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Estimated decrease in productivity for pacific silver fir as elevation increases Klinka, Karel 1998

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Scientia Silvica Extension Series, Number  12, 1998Estimated Decrease in Productivity for Pacific Silver Firas Elevation IncreasesIntroductionWhen making decisions on which areas to harvest in a sustainedyield, even-flow manner in mountainous areas such those in coastalBritish Columbia, it is important to know how timber productivitychanges with elevation.  This information allows foresters to decideat what elevation to start increasing the rotation age and to decide atwhat elevation sustainable harvesting becomes infeasible due to lowproductivity.  Since Pacific silver fir (Abies amabilis Dougl. exForbes) has an elevation range that extends from sea level nearly tothe tree line (0 m to approximately 1,650 m; from the Coastal WesternHemlock zone, through the Mountain Hemlock zone; to the lowerlimits of the Alpine Tundra zone), productivity-elevation relationshipsare especially important.To acquire quantitative measures of productivity decrease withincreasing  elevation a regression  equation relating  site  index(the height of the dominant trees at a base age of breast heightage of 50 years) to  elevation  in  southern coastal BC wasdeveloped.   In  turn,  we used this  regression  as an input  intothe  height driven yield model named the  Variable DensityYield Prediction model (VDYP).  The use of the VDYP modelallows the site  index  values to  be translated  into actualproductivity measures (e.g., volume per hectare, mean annualincrement  at  culmination age).MethodsA regression equation relating site index to elevation was developedfrom 44 stands growing on sites identified by site characteristics(e.g., slope position, soil depth, soil texture) as zonal (sites with welldrained soils and moderate nutrient availability; the vegetation onzonal sites are believed to be primarily affected by climate) locatedon Vancouver Island and the adjacent mainland.  The latitude of thearea sampled ranged from 49?10'N to 50?50'N, the longitude from121?15'W to 127?20'W, and the elevation from 220 m to 1670 m(Splechtna et al., in preparation). Stands were chosen which hadPacific silver fir as a major species, and were located in naturallyestablished, unmanaged, even-aged stands without evidence of ahistory of suppression or damage.  In each stand a 0.04 ha plot wasestablished and three of the largest diameter trees felled for stemanalysis. The site index for each plot was determined from the resultingheight growth curves.  A regression equation relating site index toelevation was developed (Figure 1) and used as input into the VDYPmodel.The VDYP model was developed from a combination of temporaryforest inventory sample plots and permanent growth sample plots innatural second-growth stands. The model is an empirical yieldprediction system for even-aged, naturally established stands, anduses height, age, and density (estimated from crown closure) as inputto predict stand heights, diameters, volumes, and mean annualincrements at different utilization levels and ages.  Yield dynamicsover time are driven by height growth. We supplied the site indexvalues directly (instead of height and age) and used the default valuesfor crown closure (57%) and stocking class 1 up to 1250 m. For1250 m in elevation we used a crown closure of 20%; for 1600 m inelevation we used a stocking class of 2 (which is approximately halfthe volume of stocking class 1).  The reductions at the higher elevationswere done to reflect the increasing patchiness as timberline isapproached.    We  chose  the  net  merchantable volumeutilization level of 17.5 cm diameter.Figure 1.  Site index-elevation regression model for Pacificsilver fir (Splechtna et al. in preparation).  The regression is:site index = 34.17 - 0.016 x elevation (m) (R2 = 0.75; root MSE= 3.80 m; P < 0.001; n = 44).ResultsSite index on zonal sites ranged from 34.2 m at sea level to 8.5 m at1600 m in the Alpine Tundra-Mountain Hemlock zone boundary(Table 1).  The predicted productivity (merchantable  volume,culmination age, mean annual increment at culmination age, and themerchantable volume at culmination age) by the VDYP modelshowed a corresponding decrease.DiscussionWe illustrated a methodology for estimating productivity trends withincreasing  elevation.   