Source effects determine hydrochorous seed dispersal of ancient forest plants

Abstract

Abstract Plant species of ancient forests tend to be poor dispersers, although field studies suggest that dispersal may be strongly accelerated in streams. To further test this idea we addressed the following two questions: 1) which traits facilitate transport and deposition of seeds by streams? 2) do ancient forest species differ from other forest species with respect to these traits? In the BovenSlinge, a forested lowland stream in the Netherlands we measured water transport and deposition and analysed which traits best explained this. We subsequently compared ancient forest species with other forest species with respect to these key traits. Our field measurements indicated that stream dispersal is strongly determined by both source related traits, i.e. upstream population size, seed production and seed release period and the path related variables buoyancy and seed weight. These traits seemed to affect transport and deposition similarly, but seed mass had a much stronger, negative impact on deposition. Ancient forest species do not differ with respect to seed release period, buoyancy and seed mass, but at a given plant height, ancient forest species produce less seeds than other forest species. This may reflect an adaptation to their relatively stable habitat which comes at the expense of (stream) dispersal capacity. In conclusion, ancient forest plants are just as well equipped for stream dispersal as other forest species, but success will critically depend on upstream population size, given their low per plant seed production.