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Relationships between site index of major tree species in the ESSF zone and ecological measures of site.. Klinka, Karel; Krestov, Pavel; Chourmouzis, Christine 1999-12-31

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Scientia Silvica Extension Series, Number  23, 1999Relationships Between Site Index of Major Tree Species in the ESSFZone and Ecological Measures of Site Quality.IntroductionKnowledge of ecological characteristics of sites and growthof trees on different sites is fundamental for silviculturaldecision-making and planning. With the biogeoclimaticecosystem classification in  place in  British Columbia,silvicultural management has been given  an ecologicalfoundation; however, relationships between growth andsite quality have not yet been fully investigated, particularlyfor  high-elevation tree  species  and sites.  One of thecontributing factors for this situation is limited knowledgeof forest productivity  in the  high-elevation MountainHemlock (MH) and Engelmann Spruce - Subalpine Fir(ESSF)  biogeoclimatic zones. Consequently,  themanagement and planning in the high-elevation forest isfraught with difficulties and uncertainties. Current harvestrates  of old-growth forest  stands  and the  method anddistribution of cuttings in these zones suggest that thereneeds to be more recognition of the uppermost elevationlimit for harvesting.Subalpine fir (Bl), Engelmann spruce (Se), and lodgepolepine (Pl) are important timber crop species in the interiorhigh-elevation forest which is represented predominantlyby the  subalpine boreal ESSF zone. This zone extendsfrom  49? to  approximately 57? N latitude  and fromapproximately 900 to 1,700 m in the north, from 1,200 to2,100 m in central BC, and from 1,500 to 2,300 m in thesouth. In view of this relatively wide climatic and edaphicamplitude, a large variability in productivity is expected.The objective of this study was to quantify relationshipsbetween site index (height @ 50 yrs @ bh) of Bl, Se, andPl, and three ecological determinants of site quality: climate,soil moisture, and soil nutrients. Quantitative relationshipsbetween site index and these measures provide predictivemodels for estimating site index. Additionally, we comparedthe site indices of the three study species to each other toexamine their early height growth performance on the samesites.Materials and MethodsTo determine potential forest productivity, measured bysite  index (m  @  50 yr bh) and its  relation to  selectedmeasures of site quality, we collected and analyzed stemanalysis and environmental data for Bl (n = 174), Se (n =90), and Pl (n  = 75) stands across the  ESSF zone. Allstands were naturally established, fully stocked, even-aged(ranging from 41 to 240 years @ bh), and without apparentdamage and suppression. The stands were distributed in10 forested and adjacent parkland subzones, and across awide range  of elevation, aspect, and soil  conditions.However,  they  were not equally distributed alongenvironmental gradients due to a low frequency of sites inextreme environmentals. Effects of climate (representedby subzone, elevation, latitude, and longitude), aspect, actualsoil moisture, and soil nutrient regimes on site index wereexamined by analysis of variance (ANOVA). To developand test predictive site index models, the data set for eachspecies was randomly split into calibration and test datasets. Multiple regression analysis was used to fit predictivemodels from climate and/or soil variables, and the precisionof fitted models was tested against independent data.Results and DiscussionEffects of regional climate, local climate, and edaphicconditions on site indexOn zonal sites in the forested subzones, mean site indexwas 12.0 m for Bl, 12.3 m for Se, and 16.0 m for Pl. Siteindex of any species on zonal sites did not vary significantly(alpha=0.05) among the forested subzones; however, site indexof Bl, which was sampled from the  lower  to  the  upperelevational limits of the ESSF zone, was significantly lowerin the parkland subzones (6.8 m) than forested subzones(12.0 m).All indirect climatic variables - elevation, latitude, longitude,and subzone - had significant influences on Bl site index.Latitude, longitude, and subzone affected Se site index,and only latitude  affected Pl site  index. Latitude wasconsistently the most influential factor on site index of allspecies explaining 34% of the variance for Bl, 38% for Se,and 19% for Pl.For every 100 m increase in elevation and 10 increase inlatitude, Bl site index is predicted to decrease about 2.4 m,Se about 3.9 m, and Pl about 1.8 m, with site  indexdecreasing more precipitously with latitude than elevationand longitude. The rate of site index decrease was Se>Bl>Pl,with Pl being the least sensitive to changes in local climate.SMR influenced site index of all study species. SNR had asignificant  influence  on Bl and Pl site  index,  and nosignificant SMR?SNR interaction was detected, i.e., theinfluence  of soil moisture and nutrients was consistentacross both gradients. Differences between Bl and Se siteindices were insignificant on any aspect except on westslopes.  