The river‐bed: a dynamic store for plant propagules?

Abstract

AbstractThis paper investigates temporal variations in the abundance, species composition and richness, and spatial distribution of plant propagules stored on river‐beds.Samples of the surface layers of river‐bed sediments were obtained from two reaches of the River Frome, Dorset, UK, and one reach of the River Tern, Shropshire, UK. Samples were taken from the same bed locations on four occasions spaced at approximately four monthly intervals. Following greenhouse germination trials, 11 384 viable propagules of 105 species were identified in 192 channel bed samples. On average, 32% of the species found in the propagule samples were not present in the channel and riparian vegetation. Most of these additional species were wetland and terrestrial rather than aquatic species. The number of species and density of propagules stored on the river‐bed varied through time and between reaches. These findings indicate that the channel bed is a dynamic store for viable propagules, including a large proportion of species that are not found locally. These propagules are available for redistribution across river‐banks and floodplains during high river flows.Propagules were preferentially stored within particular bed habitats. On the Tern, the most retentive habitats were close to the channel margins and adjacent to riparian vegetation and trees. On the Frome, stands of emergent vegetation along the channel margin were the most retentive habitat, and were also associated with trapping of finer sediment and organic matter. This habitat provides an aggrading environment at the bank toe that could potentially offer germination sites for the non‐aquatic propagules that are stored. Germination and growth of wetland and terrestrial plants during bank profile aggradation and lateral extension would provide root reinforcement and flow resistance across the changing bank profile. The preferential accumulation of sediment and species‐rich propagules around stands of marginal emergent vegetation is indicative of a subtle coupling of geomorphological and ecological systems. Copyright © 2007 John Wiley & Sons, Ltd.