), inditing non-onstnt over-estimtion on the photos. Clerly, the photo mesurements re sensitive enough to revel minor proedurl disrepnies. Further exmples of is used y disrepnies in mesurement proedures or times hve een identified y Pitt (1994). CONCLUSIONS This pper hs explored the extent to whih low-ost lrge-sle eril photogrphs (LSP) might e used to ugment field evlutions of woody plnt ttriutes in vegettion surveys of newly regenerting forest sites. Stereo LSP ws quired y suspending stndrd 35-mm mer system from lloon or tripod pltform. The lloon ws suitle for lrge res ontining severl smple plots in lose proximity or where lrge mer heights (> 7 m) were required. The tripod ws suessfully used for photogrphing smll plots (< 3 x 3 m) nd ws found to e more portle nd less ostly to own nd operte thn the lloon. Photo evlutions were onduted on 10 x 15-m prints enlrged from imges ptured on 125, 200, or 400 ASA print film. A BASIC omputer progrm stremlined photo dt olletion y reording height nd rown mesurement dt, grphing stem mps of plnts in reserh plots, nd performing ll neessry lultions. The primry dimensions of visile individul woody plnts (totl height, mximum rown dimeter, nd rown re) ould e estimted to within 10% of their men vlues on the LSP otined. Photo mesurements of these response vriles were generlly unised nd onsistent. For most purposes, the levels of preision nd ury otined re likely to e within eptle limits. For rown re, diret photo mesurements were more urte thn trditionlly used indiret estimtes sed on two field mesurements of rown dimeter. This result suggests tht photo estimtes of rown re my provide more sound index of plnt ompetitive sttus thn trditionl field estimtes. For the response vriles exmined, LSP preision nd ury n e ensured or enhned y dhering to the following guidelines: (1) Use the lrgest photo sles tht logistis will llow. (2) Optimise mer seprtion reltive to mer height (se-to-height rtio) to provide dequte vertil exggertion in the stereo model ( differentil prllx of 2 to 5.5 mm should provide results similr to those reported), (3) Confine mesurements to the inner 70% of the stereo (overlp) portion of eh photo pir.

18 70 New Zelnd Journl of Forestry Siene 26(1/2) (4) Corret photo sle, on n individul-plnt sis, for the tul height t whih mesurements of rown dimensions re mde. (5) Use the most preise vlues of mer height, fol length, irse, nd print enlrgement ftor ville in ll lultions. (6) Personnelshouldplnonspending 15to30hoursintrhiingeforereliingmesurement preision levels similr to those reported (Pitt 1994). Proper use of the prllx r requires prtiulr ttention. (7) Avoid using LSP for evlutions of () tree height on speies with tll slender terminl leders, () tree height on very uneven terrin, () deiduous speies in lef-off ondition, nd (d) vegettion with high levels of rown losure. (8) In (Id), where miro-relief is miniml, use the o-ordinte plne method for interpolting se prllx redings. Also, onsider shifting fous from individul plnts to vegettion groups of similr omposition nd struture, for whih verge height nd perentge over vlues n e otined. (9) Avoid "uilt-in" ises y striving to synhronise ground tmthing nd LSP mesurements in terms of timing nd proedures. When orretions for is re ntiipted or suspeted, minimum of 30 individuls, representing the entire rnge of vrites in the popultion of interest, should e onsidered for dequte sttistil power. This investigtion hs demonstrted tht relile mesurements of individul woody plnt ttriutes n e otined diretly from low-ost LSP. LSP my e effetively employed in two-phse or doule smpling design, whih will permit orretion for is ssoited with the omission of smller plnts hidden in n understorey nd enhne smple distriution nd intensity (Pitt et l. 1996). ACKNOWLEDGMENTS The uthors re indeted to Roert Tufts (Auurn University) for his ssistne in omputer progrmming, Dik Cre (Ntionl Reserh Counil) for lon of the lloon, nd the stff of Cmer Grphis (Auurn) for their ptiene nd qulity photo finishing. Brue Zutter, Dwight Luer, nd Roert Jones (ll of Auurn University) re reognised for their ssistne in the quisition of some of the photogrphs nd ground-truth dt used in this investigtion. Evert Johnson, Greg Somers, Rlph Meldhl, nd Den Gjerstd (ll of Auurn University), Roert Fleming, nd Rihrd Sims (of the Cndin Forest Servie), nd two nonymous referees provided mny helpful omments to improve this mnusript. REFERENCES ALDRED, A.H.; HALL, J.K. 1975: Applition of lrge-sle photogrphy to forest inventory. Forestry Chronile 51(1): ALEMDAG, LS. 1986: Estimting oven-dry mss of tremling spen nd white irh using mesurementsfromeril photogrphs. Cndin Journl of Forest Reserh 16(1): ALLISON, G.W. 1956: The ury of tree height mesurements on vrious qulities of eril photogrphs. Forestry Chronile 32: ANTON, H. 1988: "Clulus with Anlyti Geometry". John Wiley & Sons In., New York. BONHAM,. 1989: "Mesurements of Terrestril Vegettion". John Wiley & Sons In., New York.

