Modeling individual tree mortality for white spruce in Alberta

An individual tree mortality model, based on the provincial PSP data, was developed for white spruce (Picea glauca (Moench) Voss), an important tree species in Alberta, Canada. Annual probability of survival was modeled by a generalized logistic function with the measurement interval length as a predictor variable to overcome the problem of unequal measurement intervals. Potential predictor variables were selected based on their ecological importance to tree mortality and unknown parameters were estimated using the maximum likelihood method. A U-shaped mortality trend was captured by diameter and diameter squared. Annual diameter increment was used to indicate tree vigor. Basal area of larger trees and a ratio of basal area of larger broadleaf trees to stand total basal area were both used to capture competition from neighboring trees. The newly developed mortality function outperformed the old one previously used in the Mixedwood Growth Model (MGM) based on both goodness-of-fit and prediction statistics.