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Ecological and height growth analysis of some sub-boreal immature lodgepole pine stands in central British Columbia Wang, Qingli


This study investigated relationships between lodgepole pine {Pinus contorta Dougl. ex Loud.) height growth and ecological site quality. Vegetation, environmental, and stand data, obtained from seventy-two sample plots established in immature stands over wide range of soil moisture and soil nutrient conditions in the montane boreal climate in central British Columbia, were analyzed using the methods of biogeoclimatic ecosystem classification and numerical analysis. The analysis produced categorical and continuous measures of ecological site quality which were then related to measures of height growth obtained from stem analysis of one hundred and sixty-two site trees. The seventy-one diagnostic species and ten vegetation units identified by tabular analysis were strongly correlated with, and occupied relatively narrow segments of climatic, soil moisture, and soil nutrient gradients. Heat index was used to characterize the climatic gradient represented by three biogeoclimatic subzones. Actual/potential evapotranspiration ratio and the depth of the growing season water table or gleyed soil horizons were used to characterize the soil moisture gradient and to classify the study plots into eleven soil moisture regimes. Soil mineralizable-N and the sum of exchangeable bases were used to characterize the soil nutrient gradient and to classify the study plots into five soil nutrient regimes. Correlations between vegetation and categorical or continuous measures of ecological site quality implied that these measures had a meaning relative to moisture and nutrient conditions experienced by plants. Eleven site associations circumscribed by vegetation units and characterized by a range of climatic, soil moisture, and soil nutrient regimes, stratified the study plots into qualitatively and quantitatively distinct, field recognizable, segments of regional gradients of ecological site quality. Regression analysis showed that the most strongly related ecological variables to lodgepole pine site index were: (1) ecotopes, defined either by a combination of categorical variables (biogeoclimatic subzone, soil moisture regime, and soil nutrient regime) (adj. R² = 0.85) or by a combination of continuous variables (potential evapotranspiration, and the depth of water table or gleyed soil horizons, and soil mineralizable-N) (adj. R² = 0.82), (2) site associations (adj. R² = 0.81), (3) site series (adj. R² = 0.84), and (4) vegetation units (adj. R² = 0.83). Lodgepole pine appears to have a potentigd to grow on nitrogen-rich sites with pH < 7. The three-parameter Chapman-Richards growth fimction precisely described height growth of site trees over a wide range of sites. The pattern of height growth changed with ecological site quality. Site series and ecotope (defined either by a combination of categorical or continuous variables) had a stronger relationship with the function parameters than site index. The two site-specific height growth models developed—the site unit model and the ecotope model—were more effective than an existing site-index driven growth models. The above results support the use of either categorical or continuous synoptic ecological variables in describing the variation of lodgepole site index in relation to ecological site quality, which can be inferred from the understory vegetation developed in mid-seral stands. The derived site index and site-specific height growth models showed strong relationships between height growth and several measures of ecological site quality produced by biogeoclimatic ecosystem classification. In consequence, categorical or continuous ecological variables could be used in polymorphic growth modelling to predict lodgepole pine height growth so that the effects of site, and environmental changes, including management practices, on forest productivity can be better understood.

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