Growth and Management of Mixed-Specie

Abstract

 

Ju ra mountains

Ju ra mountains

The forests of the Ju ra 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 ecolog- ical criteria were Shannon’s index of tree diversity by spedes 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, (‘rii) achieve economic effidency, 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 effidency 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 investigate

Since then, uneven-aged management has been a common practice in the French Jura and neighboring Switzerland. But few formal tools are available to guide this practice. Until 1950, the application of too conservative norms (Liocourt 1901, Schaeffer et al. 1930) led to excessive volumes per ha, insuffident regen- eration, and, in fact, old even-aged stands. More recently, the policy has been to rejuvenate these stands, by cutting sufficiently at regular intervals to insure continuous regeneration. In this situation, more research and studies became necessary to better un- derstand stand growth and predict the effect of alternative management practices. A first step was accomplished in some areas by a classification of main stand types, with a description of the likely evolution between types (Bruciamacchie 1993). Nevertheless, quantitative information on stand growth, and on the con- sequences of alternative management regimes, is still scarce.

The present study does not use a stand classification, but describes uneven- aged stands by diameter distributions specifying the number of trees by size classes and species. This description has sufficient detail to model stand dynam- ics, to define meaningful criteria of stand diversity, and to assign accurate market values to stands and harvests. Stand states described in this way can then be classified easily according to systems appropriate for different tasks.

In the Jura context, uneven-aged silviculture is attractive because it keeps a natural look to the forest, with a mix of firs, spruces, and beeches of various sizes on the same area. It also protects the soil against erosion by maintaining a continuous forest cover. Moreover, it is finandally rewarding because it relies on natural regeneration and yields high-value products. Technically, it is judged more difficult to implement because it requires the marking of individual trees by skilled foresters, but simple guidelines could facilitate the marking operation, which is one of the objects of this paper.

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