Field Validation and Sensitivity Analysis of a Mechanistic Model for Tree Seed Dispersal by Wind

Abstract

We present a temporally and spatially explicit mechanistic model of tree seed dispersal by wind, incorporating full stochasticity based on natural variation. The model simulates the dispersal of each individual seed by integrating the temporal effects of climatic conditions on the rate of seed release, and the spatial effects of wind direction and horizontal and vertical velocities, the terminal velocity of seeds (i.e., the constant descent velocity in calm air), and the height of seed release, partitioned into tree height and the distribution of seeds with tree height. The model was tested for two Pinus halepensis stands within the Mediterranean region of Israel, in which seed dispersal has been extensively monitored by seed traps. The predicted dispersal curve verified expectations of a positively skewed leptokurtic distribution and of peak location at some distance from a point source and at zero distance from an area source. Long‐distance dispersal events occurred with very low frequency, but given the large seed crop in P. halepensis, 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 m2 (R2 between 60% and 90%), as revealed from comparisons of the predicted and observed proportions of seed dispersed to seed traps. A sensitivity analysis using Latin hypercube sampling along with stepwise multiple rank regression showed that the effects of the horizontal and vertical wind velocities on the dispersal distance override those of the biotic factors. This suggests that the synchronization of seed release with favorable winds is the most effective plant‐controlled mechanism to increase the distance of dispersal in wind‐dispersed species such as P. halepensis.

Assessment of Wind Dispersal Potential in Plant Species

“…The enormous differences in WDP under different weather conditions shown in Fig. 3 support the hypothesis of Nathan et al (2001b), who suggest that the synchronization of diaspore release with favorable weather conditions is a very effective means of increasing WDP. However, favorable weather conditions are not necessarily characterized by high horizontal wind speeds, but, particularly for low-growing species (H rel Ͻ 0.8-1.6 m), by an unstable atmosphere with low horizontal wind speeds but strong thermal updrafts (Fig.…”

“…This example demonstrates that the importance of a low V term for achieving a high WDP increases, whereas the importance of a high H rel decreases with increasing reference distance. Diaspores of low-growing species can only achieve long-distance dispersal by wind if they are lifted, at least temporarily, by a vertical updraft (Bullock and Clarke 2000, Nathan et al 2001b, Tackenberg 2003; see also Greene and Johnson 1997). The lower the V term , the higher the chance of the diaspore being lifted and the higher the proportion of diaspores traveling long distances.…”

Macroalgal Spore Dispersal in Coastal Environments: Mechanistic Insights Revealed by Theory and Experiment

“…There is another level of understanding that is also vital but has remained even more elusive. Unlike with dispersal of seeds on land where there is a tradition of incorporating physical elements in kernel parameterizations (e.g., seed release heights and fall velocities, air currents, statistics of wind gusts, Okubo and Levin 1989, Greene and Johnson 1989, Andersen 1991, Nathan et al 2001, 2002, Tackenberg 2003, Soons et al 2004, phenomenological representations of marine dispersal have not been as effectively linked to nearshore ocean processes. A number of important physical factors have been identified, including upwelling/relaxation phenomena (Roughgarden et al 1988, Wing et al 1995, 1998, Garland et al 2002 and internal waves and bores (Pineda 1991, Shanks 1995, Leichter et al 1996, but it has not been generally possible to develop mechanistic descriptors of propagule transport that can be reduced to simple mathematical forms suitable for guiding ecological theory.…”

Sex, isolation and fidelity: unbiased long‐distance dispersal in a terrestrial amphibian

“…It is notoriously difficult to track large numbers of individuals accurately enough to determine a dispersal curve (Nathan et al 2001, Largier 2003, Cain et al 2003, Gaines et al 2003). Fowler's toad (Bufo fowleri Á a medium-sized toad whose adults are 50 to 80 mm from snout to vent, with females slightly larger than males) exists in a landscape with near ideal conditions for measuring dispersal.…”

“…To test the hypothesis of sex-biased dispersal we compared site-specific and known-point mark-recapture distance records for adult males and females as dispersal frequency distributions (dispersal curves i.e. Nathan et al 2001).…”