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Predicting site index of Lodgepole pine and interior spruce in the sub-boreal spruce zone Klinka, Karel; Wang, Qingli; Wang, G. G.; Coates, K. Dave; Chourmouzis, Christine 2001

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Predicting Site Index of Lodgepole Pine and Interior Sprucein the Sub-boreal Spruce ZoneScientia Silvica Extension Series, Number  39, 2001IntroductionKnowledge of ecological site characteristics and tree growth on different sites is fundamental for silvicultural decision-making and planning. With biogeoclimatic ecosystem classification in place in British Columbia, silvicultural managementhas been given an ecological foundation; however, relationships between growth and site quality have not yet been fullyinvestigated. The purpose of this study was to determine how site conditions within the SBS zone affect the height growth oflodgepole pine (Pl) and interior spruce (Sx).We adopted site index (ht @ 1.3m age 50) as a species-specific measure of forest productivity, recognizing that it gives atemporal indication of height growth. Site index cannot be considered a true measure of the ecological quality of the site norcan ecological site quality be considered a true measure of forest productivity as two different species on the same sitemay have different site indices, or the same tree species may have the same site index on two ecologically different sites.If forest productivity is correlated with ecological measures of site quality, what site factors should be used quantify therelationships? Because of compensating effects, the many possible site factors can be reduced to four key determinants thatimpact plant establishment and growth: climate (light and temperature), soil moisture, soil nutrients, and soil aeration (notused in this study).Study Stands and MethodsNinety three Pl plots and 77 Sx plots were located in naturally established, unmanaged, even-aged (from 25 to 154 years @1.3m) stands without a history of damage or supression. The stands were located in the Dry Cool and Moist Cold subzonesof the SBS zone (SBSdk and SBSmc subzones, respectively) across a wide range of elevation, aspect, and soil conditions.In each stand, a 20 x 20 m plot was established to represent an individual ecosystem with relatively uniform vegetation andtopography. The site relative soil moisture regime (SMR) and soil nutrient regime (SNR) were estimated in the field andthen  converted to actual qualitative values.In each plot, the five largest diameter trees of the study species were measured for age @ 1.3m with an increment borer,and top height with a clinometer. Site index for both species was taken from height growth tables. Biogeoclimatic subzone(identified from maps and representing climate), actual SMR, and SNR were used as independent categorical variables.Multiple regression analysis was used to examine relationships between site index, climate, actual SMR, and SNR.ResultsRelationships between site index and climateStratification of study stands by subzones did not indicate any significant difference in site index of either species betweenclimates. Comparison using zonal (intermediate) sites indicated a trend of increasing Pl site index from the SBSdk toSBSmc subzone (from 18.9 to 21.1 m), and decreasing Sx site index from the SBSdk to SBSmc (17.8 to 17.4 m).Relationships between site index and soil moistureThere were similar and strong relationships between site index of both species and actual SMR. Mean site index of bothspecies increased from very dry to fresh sites, reached a plateau (approximately at 20 m) on fresh and moist sites - theedaphic conditions of highest growth for both species - and decreased from moist to very wet sites. Although the Sx siteindex was lower than that of Pl on very dry and wet sites, interspecific differences were not significant for any SMR (Table1).Relationships between site index and soil nutrientsStratification of study stands by SNRs showed different  relationships  for  each species (Table 2).  Mean Pl  site indexincreased from very poor  to medium sites but there were no significant differences (alpha = 0.05) between medium, rich, andvery rich  sites. In  comparison, the  Sx site index  increased  consistently from  very poor to  very rich  sites, reaching  amaximum of approximately 23 m.G36G53G48G46._G4cG48G56G3 G39G48G55G5cG3G53G52G52G55G3 G33G52G52G55G3 G30G48G47G4cG58G50G3 G35G4cG46._G4bG3 G39G48G55G5cG3G55G4cG46._G4bG3G33G4fG3 G14G15G11G16G26GfG44G3GbG14G11G16GcG3G51G3G20G3G1bG3G14G19G11G14G25GfG44G3GbG15G11G16GcG3G51G3G20G3G15G1cG3G14G1cG11G15G24GfG44G3GbG14G11G18GcG3G51G3G20G3G16G13G3G15G13G11G19G24GfG44G3GbG15G11G15GcG3G51G3G20G3G15G16G3G15G13G11G15G24GfG44G3GbG17G11G14GcG3G51G3G20G3G16G3G36G5bG3 G19G11G15G28GfG45G3GbG13G11G15GcG3G51G3G20G3G15G3G14G16G11G18G27GfG45G3GbG16G11G13GcG3G51G3G20G3G14G15G3G14G19G11G1cG26GfG45G3GbG14G11G1bGcG3G51G3G20G3G15G16G3G14G1cG11G16G25GfG44G3GbG16G11G15GcG3G51G3G20G3G16G15G3G15G15G11G1cG24GfG44G3GbG15G11G18GcG3G51G3G20G3G1bG3G3Table 2. Number of stands (n) and standard deviations (in parentheses) of site index (m @ 50 yr bh) of Pl and Sxstratified by SNR. Values in the same row with different uppercase superscripts or in the same column with differentlowercase superscripts are