Relationships between site index and synoptic environmental factors in immature coastal Douglas-fir stands

Indirect synoptic measures of climate, soil moisture,and soil nutrients are used to characterize site quality in the biogeoclimatic ecosystem classification. To assess relationships of these measures to potential forest productivity, environmental and site index data from 133 sample plots established in immature coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) stands were utilized. The stands were distributed across three climatic regimes (biogeoclimatic units), five soil moisture regimes, and five soil nutrient regimes. These regimes were used as categorical variables in stratification of the data and regression analysis. Direct measures of these regimes, i.e., components of annual water balance and mineralizable-N in the forest floor and mineral soil, were determined for each stand and used as continuous variables in regression analysis. Douglas-fir site index (m/50 yr) and these direct measures were significantly different between groups of stands stratified according to soil moisture and soil nutrient regimes. Five regression models indicated good relationships between site index and both categorical and continuous variables. The analytical models (based on actual evapotranspiration during May and June, growing season water deficit, and/or mineralizable-N in the forest floor and mineral soil) were slightly inferior (R2 = 0.72) to the categorical models (based on soil moisture and nutrient regimen (R2 = 0.84). Testing on an independent data set showed similar results, i.e., the categorical models continued to have a better relationship with site index than the analytical models. It is concluded that the indirect synoptic measures of climate, soil moisture and soil nutrients were good predictors of Douglas-fir site index over a large area and provided useful estimates of the direct measures.