Seed dispersal, breeding system, tree density and the spatial pattern of trees -: a simulation approach

Tropical tree populations with low densities and with clumped spatial distributions are at risk in the face of fragmentation. It is therefore important to understand factors driving spatial patterns of tree populations. An important set of factors which may influence the spatial pattern of plants, could be the spatial distribution of possible seed sources such as the breeding system (monoecious, dioecious), tree density and the spatial pattern of the parent population. Another set of important factors might be dispersal of seeds away from the parent plants characterised by the mean dispersal distance and by the distribution of dispersal distances (either negative exponential or lognormal). However, relevant field studies investigating these processes are scarce due to the long life span of trees. We studied the effect of these factors on the spatial distribution of trees over time using an individual-based, spatially explicit simulation model. The results demonstrated that dispersal distance and tree density were the main factors influencing spatial patterns. Low to medium dispersal distances always resulted in highly clumped tree populations. Populations with low tree density developed highly clumped patterns, too, and an increase in density lead to a decline in clumping. Breeding system had a medium effect on spatial patterns with dioecious populations usually developing higher clumping than monoecious ones. The distribution of dispersal distances and the spatial Pattern of the Patent population had only small effects on the spatial pattern of populations. With a good understanding of these factors it is now possible to study the effect of post-dispersal factors such as e.g. predation and herbivory on spatial patterns of trees.