A MODEL OF EVEN-AGED BEECH STANDS PRODUCTIVITY WITH PROCESS-BASED INTERPRETATIONS

In order to describe the productivity of pure even-aged stands of comm on beech, a system of three differential equations is proposed for dom inant height, basal area and total volume growth. The model was derive d and fitted to 317 observation periods in 29 long-term experimental p lots ranging from northwest to northeast France. It involves parameter s at the forest and stand levels. Site index is the asymptote of the h eight-age curve. Model structure is such that, for any given height, s ome differences in total volume yield exist between stands of differen t productivities. This result is in contradiction with Eichhorn's rule . However, in our model, no parameter other than site index is necessa ry to characterize stand productivity. The possibility to generalize t he model to a larger range of ecological conditions is discussed by a process-based interpretation. The site dependence of the parameters ca n be understood by reference to carbon-balance models. A linear relati onship between basal area and height-growth rates is investigated by a separate model of sapwood geometry and dynamics.