GROWTH AND MANAGEMENT OF MIXED-SPECIES, UNEVEN-AGED FORESTS IN THE FRENCH JURA - IMPLICATIONS FOR ECONOMIC RETURNS AND TREE DIVERSITY

The forests of the Jura mountains are among the most productive in France. This study developed quantitative models to improve the management of these mixed forests of fir, spruce, and beech. A nonlinear matrix model of stand growth was estimated and validated with medium-term projections and simulations of long-term stand dynamics. The ecological criteria were Shannon's index of tree diversity by species and size, the minimum number of trees in any species-size class, and basal area. The economic criteria were the present value of harvests, gross or net of the value of the growing stock, and the rate of return on the capital invested in the growing stock. Linear and goal programming were used to determine the effects, on all criteria, of managements that would maintain the stand in a steady state, and either (i) remove only the dead trees, (ii) maintain the forest close to its current state, (iii) achieve economic efficiency, subject to a desired level of diversity, (iv) maximize diversity, with or without constraints on economic returns. The results suggested that a light management could improve tree diversity, relative to natural stand growth. Maximum economic efficiency would require a substantial reduction in basal area and average size of trees, and it would lead to low tree diversity. Maximizing unconstrained tree diversity would be costly. Managements that conciliated diversity and economic efficiency all required a decrease in the basal area of stands, relative to their current state. A simple approach to convert a stand from its current condition to a desired steady state was investigated.