However,  caution should be used whenconsidering the productivity values as predicted by the VDYP model.Elevation (m)0 500 1000 1500Site Index (m @ breast height age 50) 0510152025303540Constraints to the model when used for Pacific silver fir are:1. The site index values we used were derived from stem analysiswhereas the height curves used in the VDYP model are based onKurucz (1982);2. The underlying data used to develop the model are not evenlydistributed across age classes and site classes.  There was very littledata under 50 years, while there is a disproportionately high numberin ages greater than 100 years.3. There was very little data for low crown closure conditions (lessthan 50%).4. There is a lack of validation plots, again since pure even-agedPacific silver fir stands are uncommon. There is especially a lack atthe higher elevations where the logging history is more recent, butwhere factors such as regeneration delay may be particularlyimportant.Nevertheless, a technical review concluded that ?in general, the VDYPvolume-prediction models produce reasonable estimates  of yield andaverage diameter over time.  With the exception of low sites (site indices<10 m), the yield projections fit the data quite well.? (D.R. Systems Inc.,1993:4). Considering the above constraints and the technical review,Table 1.  Site index (m @ 50 years breast height age) and productivity (using the VDYP model) changes with increasingelevation.  Values are given for the boundaries of the biogeoclimatic zones, subzones, and variants in southern coastalBritish Columbia.  Volumes are based on a utilization level of 17.5 cm.1250 m950 m600 m0 m1600 mAlpineTundraMountainHemlockCoastalWesternHemlockparklandforestedmontanesubmontaneElevation Site Index(m@ 50 yrs)Volume (m3 ha-1) Culminationage(yrs)100(yrs)200(yrs)m.a.i.(m3 ha-1 yr-1)volume(m3 ha-1)352924896938831325628071025120821513410176590.73.35.27.610.41424044935535948.514.118.924.534.2ConclusionsThe methodology developed here gives foresters a tool to evaluatecut control and harvesting options for pre-harvest decisions in highelevation sites when timber production is the primary objective.ReferenceSplechtna, B. 2000. The growth of Abies amabilis (Dougl. Ex Forbes)in relation to climate and soil in southwestern British Columbia. Ph.D.thesis, Forest Sciences Department, University of British Columbia,Vancouver, BC. In progress.Scientia Silvica  is published by the Forest Sciences Department,The University of British ColumbiaISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Gordon Kayahara (gordon.kayahara@mnr.gov.on.ca) andBernhard Splechtna (b.splechtna@utanet.at)Production and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Forest Renewal British Columbia and BC Ministry ofForestsCopies available from: www.forestry.ubc.ca/klinka, orK. Klinka, Forest Sciences Department, UBC,3036-2424 Main Mall, Vancouver, BC, V6T 1Z4h).eipp.hitKecKh)o@eSaebkaga@eiceiemk.cueipp.hMkliukhreuhetKir(acekrmh.acuep.h@)tukbkuGegkuKeaoabiukhrsee4celh.ae@iuieSathlaceibikoiSoadFitkmkteckoba.emk.ep.h@)tukbkuGeacukliuacde.aoiukbaeuheaoabiukhrdegkooeSae.amkra@sCKaep.h@)tukbkuGep.a@ktukhre.aph.ua@eKa.ae.ap.acaruceriu).ioe.a(ara.iukhrRgkuKh)uekruarckbaeckobkt)ou).ioep.ituktac/ehre-hrioeckuacseeCKaep.h@)tukbkuGhmeuKaec)SlhruiraeB 6emh.acueaickoGelaauceuklSa.eliri(alaruhS4atukbaceRiSoaeD e6hgaba.dehrtaekreuKaeB 6elhruiraemh.acudeuKap.h@)tukbkuGeippai.ceuhe@at.aicaearh)(Keuhe.athrck@a.e@u.i@kukhrioBeD,,yGai.e.huiukhrcsee4cc)lkr(egaei.aehpa.iukr(ehreiec)cuikra@eGkao@eabarymohgeSickcdep.h@)tukbkuGekreuKaeB 6elhruiraemh.acueliGeSaeohgearh)(Kuhe.aV)k.aeohr(a.e.huiukhrei(acseeCKae26emh.acueR53,yD93,el/ecui.uceuhp.acaruep.hSoalcekreua.lcehmeohgep.h@)tukbkuGdeacpatkiooGeiueuKaeKk(Ka.aoabiukhrcseeDbareimua.e9,,eGai.cdehroGe3U9elGei.ae.aiok-a@eiueD93,elaoabiukhrsee4ShbaeuKkcephkruekreuKaepi.noir@e26emh.acudep.h@)tukbkuGekcoknaoGeuhheohgeuhelaauec)cuikriSoaemh.acu.GehS4atukbacseeHrael)cueiochthrck@a.euKiueiueuKaeKk(Ka.eaoabiukhrcdeuKa.aei.aemaga.eckuacegkuKechkoer)u.karu.a(klaceKk(Ka.euKire@la@k)lsBeeCKaep.h@)tukbkuGehme-hrioeckuaceiueKk(Ka.aoabiukhrceuKa.amh.ae.ap.acaruceuKaeKk(Kyar@ep.h@)tukbkuGseeEoukliuaoGdeirathrhlkteirioGckcegkooeSaeraa@a@euhe@aua.lkraeuKaephkruegKa.aeuklSa.p.h@)tukbkuGekceuhheohgeuheSaec)cuikriSoGeir@eathrhlktiooGeKi.bacua@s

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