When Will Plant Morphology Affect the Shape of a Seed Dispersal “Kernel”?

Most models of dispersal assume that plants are point sources. In reality, the scale in height over which seed sources are distributed is often of the same order as the scale in distance over which most individual seeds are dispersed. But is this sufficient to affect the fundamental shapes of dispersal frequency distributions? Most published conclusions about the effects of canopy structure on dispersal are subjective. A model is developed to explore the consequences of plant canopies for the shapes of whole-plant seed dispersal “kernels”. The canopies were described by simple geometric shapes, while an empirical probability density function (PDF) was used for dispersal from a point source. It was found that the resulting whole-plant PDF for dispersal distance was almost invariably peaked, whereas the PDF for the density of seed rain (as would be measured by pitfall traps) could either be peaked or monotonic according to the canopy shape, position of seeds in the canopy, and mean dispersal distance. The shapes of kernels from whole plants (distributed seed sources) can be very different from those derived from a point source under certain circumstances.