significantly different (p <0.05; Tukey?s test).Relationships between site index, climate, soil moisture, and soil nutrientsRegression models were developed to quantify relationships between site index and categorical measures of site quality(Table 3). Climatic models showed no relationships with site index, and inclusion of subzone into other models had noeffect. Each SMR and SNR model explained some proportion of site index, but the combined SMR & SNR (edatope) modelaccounted for the largest proportion of the variation in site index (R2  >0.80) and gave the lowest standard error (from 1.4to 1.6 m).Species Very dry Moderately dry  Slightly dry  Fresh  Moist  Very moist  Wet  Very wet Pl  14.2B,a (6.1) n = 2 15.2B,a (2.2) n = 18 17.9A,a (1.2) n = 25 20.8A,a (2.5) n = 22 20.1A,a (2.0) n = 10 17.0B,a (3.6) n = 13 15.2B,a (2.1) n = 2 8.9  n = 1 Sx  8.0D,a (1.6) n = 3 14.4C,a (1.7) n = 12 17.9B,a (2.0) n = 16 20.4A,a (2.6) n = 25 19.7AB,a (1.2) n = 8 18.8AB,a (3.4) n = 10 10.9CD,a (6.9) n = 2 4.3 n = 1 G3Table 1. Number of stands (n) and standard deviations (in parentheses) of site index (ht @ 1.3m age 50) of Pl and Sx stratified byactual SMR. Values in the same row with different uppercase superscripts or in the same column with different lowercasesuperscripts are significantly different (p <0.05; Tukey?s test).Site index model Lodgepole pine [1] SI = 8.90 + 5.25(VD) + 6.68(MD) + 9.04(SD) +11.92(F) + 11.18(M) + 8.08(VM) + 6.30(W) + 0.00(VW)   Adjusted R2 = 0.45, SEE = 2.4 m [2] SI = 20.23 - 7.98(VP) - 4.11(P) - 1.09(M) -0.51(R) + 0.00(VR)  Adjusted R2 = 0.60, SEE = 2.0 m [3] SI = 14.17 - 9.27(VP) - 5.27(P) - 3.11(M) - 1.72(R) - 0.00(VR) + 4.17(VD) + 6.85(MD) + 8.13(SD) + 9.33(F) + 8.48(M) + 6.97(VM) + 1.89(W) + 0.00(VW)  Adjusted R2 = 0.82, SEE = 1.4 m Interior spruce  [4] SI  = 4.30 + 3.67(VD) +10.08(MD) + 13.59(SD) +16.05(F) + 15.38(M) + 14.45(VM) + 6.55(W) + 0.00(VW)   Adjusted R2 = 0.65, SEE = 2.5 m [5] SI = 22.89 - 16.74(VP) - 9.43(P) - 6.03(M) -3.58(R) + 0.00(VR)  Adjusted R2 = 0.58, SEE = 2.7 m [6] SI = 7.13 - 10.09(VP) - 7.17(P) - 5.05(M) - 2.83(R) - 0.00(VR) + 9.25(VD) + 13.01(MD) + 15.37(SD) + 16.10(F) + 15.58(M) + 14.97(VM) + 10.58(W) + 0.00(VW)  Adjusted R2 = 0.85 SEE = 1.6 m G3Table 3. Models for the regression of Pl (n = 93) and Sx (n = 77) site index (m @ 50 years bh) on SMR and SNR. All models aresignificant at alpha = 0.05. SEE - standard error of estimate; abbreviations for SMRs: VD ? very dry, MD ? moderately dry, SD ? slightly dry,F ? fresh, M ? moist, VM ? very moist, W ? wet, VW ? very wet; abbreviations for SNRs: VP ? very poor, P ? poor, M ? medium, R ? rich,VR ? very rich.DiscussionDespite a limited representation of certain edatopes, the large amount of variation explained by the edatope model (Equations[3] and [6]) revealed strong relationships between site index of Pl and Sx and selected measures of site quality. Productivityof these species is expected to increase from cool to warm and from dry to wet climates. Insignificant differences in siteindex between zonal sites of both species indicated that the two study subzones either are climatically similar or, because ofcompensating effects, have similar actual evapotranspiration. In consequence, site index - site quality relationships can beexamined within the study area without considering climate.Sx site index was significantly lower than Pl on all but the richest sites. On all other sites, Pl site index was higher, with thedifference in favour of Pl increasing with decreasing nutrient supply and increasing water deficit or surplus. These relationshipssuggest Pl is less nutrient-demanding than Sx, which is more sensitive to extremes in water supply and soil nitrogen than Pl.The best growth of both species occurred on fresh or moist, very rich sites where Pl and Sx attained site indices of 25 and24 m, respectively.ConclusionsSite index of Pl and Sx varied with soil moisture and soil nutrients, but not with climate represented by two adjacent,climatically similar biogeoclimatic subzones. The pattern of change in site index relative to soil moisture was similar for bothspecies but differed relative to soil nutrients. Sx site index was significantly lower than Pl on nitrogen-deficient sites. Theregression model using soil moisture and nutrient regimes as categorical variables had the strongest relationships with siteindex of both study species (R2 >0.80; SEE <1.6 m).ReferenceWang, Q., G.G. Wang, K.D. Coates, and K. Klinka. 1994. Use of site factors to predict lodgepole pine and interior sprucesite index in the Sub-boreal Spruce zone. Research Note No. 114, BC Ministry of Forests, Victoria BC. 26 pp.Scientia Silvica  is published by the Forest Sciences Department,TheUniversity of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Q. Wang (charlw@mail.sy.ln.cn); G.G. Wang (g.wang@uwinnipeg.ca), K.D. Coates(Dave.Coates@gems7.gov.bc.ca), and K. KlinkaProduction and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: G.F. Dodd Operational Research Chair, Prince Rupert Forests Region, BC Ministry of Forests.For more information contact:  K. KlinkaCopies available from: www.forestry.ubc.ca/klinka or K. Klinka,Forest Sciences Department, 3036-2424 Main Mall,University of British Columbia, Vancouver, BC, V6T 1Z4

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