19 Pitt & Glover Mesurements of woody plnts from eril photogrphs 71 BORDERS, B.E.; SHIVER, B.D. 1989: Heriide field studies in forestry: sttistil nd other onsidertions. Cndin Journl of Forest Reserh 19: BRINKER, R.C.; WOLF, P.R. 1994: "Elementry Surveying". Hrper Collins College Pulishers, New York. DRAPER, N.R.; SMITH, H. 1981: "Applied Regression Anlysis". John Wiley & Sons In., New York. HALL, R.J.; ALDRED, A.H. 1992: Forest regenertion pprisl with lrge-sle eril photogrphs. Forestry Chronile 68: KNOWE, S.A. 1991: Comprison of expressions for rown size of woody ompetitors in heriide effiy studies. Forest Siene 3 7(6): LYONS, E.H. 1966: Fixed-irse 70-mm photogrphy, new tool for forest smpling. Forestry Chronile 42: : Forest smpling with 70-mm fixed irse photogrphyfromheliopters. Photogrmmetri 22: MITCHELL, R.J.; ZUTTER, B.R.; GREEN, T.H.; PERRY, M.A.; GJERSTAD, D.H.; GLOVER, G.R. 1993: Sptil nd temporl vrition in ompetitive effets on soil moisture nd pine response. Eologil Applitions 3(1): A. NEEDHAM, T.D.; SMITH, J.L. 1987: Stem ount ury nd speies determintion in lololly pine plnttions using 35-mm eril photogrphy. Photogrmmetri Engineering nd Remote Sensing 53(12): NELDER, J. A. 1962: New kinds of systemti designs for sping experiments. Biometris 18: PAINE, D.P. 1981: "Aeril Photogrphy nd Imge Interprettion for Resoure Mngement". John Wiley & Sons In., New York. PITT, D.G. 1994: Use of lrge-sle 35-mm eril photogrphs for ssessment of vegettion mngement reserh plots. Ph.D. disserttion, Shool of Forestry, Auurn University, Alm. PITT, D.G.; GLOVER, G.R. 1993: Lrge-sle 35-mm eril photogrphs for ssessment of vegettionmngement reserh plots in estern Cnd. Cndin Journl of Forest Reserh 23: PITT, D.G.; GLOVER, G.R.; JONES, R.H. 1996: Two-phse smpling of woody nd hereous plnt ommunities using lrge-sle eril photogrphs. Cndin Journl of Forest Reserh 26(4): PITT, D.G.; THOMPSON, D.G.; PAYNE, N.J.; KETTELA, E.G. 1993: Response of woody estern Cndin forest weeds to fll folir tretments of glyphoste nd trilopyr heriides. Cndin Journl of Forest Reserh 23: SMITH, J.L.; ZEDAKER, S.H.; HEER, R.C. 1989: Estimting pine density nd ompetition onditions in young pine plnttions using 35mm eril photogrphy. Southern Journl of Applied Forestry 13(3): WOLF, P.R. 1974: "Elements of Photogrmmetry". MGrw-Hill In., New York. ZEDAKER, S.M.; GREGOIRE, T.G.; MILLER, J.H. 1993: Smple-size needs for forestry heriide trils. Cndin Journl of Forest Reserh 23:

