Plant species around the world invest in seed dispersal by producing large numbers of seeds, with a wide range of morphological adaptations that facilitate dispersal. Not all dispersed seeds reach suitable sites, however, and plants can significantly improve their fitness by increasing the proportion of seeds arriving at suitable sites for germination and establishment. Disproportionate dispersal to suitable sites is known as 'directed dispersal'. Yet, mechanisms of directed dispersal are only known for a limited number of animal-dispersed plant species. We tested the hypothesis that directed dispersal can also be driven by abiotic vectors, such as water or wind. We used a tiered approach, combining analyses of experimental, field and literature data on wetland plant species and evaluating the potential for evolution of directed dispersal with a spatially explicit individual-based model. The data collected demonstrate that wetland plants produce seeds with adaptations to promote transportation and deposition by water towards microsites along the hydrological gradient where they germinate and establish best. Aquatic species produce seeds that sink and are transported by water as bed load towards inundated sites. In contrast, shoreline species produce seeds that float for very long periods of time so that they are eventually entrapped by shoreline vegetation or deposited at the waterline. Our model simulations confirm that the patterns we observed in nature can evolve under natural selection through adaptations in seed buoyancy. For wind dispersal, the situation is more complex. Wind does not provide directed dispersal in the strictest sense but, rather, simply appears to be the best available dispersal vector for more terrestrial wetland plant species to reach drier areas in a wet environment. Synthesis. We show that directed dispersal towards specific, suitable microsites is not exclusive to animal-dispersed plant species, but may be far more common in plants - also in species dispersed by abiotic vectors, in particular water. As water and wind are very common dispersal vectors throughout the plant kingdom, directed dispersal (and not just dispersal distance) seems to be of general importance for the ecology of plants.
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