The differential effect of updrafts, downdrafts and horizontal winds on the seed abscission of Tragopogon dubius

P>1. While it is well known that many plant species enhance the dispersal of seeds by wind via traits such as lift-promoting wings and drag-producing fibres, we hypothesized that natural selection would also increase dispersal capacity through the evolution of mechanisms that promoted abscission by updrafts rather than downdrafts. 2. An experiment with the cosmopolitan weed, Tragopogon dubius, showed that a combination of simple morphological traits and achene orientation made updrafts from three to five times more likely than downdrafts to abscise a seed over the vertical wind speed range of 0 center dot 2-0 center dot 5 m s-1. (Abscission will not occur at speeds lower than about 0 center dot 2 m s-1 while vertical speeds > 0 center dot 5 m s-1 near ground level would be extremely rare.) Horizontal winds were even more effective than updrafts at abscising seeds at any wind speed. 3. We speculate that mechanisms causing an updraft abscission bias are quite common and will eventually be seen as a crucial component of long distance seed movement for almost all wind-dispersed species.