UBC Faculty Research and Publications

Relationships between site index of major tree species in the ESSF zone and ecological measures of site.. Klinka, Karel 2008

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Scientia  Silvica Extension Series, Number  23, 1999 Relationships Between Site Index of Major Tree Species in the ESSF Zone and Ecological Measures of Site Quality. Introduction Knowledge of ecological characteristics of sites and growth of trees on different sites is fundamental for silvicultural decision-making and planning. With the biogeoclimatic ecosystem classification in place in British Columbia, silvicultural management has been given an ecological foundation; however, relationships between growth and site quality have not yet been fully investigated, particularly for high-elevation tree species and sites. One of the contributing factors for this situation is limited knowledge of forest productivity in the high-elevation Mountain Hemlock (MH) and Engelmann Spruce - Subalpine Fir (ESSF) biogeoclimatic zones. Consequently, the management and planning in the high-elevation forest is fraught with difficulties and uncertainties. Current harvest rates of old-growth forest stands and the method and distribution of cuttings in these zones suggest that there needs to be more recognition of the uppermost elevation limit for harvesting. Subalpine fir (Bl), Engelmann spruce (Se), and lodgepole pine (Pl) are important timber crop species in the interior high-elevation forest which is represented predominantly by the subalpine boreal ESSF zone. This zone extends from 49° to approximately 57° N latitude and from approximately 900 to 1,700 m in the north, from 1,200 to 2,100 m in central BC, and from 1,500 to 2,300 m in the south. In view of this relatively wide climatic and edaphic amplitude, a large variability in productivity is expected. The objective of this study was to quantify relationships between site index (height @ 50 yrs @ bh) of Bl, Se, and Pl, and three ecological determinants of site quality: climate, soil moisture, and soil nutrients. Quantitative relationships between site index and these measures provide predictive models for estimating site index. Additionally, we compared the site indices of the three study species to each other to examine their early height growth performance on the same sites. Materials and Methods To determine potential forest productivity, measured by site index (m @ 50 yr bh) and its relation to selected measures of site quality, we collected and analyzed stem analysis and environmental data for Bl (n = 174), Se (n = 90), and Pl (n = 75) stands across the ESSF zone. All stands were naturally established, fully stocked, even-aged (ranging from 41 to 240 years @ bh), and without apparent damage and suppression. The stands were distributed in 10 forested and adjacent parkland subzones, and across a wide range of elevation, aspect, and soil conditions. However, they were not equally distributed along environmental gradients due to a low frequency of sites in extreme environmentals. Effects of climate (represented by subzone, elevation, latitude, and longitude), aspect, actual soil moisture, and soil nutrient regimes on site index were examined by analysis of variance (ANOVA). To develop and test predictive site index models, the data set for each species was randomly split into calibration and test data sets. Multiple regression analysis was used to fit predictive models from climate and/or soil variables, and the precision of fitted models was tested against independent data. Results and Discussion Effects of regional climate, local climate, and edaphic conditions on site index On zonal sites in the forested subzones, mean site index was 12.0 m for Bl, 12.3 m for Se, and 16.0 m for Pl. Site index of any species on zonal sites did not vary significantly (α=0.05) among the forested subzones; however, site index of Bl, which was sampled from the lower to the upper elevational limits of the ESSF zone, was significantly lower in 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 was consistently the most influential factor on site index of all species explaining 34% of the variance for Bl, 38% for Se, and 19% for Pl. For every 100 m increase in elevation and 10 increase in latitude, Bl site index is predicted to decrease about 2.4 m, Se about 3.9 m, and Pl about 1.8 m, with site index decreasing more precipitously with latitude than elevation and 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 a significant influence on Bl and Pl site index, and no significant SMR´SNR interaction was detected, i.e., the influence of soil moisture and nutrients was consistent across both gradients. Differences between Bl and Se site indices were insignificant on any aspect except on west slopes. Site index gradually decreased from flat>east>west>south slopes>cool-air drainage>snow pocket = frost pocket>north slopes>ridges, with site index on ridge sites being significantly lower than on any other aspects. Aspect had the most pronounced influence on Se site index, while Bl and Pl site indices were much less affected. This result indicates a high sensitivity of Se to local climate, specifically to temperature as influenced by topography (aspect and slope gradient) and depth and duration of