The role of abscission in long-distance seed dispersal by the wind

It has become clear that long-distance seed dispersal plays a crucial role in plant metapopulation persistence and response to rapid climate change. Recent studies of the role of convective vs. shear-generated updrafts in prompting long-distance dispersal using Taraxacum officinale as an example, suggest that (1) the probability of abscission is independent of horizontal speed and thus (2) shear-induced vertical turbulence is low, and so by default the bulk of updrafts must be due to convection, especially in open habitats such as grasslands. In this paper, I directly test the first hypothesis, and indirectly test the second via a modeling exercise. Employing shorter averaging times than used previously, it is shown that abscission in T. officinale is controlled by horizontal wind speed. Indeed, it is related to the square of the wind speed, as might be expected if drag is the motive force. I also show that this augmentation of wind speed by the abscission bias should sufficiently increase the shear-induced turbulence so that shear rivals convection as a source of updrafts in open habitats. In conclusion, long-distance dispersal by wind will not be successfully modeled until we couple the abscission and subsequent dispersal processes.