Use of synoptic variables in predicting white spruce site index

Based on data from 102 white spruce-dominated stands in the sub-boreal spruce zone of British Columbia, forest site quality was evaluated using both species-specific and ecological approaches. White spruce site index, determined from stem analysis data, was used as a species-specific measure of site quality. Climate regimes and soil moisture, aeration and nutrient regimes were used as ecological measures of site quality. Subzones and variants, differentiated by zonal vegetation, were used as synoptic measures of climate. Soil moisture regimes, differentiated by actual/potential evapotranspiration ratio, depth to a gleyed layer or prominent mottles, and depth to a groundwater table, were used as synoptic measures of soil moisture. Soil aeration regimes, differentiated by soil water saturation, slope, drainage, and soil texture, were used as synoptic measures of soil aeration. Soil nutrient regimes, differentiated by mineralizable nitrogen (kg ha(-1)) and carbon/nitrogen ratio, was used as synoptic measures of available soil nutrients. These measures were related to white spruce site index through regression analysis and limiting factor analysis. The results showed that soil moisture, aeration, and nutrients were growth-limiting factors while climatic variations within the sub-boreal spruce zone were not important. The best regression model explained about 90% of the total variance in site index, with standard error of estimates of 1.2 m. This strong relationship suggests that ecological site classifications can be used to evaluate both ecological site quality and forest productivity.