Field validation and sensitivity analysis of a mechanistic model for tree seed dispersal by wind

We present a temporally and spatially explicit mechanistic model of tree se ed dispersal by wind, incorporating full stochasticity based on natural var iation. The model simulates the dispersal of each individual seed by integr ating the temporal effects of climatic conditions on the rate of seed relea se, and the spatial effects of wind direction and horizontal and vertical v elocities, the terminal velocity of seeds (i.e., the constant descent veloc ity in calm air), and the height of seed release, partitioned into tree hei ght and the distribution of seeds with tree height. The model was tested fo r two Pinus halepensis stands within the Mediterranean region of Israel, in which seed dispersal has been extensively monitored by seed traps. The pre dicted dispersal curve verified expectations of a positively skewed leptoku rtic distribution and of peak location at some distance from a point source and at zero distance from an area source. Long-distance dispersal events o ccurred with very low frequency, but given the large seed crop in P. halepe nsis, even a small fraction should result in a considerable number of seeds dispersed far from their source. The model reliably simulates the observed dispersal pattern in a spatial resolution of 1 m(2) (R-2 between 60% and 9 0%), as revealed from comparisons of the predicted and observed proportions of seed dispersed to seed traps. A sensitivity analysis using Latin hyperc ube sampling along with stepwise multiple rank regression showed that the e ffects of the horizontal and vertical wind velocities on the dispersal dist ance override those of the biotic factors. This suggests that the synchroni zation of seed release with favorable winds is the most effective plant-con trolled mechanism to increase the distance of dispersal in wind-dispersed s pecies such as P. halepensis.