Investigating canopy regeneration processes through individual-based spatial models: application to a tropical rain forest

Observations of circular structures in a permanent plot in a natural dense tropical forest of the Western Ghats (India) led to formulation of new hypotheses on the spatial organization resulting from interactions between trees. Here we propose a canopy regeneration process that does not involve treefall gaps. The peculiar spatial circular structure is composed of a central dominant tree with less than two trees under its crown and five or more other trees forming a ring at the limit of its crown. Such a structure is supposed to occur under certain conditions and, in particular, under strong competition for light. We describe a regeneration mechanism based on replacement of the central tree by a shift to one of the trees in its peripheral ring so that progressive recovery of the canopy occurs without large gap dynamics. We built a spatial simulator of forest dynamics in order to investigate the occurrence of such spatial structures and to examine their role in canopy regeneration. The model is an individual-based spatial model for mixed uneven-aged forest stands and accounts for annual evolution of the trees in the simulated stand, incorporating mortality, recruitment, growth and competition processes. The main originality of this model lies in the description of crown growth in eight cardinal directions, and in the use of neighbor position and size to express competitive interactions. The model demonstrates the development of the circular spatial structures in simulated stands as those observed in the permanent plot and shows that the proposed canopy regeneration mechanism can occur in the absence of perturbation.