Modeling Long-Distance Dispersal of Plant Diaspores by Wind

We developed PAPPUS, a trajectory model for wind dispersal of plant diaspores under field conditions. The model considers the effects of topography, turbulence (including thermal updrafts), and different weather conditions on the dispersibility of diaspores by wind. In the model, the plant species are characterized by the initial release height and the falling velocity of their diaspores. The common problems in modeling turbulence, which limit the applicability of existing models in predicting long-distance dispersal, are avoided by measuring the high-frequency fluctuations of the wind vector and using these data to simulate the course of the wind vector in the model. Using PAPPUS, we simulated dispersal distance spectra and compared them with those observed during field experiments executed in open habitats. Within a broad range of landscapes and under different weather conditions, the results of the model represent the observed spectra reasonably well. Additionally, we compared the observed dispersal distance spectra with the predictions of an existing plume and trajectory model, respectively. PAPPUS was the only model capable of predicting the proportion of diaspores dispersed over long distances; the existing models were unable to predict that proportion. This is particularly true for sunny weather conditions with thermal turbulence and updrafts, because these conditions may result in a high proportion of long-distance dispersal, even if the horizontal wind speed is low. In contrast, windy or stormy weather may be of much smaller importance for long-distance dispersal than is commonly assumed, especially for non-tree species with $V_{term}100 m, whereas the frequency of updrafts was. Furthermore, the effects of landscape topography on dispersal distances were examined in simulations using PAPPUS. Differences in elevation and slope may affect the dispersal distance spectra considerably. The superior performance of PAPPUS in predicting long-distance dispersal is mainly due to the new method of incorporating turbulence (especially thermal updrafts) and the consideration of topographic effects. Because other wind dispersal models applied to diaspore dispersal thus far do not consider thermal updrafts or topography, they may considerably underestimate the dispersibility of plant diaspores by wind.