Growth models of silver birch (Betula pendula Roth.) on two volcanicmountains in the French Massif Central

摘要

Silver birch woodlands of two volcanic mountains (altitude 850 m and 1450 m) were studied in order to establish a growth model of birch. Height, radial increment and crown width were measured on both sites taking into account two situations: isolated birch or birch within a stand. For the latter case two categories were sampled considering the competition status of the tree: birch suffering the most severe competition (suppressed tree) were distinguished from trees facing the lowest competition (dominant tree). Measures of competition were also made using plots where each tree was located by its coordinates and its height, trunk circumference and crown width recorded. Examination of radial growth curves showed similar patterns for the two sites for the same category of tree. Radial growth was always inferior for the site located at the highest altitude but tree lifespan was about two-fold longer. Radial increment data were used to calculate circumference as function of tree age. Models predicting circumference with time were then established for each category of tree (dominant, suppressed or isolated) using Chapman-Richards'model. Height and crown width models were produced using circumference as the predictor. Competition indices based on vertical or horizontal angles weighted by the distance were calculated for birches in stands. Indices using vertical angles appeared to be more resistant and robust to characterize local competition. Competition index was then associated with the parameters of Chapman-Richards' growth circumference model for the two sites and models predicting the circumference from the age of the tree and its competition index are proposed.