Print PageSeed dispersal is the main movement process in plants. As such, it generates the spatial template for the subsequent demographic processes. Furthermore, the rare long-distance dispersal events have a crucial role in determining population spread rates, inter-population connectivity, and gene flow. Landscape heterogeneity is a basic characteristic of any ecosystem, further accentuated by anthropogenic habitat fragmentation. For dispersal by wind, both topography and sharp transitions in vegetation stature profoundly affect wind flow. Yet, most current mechanistic dispersal models, which are central to quantitative prediction of dispersal patterns and understanding their underlying mechanisms, assume homogeneous environment.
Our main goal was to incorporate landscape heterogeneity effects into mechanistic models of seed dispersal by wind. We have focused on simplified models which enable to treat large-scale and long-term dispersal processes, and are applicable to ecosystem management. Towards this end, we modified the Coupled Eulerian-Lagrangian closure (CELC) mechanistic model to represent scenarios of wind flow over a sharp transition from short to tall vegetation or over forested hilly terrain, and simulated the resulting dispersal distances and direction. We parameterized the wind and vegetation factors using measurements taken on a hill with sharp transitions between Mediterranean shrubland and a pine forest at Mt. Pithulim, Israel.
Results/Conclusions
The modeling results showed that landscape heterogeneity, by altering the wind flow patterns, had a major effect both on the dispersal distances and, for topography, on dispersal directionality. For the short-to-tall vegetation transition scenario, the main features of the modeled wind field were an exponential decay of the mean horizontal wind velocity, and a consequent strong upward mean vertical velocity component. We found that for seeds released downwind of the transition, the simulated median (short) and 99-th percentile (long) distances were longer than those for the homogeneous tall vegetation scenario. For seeds released upwind of the transition, the effects on dispersal distance were more complex, and depended on the seed motion capacity.
The main attributes of the modeled wind field over a forested hill were acceleration of the topography-following mean wind component uphill, and recirculation zone formation within the canopy downhill. For seeds released uphill, the modeled median dispersal distances were longer than for the flat-terrain scenario, while for release downhill the median distances were lower than for the flat scenario. Median dispersal direction fitted the local wind direction; hence, seeds released on the lee side of the hill dispersed mainly uphill, opposing the regional wind direction.