20 72 New Zelnd Journl of Forestry Siene 26(1/2) APPENDIX 1 SUMMARY OF FIELD MEASUREMENTS CONDUCTED BY FIELD STUDY (1) Two growing sesons fter tretment (July 1991), woody plnts inluded in the study's vrile-rdius smple units (SUs) were mesured for totl height (HT g ; "g" denoting ground mesurement) to the nerest entimetre nd rown dimeter (l g nd 2 g, lef tip to lef tip, Fig. 2). Within 372 rndomly seleted SUs, the position of ll hrdwoods nd rop trees inluded in the originl effiy evlution ws lso reorded y mesuring the distne nd ering from the SU to the entre of eh rown. (Study referene: Pitt et l. 1993). A suset of 91 stem-mpped hrdwoods were seleted for detiled rown re mesurement. Seleted rowns were not interloked or overlpped y other rowns nd inluded the full rnge of rown size oserved in the originl smple. Crown re (CA g ) ws determined y projeting on to the ground the mjor points of hnge in rown's perimeter nd then onduting losed trverse (with ompss nd metre stik) round the projeted points (Fig. 2). Depending on the ross-setionl shpe of the rown, etween 5 nd 10 legs were mesured for eh trverse. (2) Two nd three growing sesons fter site preprtion (Septemer 1992 nd 1993), rodleved woody plnts nd rop trees flling within SUs (0.5-m rdius plots) were mesured for HT g, l p nd 2 g, in the mnner desried for field study 1, ove. (3) Two-growing sesons fter tretment (Septemer 1991), rop trees situted in 2.1 x 2.1-m SUs on edded surfes were mesured for HT g, l g, nd 2 g, s desried for the two previous studies (exept tht rown dimeter mesurements were ud tip to ud tip). (4) in 1.5-m rdius neighorhoods entered on 32 pines were mesured for HT g, l gi nd 2 g on 21 My, 19 June, nd 23 Septemer 1991, nd 12 Mrh nd 18 My Fol pines were lso mesured t these times. With the exeption of the Mrh sweetgum mesurements, ll rown dimeter mesurements were lef tip to lef tip. Distne nd ering were lso ville for eh sweetgum reltive to eh fol pine on plot. (5) Mesurements of HT g, l g, nd 2 g (lef tip to lef tip on pine, ud tip to ud tip on sweetgum) were onduted in Jnury The position (x,y o-ordintes) of eh plnt ws lso ville. (Study referene: Mithell et l 1993). (6) Mesurements of HT g, l g, nd 2 g (lef tip to lef tip on pine, ud-tip to ud-tip on sweetgum) were onduted on ll plnts within eh Nelder plot in Jnury 1992.

21 Pitt & Glover Mesurements of woody plnts from eril photogrphs 73 APPENDIX 2 EQUATIONS USED FOR PHOTO MEASUREMENTS OF PLANT HEIGHT AND CROWN AREA (see Pitt 1994 for derivtions) HT n where: HT p H AP f B EF HxAP [4] fxbxef - + AP H = photo estimte of plnt height (m), = mer height ove the ground (m), = differene in solute prllx etween the top nd ottom of the plnt (mm), = tul fol length of the lens (mm), = irse (m), nd = print enlrgement ftor (4.576), H-HTJmid) CA n CAJrw) x [5] fxef where: CA p = photo estimte of rown re (m 2 ), djusted for photo sle t HT p (mid), CA p (rw) = rown re, undjusted for photo sle, nd HT p (mid) = pproximte height of the rown t its widest point, omputed vi Eqution 4.