Willows (Salix spp.) produce many small seeds that are dispersed primarily through the air by winds (anemochory) and sometimes secondarily by flowing water (hydrochory). In this paper I identify another way willow seeds are dispersed – being blown by winds while sailing or floating on the surface of standing water, here termed pleustochory. In experiments, seeds of Salix gooddingii C.R. Ball floated on water for four days and sailed swiftly on the surface of pools when light winds blew, reaching speeds in excess of five meters per minute. Field observations showed that S. gooddingii seeds sailing on water were blown downwind and soon came to rest at the edge of the pool, in their preferred safe sites. This dispersal via pleustochory from unsuitable sites (middle of a pool) into safe sites (edge of pool) is therefore a new example of directed dispersal. Salix gooddingii seed dispersal and seedling densities were studied at a large, remote pool in the Tijuana River Valley, San Diego Co., to examine the influence of pleustochory on S. gooddingii seedling densities. The study focused on a 15-day period when the pool was slowly drying and S. gooddingii seeds were dispersing from a distant, isolated stand. The S. gooddingii seedlings that established during the 15-day period formed a band that encircled the entire pool with the highest densities (3140 seedlings per m2) occurring along the On Shore. The numbers of S. gooddingii seeds arriving in the seedling sites via three dispersal routes – hydrochory, anemochory, and pleustochory – were measured or estimated. A model combining all three dispersal routes accurately predicted the pattern of seedling densities around the pool and estimated that pleustochory accounted for more than 99% of the seedlings along the On Shore. Together these results showed that pleustochory played a vital role in the transport of S. gooddingii seeds to safe sites and was the underlying mechanism producing the pattern of seedling densities around a large pool.
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Vol. 61 • No. 4