TREE SIZE DIVERSITY AND ECONOMIC RETURNS IN UNEVEN-AGED FOREST STANDS

Methods are presented to predict some of the effects of management on the tree size diversity of forest stands, measured with the Shannon-Wi ener index applied to diameters. The diameter distribution is an indic ator of stand structure and a determinant of biological diversity. The methods, which involve linear and nonlinear programming models of man aged stands, were applied to northern hardwood forests. The attendant stand growth model was calibrated and validated with remeasured plots. The results suggest that, in this specific case, a natural, undisturb ed stand would reach the highest possible sustainable diversity of tre e size. Any intervention would decrease that diversity. In particular, economic harvesting policies would reduce tree size diversity by 10 t o 20%, depending on the length of the cutting cycle. However, economic s and diversity did not necessarily conflict. With the data and model used here, lengthening the cutting cycle up to 30 yr led to higher tre e size diversity, while it also increased the soil rent of a stand by decreasing the present value of the cost of re-entry. Similarly, for a given cutting cycle, the soil rent increased with tree size diversity up to about 90% of the sustainable maximum, but decreased sharply the reafter. Real internal rates of return of 2 to 4% could be earned with stands that had either mediocre or very good levels of diversity. Lik ewise, starting from the same initial stand state, proper choice of a cutting guide could lead to much higher levels of tree size diversity, without decreasing forest value.