HYPOTHESES ABOUT COMPETITION FOR LIGHT AND WATER IN EVEN-AGED COMMON BEECH (FAGUS-SILVATICA L)

A distance-independent tree model was built, in order to simulate the dynamics of pure even-aged stands of beech, under various silvicultura l scenarios. The formulation and calibration of this model were based on data from permanent plots, installed around the beginning of the ce ntury. The modeling method was largely empirical. However, to overcome some limitations of the database, the main features of the model were justified by means of simple biological assumptions. The influence of tree social status on growth is featured with a segmented linear diff erential equation, organized around a threshold size. The properties o f this threshold size (increasing with age, independent of stand densi ty and structure) are interpreted as the consequence of a predominant aerial competition for light, between trees vertically structured in t wo storys (upper and lower storys). Additional trials performed to tes t the validity of the model reveal that the threshold may have much mo re complex temporal patterns; in the case of remarkable climatic seque nces the possibility of an underground competition for water uptake is discussed. Finally some reflections about the attitude of growth and yield models with regard to the physiological and ecological knowledge are proposed. Because all the complexity of basic mechanisms is not f ully described, or described at time and spatial scales which are not appropriate for growth and yield problems, the empirical phase can pro bably not be avoided in the building of growth models. At least, an ef fort for simplifying their mathematical machinery win allow us to be m ore lucid with regard to their biological content and more confident i n their qualitative behavior.