The effect of vertical and horizontal wind speeds on seed abscission

Background/Question/Methods A comprehensive model of seed dispersal by wind awaits a more complete understanding of the role of wind speed and direction in seed abscission. Especially, our prediction of long distance dispersal requires that we understand any abscission bias that might exist in relation to updrafts. In a wind tunnel we examined the release of seeds from drying fruits of mahogany (Swietenia humilis) and ovulate cones of jack pine (Pinus banksiana), and compared the probability of release as a function of both wind speed and orientation.

Results/Conclusions Swietenia fruits point upward from the branches; as the woody pericarp dries, curls and abscises, the winged seeds are then directly exposed to the wind and initially hanging downward from the vascular bundle. Pinus ovulate cones also point upward but the vascular bundle is below the winged seed, and the flexing scales form a cup; escape following abscission is difficult. Following scale flexing, seed abscission in the absence of wind could certainly occur for Pinus; however the seeds would not subsequently disperse from the opened, upward-pointing cones. More typical of other species, seeds could not passively abscise from Swietenia. For both species we found that, no matter what the orientation, the seeds preferentially released at higher wind speeds with a slope of around 2.0 (as expected from the argument that drag is the motive force) in a power-law regression of abscission probability on wind speed. Finally, for Swietenia, fruit orientation mattered greatly: updrafts were far more effective at releasing seeds than were horizontal winds; the latter in turn were more effective than downdrafts. By contrast, for Pinus, updrafts were only modestly more effective than either side winds or downdrafts. We conclude that (1) the bias toward updrafts observed here is likely common among wind-dispersed plant species, and thus our present mechanistic models of seed dispersal are badly underestimating long distance dispersal; and (2) the magnitude of the bias depends on the cone/fruit morphology.