Site  index  gradually  decreased  fromflat>east>west>south  slopes>cool-air  drainage>snowpocket = frost pocket>north slopes>ridges, with site indexon ridge sites being significantly lower than on any otheraspects. Aspect had the most pronounced influence on Sesite index, while Bl and Pl site indices were much lessaffected. This result indicates a high sensitivity of Se tolocal climate, specifically to temperature as influenced bytopography  (aspect  and slope  gradient) and depth andduration of snowpack.Site indices of all study species in forested subzones (i)increased from moderately dry to moist sites and decreasedfrom moist to wet sites; and (ii) increased consistently fromvery poor through very rich sites (except for lodgepolepine). The site indices of Bl and Se were similar acrossstudy sites, while Pl had higher site indices than Bl and Seon all sites except on rich and very rich sites. The mostproductive edaphic region for all study species with siteindices ranging from 16 to 18 m was on fresh to moist,rich to very rich sites.Can site index be predicted from a few, easily ob-tainable, climatic and edaphic properties?We developed regression models using climatic, edaphic,and climatic-edaphic measures of site quality as predictorsof site index but present only the results from the climatic-edaphic models (Table 1). The climatic models, which usedelevation, latitude,  and/or longitude  as the  predictorsexplained less of the variation in site index than the climatic-edaphic models. The accountability of these climatic modelsincreased in order from Pl (R2 = 0.24) to Bl (R2 = 0.39) toSe (R2 = 0.57). Similarly, the edaphic models, which usedSMR, SNR, and aspect for Bl and Se, and only SMR forPl as the predictors, had lower levels of accountability forvariation in site index than the climatic-edaphic models.The accountability of the edaphic models including aspectincreased in order from Bl (R2 = 0.43) to Se (R2 = 0.51) toPl (R2 = 0.52). The climatic-edaphic models, using ELE,LAT, LON, SMR, and SNR either with or without aspectas predictors, explained similar amounts of variability insite index of all study species. For brevity, we present onlythe  models without aspect (Table 1). When tested  withindependent  data sets,  the  models were unbiased inpredicting site index and had similar values of the meansquare of prediction error.Site index relationships between the three speciesUsing the data from mixed-species stands, a correlationanalysis was carried out to examine relations between siteindex of Bl and Se (pair 1), Pl and Se (pair 2), and Pl andBl (pair 3) (Figure 1). The analysis showed that site indicesof these 3 pairs were positively correlated, with the strengthof correlation decreasing from pair 1 (r = 0.91, n = 84) topair 3 (r = 0.79, n = 26) to pair 2 (r = 0.58, n = 17).There was approximately the same number of stands inwhich Bl performed as well as Se. Bl tended to have ahigher site index than Se on less productive sites, and Seoutperformed Bl at the lower elevation limits of the ESSFzone. Correlation between Bl or Se and Pl site  indexindicated  that  at the  lower  elevations Pl site index  wasTable 1.  Models for the regression of subalpine fir (Bl), Engelmann spruce (Se), and lodgepole pine (Pl) site index (m @ 50 ye ars bh)on soil nutrient regimes (SNR), soil moisture regimes (SMR), elevation (ELE, m), and latitude (LAT,. degrees and minutes in metricscale), SEE - standard error of estimates. Abbreviations for SMRs: MD - moderately dry, SD - slightly dry, F - fresh, M - moist ,abbreviations for SNRs: VP very poor, P - poor, M - medium, R - rich, VR - very rich.Species Models Adjusted R2 SEE p n     Bl  SI = 91.1099 - 5.5918 (MD-SMR) - 2.9927 (SD-SMR) - 5.0270 (VP-SNR) + 1.9558 (R-SNR) + 3.4605 (VR-SNR) - 1.2713 (LAT) - 0.0088 (ELE) 0.50 3.07 <0.001 100 Se  SI = 116.9215 + 2.9072 (M-SMR) + 3.3372 (VR-SNR) - 1.6882 (LAT) - 0.0119 (ELE)  0.49  3.37  <0.001  59 Pl  SI = 65.2362 - 7.0744 (MD-SMR) - 2.8894 (SD-SMR) + 1.5553 (M-SMR) - 0.7673 (LAT) - 0.0067 (ELE)  0.59  1.86  <0.001  40            Subalpine fir site index (m)0 4 8 12 16 20 24 28Engelmann spruce site index (m)0481216202428Subalpine fir site index (m)4 8 12 16 20 24Lodgepole pine site index (m)4812162024Engelmann spruce site index (m)8 12162024Lodgepole pine site index (m)812162024usually higher than the other two species. The superiorperformance of Pl is not surprising, considering its earlygrowth rate. However, at later stages of stand development(>100 yrs) these differences will diminish. Thus, in earlygrowth stages (approximately to 70 years @ bh) (1) Bland Se mixtures will develop a non-stratified (single-storied)structure, and (2) Pl mixtures with either Bl or Se willdevelop a stratified (two-storied) structure.Figure 1.  Correlations between site index of subalpine fir andEngelmann spruce (A), lodgepole pine and subalpine fir (B),and Engelmann spruce and lodgepole pine (C).BCAConclusionsMean site indices on zonal sites in forested subzones ofthe ESSF zone were 12 m for Bl and Se, and 16 m for Pl.The influence of regional climate, delineated by subzone,on forest productivity was detected between parkland andforested  subzones,  but not among forested  subzones.Elevation, latitude, and longitude were weak surrogatesfor  local  climate. The site  indices  of all study  speciesdeclined most strongly with increasing latitude, and thesensitivity to change in climate decreased in order from Se> Bl > Pl. Variation in the site indices along soil moistureand nutrient gradients followed expected trends: an increasefollowed by decrease with increasing soil moisture, andconsistent increase with increasing soil nutrients. On freshsites, the change from very poor to very rich sites resultedin 1.5-fold increase in Bl site index, a 2-fold increase in Sesite index, and a marginal increase in Pl site index. Theregression models using continuous climatic and categoricaledaphic variables as predictors of site index had acceptableaccountability and precision, and are recommended forestimating site index of the study species in the interiorsubalpine forest.ReferenceKlinka, K., H.Y.H.  Chen, and L. de Montigny.  2001.Potential productivity of the interior subalpine forest ofBritish Columbia. Submitted for  publication to  PlantEcology 01/01/30.Scientia Silvica is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Pavel Krestov (farrex@vtc.ru)Production and design: C. Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Forest Renewal British ColumbiaFor more information contact: Karel KlinkaCopies available from: www.forestry.ubc.ca/klinka or K. Klinka,Forest Sciences Department,UBC, 3036-2424 Main Mall,Vancouver, BC  V6T 1Z4

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