Quantifying size-asymmetric growth among individual beech trees

Abstract

We modelled the growth of individual trees in populations of European beech (Fagus sylvatica L.) grown under different thinning regimes using a modified Richards equation. The effect of competition on growth was modelled by coupling the n individual equations simultaneously with a saturation term. By assuming that the growth of an individual within the population is a function of its size to a power a, a measure of the growth advantage of larger individuals (size-asymmetric growth) is provided. If a > 1, larger trees have a disproportionate advantage in growth and by inference, in competition. The degree of size-asymmetric growth, a, exceeded one in stands with large size variability and increased significantly at increasing density. This suggests that the predominant mode of competition is size asymmetric and that this size asymmetry increases with density. A measure of growth asymmetry is more informative than static measures of size inequality in understanding the growth dynamics of managed forest stands. Since a provides a measure of the relative importance of above- versus below-ground competition, it may be useful in interpreting the growth dynamics of forest stands and may provide an additional level of information for modellers of forest growth.