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 m aking survivor growth and mortality of trees functions of stand basal area. The functions, estimated from re-measured plots in Wisconsin map le-birch forests, showed that basal area affected mostly the growth of small trees. However, even when the effects were statistically signif icant, they were small. Fifteen-year forecasts with the variable-param eter model were almost identical to those of the fixed-parameter model for a set of 37 random plots. Long-term, steady-state predictions gav e similar diameter distributions for both models, but a 20% lower basa l area for the variable-parameter model. Steady-state management regim es that maximized soil rent were identical for both models in terms of intensity of harvest and length of cutting cycle. The basal area, har vests, and net present values predicted by both models, in the long ru n and during conversion, were also nearly the same. For managed maple- birch forests, the matrix model with constant parameters has the advan tage of simplicity of form, estimation, and application, especially in optimization mode. (C) 1997 Elsevier Science B.V.