The Mixedwood Growth Model (MGM) was developed in the Department of Renewable Resources at the University of Alberta by Morton and Titus (1984). Initially, the emphasis was on trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench)). However, because of the interest in managing lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm) and some application problems identified by model users, there was the need to re-calibrate MGM. Four issues were considered; (1) the applicability of MGM for long-term projection applications, (2) the performance of MGM for predicting the effect of species interaction on tree growth, (3) compatibility between the component growth models and (4) the biological significance of predictor variables.

Empirical tree diameter growth models were developed for lodgepole pine, trembling aspen and white spruce using the modified three-parameter Weibull probability density function (pdf) as the base function of tree DBH (surrogate for age). Other variables reflecting competition, tree vigor and site productivity were included as modifiers. Fit and validation statistics indicated that all models were unbiased and fitted the data reasonably well. However, tree DBH seemed to be a poor substitute for tree age. This was attributed to lack of data balance across all DBH classes and possibly, unfavorable interaction of the growth predictor variables. The second study developed individual tree height growth models for each of the three species using the site index curve as the potential height growth. The modifier was assumed to be a function of basal area in larger trees and the species relative quadratic mean diameter. The models were unbiased and satisfactorily accounted for the effects of species interaction on tree height growth.

The third study developed a compatible system of height and diameter growth functions, linking tree height growth, diameter growth and the site curve, for each of the three species. The results of simulation tests using various forest stand conditions showed a significant improvement in the long-term projection accuracy of MGM. It was concluded that linking site curve to tree diameter and height growth was a reasonable approach to achieving compatibility between these growth components.