The role of spatial competitive interactions between trees in shaping forest patterns

General patterns that are observed in forests irrespective of their ecological biome or species composition are indicators of general biological processes that drive forest dynamics. Modelling can be used to infer these processes by identifying the minimum set of rules that generate the observed patterns. The regular spatial pattern of the largest trees, the decreasing skewness of the tree diameter distribution in young developing stands, and the size bimodality that may result from the tail of dominated trees are three general patterns observed in natural forests. Using a simple individual-based space-dependent growth model based on asymmetric competition between trees and space-independent approximations of this model, the objectives were (1) to identify a single mechanism that may explain these three patterns, and (2) to clarify the role of space in the emergence of these patterns. The space-dependent model was able to qualitatively generate the three patterns. Through cascading spatial interactions, competition resulted in the establishment of a spatially structured competitive hierarchy among trees. A second-order approximation of the space-dependent model was derived by structuring the population of trees according to diameter and competitive status. This second-order approximation succeeded in predicting left-skewed diameter distributions but did not predict diameter bimodality. Asymmetric competition thus seems to be an important driver of tree growth and the cause rather than the consequence of bimodality. The second-order approximation of the space-dependent model may be useful to generalize the stationary diameter distributions developed in the demographic equilibrium theory.