Age-related decline in forest production: modelling the effects of growth limitation, neighbourhood competition and self-thinning

1 In growing forest stands, above-ground net primary production peaks early in stand development and then declines. The causes for this decline are not yet well understood, but hypotheses include physiological and ecophysiological effects, as well as changes in stand structure due to local competition among neighbouring trees. 2 The majority of existing studies address mono-causal explanations of this decline. Here we study the combined effects of intrinsic growth limitation of individual trees, growth limitation due to neighbourhood competition, and self-thinning. 3 We use an individual-based model to analyse forest wood production of a mangrove species described by a sigmoidal growth function, and two hypothetical species with exponential or linear growth. The model reproduces a decline for all species investigated, even when individual growth rates did not become limited. 4 We conclude that individual, sigmoidal growth curves are sufficient but not necessary to explain the production decline in natural forests where neighbourhood competition is appreciably active. 5 We show that the causes for production decline change during forest development. Whereas growth reduction through neighbourhood competition is the main process at the beginning, imbalanced wood loss dominates the later stage of the decline.