Fixed versus variable-parameter matrix models of forest growth: the case of maple-birch forests

A matrix growth model with constant parameters has been modified, by making survivor growth and mortality of trees functions of stand basal area. The functions, estimated from re-measured plots in Wisconsin maple-birch forests, showed that basal area affected mostly the growth of small trees. However, even when the effects were statistically significant, they were small. Fifteen-year forecasts with the variable-parameter model were almost identical to those of the fixed-parameter model for a set of 37 random plots. Long-term, steady-state predictions gave similar diameter distributions for both models, but a 20% lower basal area for the variable-parameter model. Steady-state management regimes that maximized soil rent were identical for both models in terms of intensity of harvest and length of cutting cycle. The basal area, harvests, and net present values predicted by both models, in the long run and during conversion, were also nearly the same. For managed maple-birch forests, the matrix model with constant parameters has the advantage of simplicity of form, estimation, and application, especially in optimization mode.