snowpack. Site indices of all study species in forested subzones (i) increased from moderately dry to moist sites and decreased from moist to wet sites; and (ii) increased consistently from very poor through very rich sites (except for lodgepole pine). The site indices of Bl and Se were similar across study sites, while Pl had higher site indices than Bl and Se on all sites except on rich and very rich sites. The most productive edaphic region for all study species with site indices 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 predictors of site index but present only the results from the climatic- edaphic models (Table 1). The climatic models, which used elevation, latitude, and/or longitude as the predictors explained less of the variation in site index than the climatic- edaphic models. The accountability of these climatic models increased in order from Pl (R2 = 0.24) to Bl (R2 = 0.39) to Se (R2 = 0.57). Similarly, the edaphic models, which used SMR, SNR, and aspect for Bl and Se, and only SMR for Pl as the predictors, had lower levels of accountability for variation in site index than the climatic-edaphic models. The accountability of the edaphic models including aspect increased in order from Bl (R2 = 0.43) to Se (R2 = 0.51) to Pl (R2 = 0.52). The climatic-edaphic models, using ELE, LAT, LON, SMR, and SNR either with or without aspect as predictors, explained similar amounts of variability in site index of all study species. For brevity, we present only the models without aspect (Table 1). When tested with independent data sets, the models were unbiased in predicting site index and had similar values of the mean square of prediction error. Site index relationships between the three species Using the data from mixed-species stands, a correlation analysis was carried out to examine relations between site index of Bl and Se (pair 1), Pl and Se (pair 2), and Pl and Bl (pair 3) (Figure 1). The analysis showed that site indices of these 3 pairs were positively correlated, with the strength of correlation decreasing from pair 1 (r = 0.91, n = 84) to pair 3 (r = 0.79, n = 26) to pair 2 (r = 0.58, n = 17). There was approximately the same number of stands in which Bl performed as well as Se. Bl tended to have a higher site index than Se on less productive sites, and Se outperformed Bl at the lower elevation limits of the ESSF zone. Correlation between Bl or Se and Pl site index indicated that at the lower elevations Pl site index was Table 1.  Models for the regression of subalpine fir (Bl), Engelmann spruce (Se), and lodgepole pine (Pl) site index (m @ 50 years bh) on soil nutrient regimes (SNR), soil moisture regimes (SMR), elevation (ELE, m), and latitude (LAT,. degrees and minutes in metric scale), 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 28 En ge lm an n  sp ru ce  si te  in de x  (m ) 0 4 8 12 16 20 24 28 Subalpine fir site index (m) 4 8 12 16 20 24 Lo dg ep ol e pin e si te  in de x  (m ) 4 8 12 16 20 24 Engelmann spruce site index (m) 8 12 16 20 24 Lo dg ep ol e pin e si te  in de x  (m ) 8 12 16 20 24 usually higher than the other two species. The superior performance of Pl is not surprising, considering its early growth rate. However, at later stages of stand development (>100 yrs) these differences will diminish. Thus, in early growth stages (approximately to 70 years @ bh) (1) Bl and Se mixtures will develop a non-stratified (single-storied) structure, and (2) Pl mixtures with either Bl or Se will develop a stratified (two-storied) structure. Figure 1.  Correlations between site index of subalpine fir and Engelmann spruce (A), lodgepole pine and subalpine fir (B), and Engelmann spruce and lodgepole pine (C). B C A Conclusions Mean site indices on zonal sites in forested subzones of the 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 and forested subzones, but not among forested subzones. Elevation, latitude, and longitude were weak surrogates for local climate. The site indices of all study species declined most strongly with increasing latitude, and the sensitivity to change in climate decreased in order from Se > Bl > Pl. Variation in the site indices along soil moisture and nutrient gradients followed expected trends: an increase followed by decrease with increasing soil moisture, and consistent increase with increasing soil nutrients. On fresh sites, the change from very poor to very rich sites resulted in 1.5-fold increase in Bl site index, a 2-fold increase in Se site index, and a marginal increase in Pl site index. The regression models using continuous climatic and categorical edaphic variables as predictors of site index had acceptable accountability and precision, and are recommended for estimating site index of the study species in the interior subalpine forest. Reference Klinka, K., H.Y.H. Chen, and L. de Montigny. 2001. Potential productivity of the interior subalpine forest of British Columbia. Submitted for publication to Plant Ecology 01/01/30. Scientia Silvica is published by the Forest Sciences Department, The University of British Columbia, ISSN 1209-952X Editor: 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 Columbia For more information contact: Karel Klinka Copies 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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