Simulation of the effect of topography and tree falls on stand dynamics and stand structure of tropical forests

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Canopy gaps induced by tree falls have an important ecological role in forest communities since they act on forest sylvigenesis, dynamics and specific composition, mainly through modifications of the light context they involve. Some factors responsible for tree fall occurrences become more influent on sloping areas, particularly crown asymmetry. We have already shown in previous work (Robert, 2001) that slope also acts on the dynamics and structure of forests through the anisotropy of light it induces. In this paper we aim at investigating the consequences of tree falls of asymmetrical trees, and of canopy openness they create, combined with the increase of slope on tropical forest stands. We were particularly interested in the effects of slope and tree falls on tree growth, stand basal area, stand density and spatial structure of trees. This study was achieved by means of an individual-based and distance-dependent model, taking into account competition for light and morphological constraints between trees. Inter-tree competitive relationships occurred at crown level and were illustrated by a synthetical index that partially depends on their elevation. Recruitment, growth and mortality of trees of the virtual stand were computed on the basis of yearly time. The mortality process consists of standing death and tree falls caused by the asymmetry of crowns. The model shows that the effect of tree falls and of canopy gaps is more important than the effect of light anisotropy induced by slope. The presence of tree falls diminishes, and even inhibits, the positive effect of slope on competition for light. Furthermore, it totally prevents any slope effect on spatial structure of trees by maintaining it very regular. Finally, the occurrence of tree falls inverts slope effect on stand density and results in an impoverishment of trees on steep slopes. Simulation results allow us to understand how the presence of canopy gaps on sloping areas participates in maintaining the phytosanitary quality of tropical stands. They also permit us to put forward the role of the presence of asymmetrical trees as a key parameter that conditions the effects of light anisotropy on forests on slopes.