Development and evaluation of a biomass increment based index for site productivity

Measures of forest productivity generally rely on site index, which can be problematic for multicohort and mixed-species stands. Using stand growth and dominant tree height-age (i.e., site tree) measurements from similar to 10 900 plot locations from Maine, Nova Scotia, New Brunswick, and Prince Edward Island, a forest productivity model for the Acadian Forest Region was developed as a function of climate, lithology, soils, and topographic metrics. Approximately 65% of variation in observed aboveground dry-biomass growth rate (BG) was explained by a Chapman-Richards function of temperature, bedrock, soil root space, slope, and depth to water in combination with stand structure and species predictors. Productivity was then defined in terms of the predicted asymptote of BG, holding structure and species constant, which was termed biomass growth index (BGI), i.e., the site-influenced component of the BG relationship. BGI was mapped on a 20 m grid throughout the region. BGI explained 0%-30% of the variability in spruce (Picea sp.) and balsam fir (Abies balsamea (L.) Mill.) site index and had similar site index predictive performance (+/- 5%) when compared with existing land productivity classifications in each province. BGI provides a direct relationship between site variables and growth and can help guide forest management decisions and future research.