Abstract: To establish a link between ecological site quality and forest productivity in the 'sub-boreal'portion of the Prince Rupert Forest Region,site index and site quality data from 93 lodepole pine stands and 77 white spruce stands were obtained and analyzed.The study stands were distributed across two climatic regimes(biogeoclimatic subzones),eight soil moisture regimes,and five soil nutrient regimes.These regimes were used as categorical variables in stratification of the stands and regression analysis.Site index of lodgepole pine and white spruce changed with soil moisture and nutrient regimes but not with climate.The pattern of change in relation to sol]moisture was similar for both species but different in relation to soil nitrogen.Compared to lodgepole pine,site index of white spruce was lower on water-deficient,water-surplus,and nitrogen-deficient(very Poor,poor f and medium)sites but the same or higher within the slightly dry to very moist and rich to very rich edatopic region.Of the five types of regression models developed,the edatope model showed strong relationships between site index and soil moisture and nutrient regimes[R²>0.80,SEE(standard error of estimates)<1.6m] for both lodgepole pine and white spruce.This model was used to construe site index isolines which were superimposed onto edatopic grids.It was concluded that the categorical measures of soil moisture and soil nutrients are good predictors of lodgepole pine and white spruce site index over a large area.

Key words: Lodgepole pine, White spruce, Ecological site quality, Soil moisture regime, Soil nutrient regime, Site index, Forest productivity