Channel dynamics and geomorphic variability as controls on gravel bar vegetation; River Tummel, Scotland

This paper presents the results of an investigation into environmental controls on vegetation dynamics on gravel bars. Such environments are a hotspot of threatened plant biodiversity and the dynamics of their vegetation reflect a range of processes that should be indicative of the integrity of the wider floodplain ecosystem. The study was undertaken on a 2 ha mid-channel gravel bar complex that evolved over two decades, in response to several high magnitude flood events (including two with a return period in excess of 25 years), on a 'wandering' reach of the River Tummel, Scotland. Over 180 plant species, including a number of national or regional scarcities, had colonized. The fluvial chronology of the site was documented via sequential sets of aerial photography that revealed a number of discrete surfaces created by individual floods. Environmental heterogeneity, both within and between fluvial units, was investigated by field sampling of vegetation and abiotic variables at 66 locations. The fluvial surfaces were assigned to five habitat classes that ranged in age from two to approximately 20 years, from fine gravel to cobbles, and maintained an elevation range of up to 2.5 metres above low flow river levels. Multivariate analysis highlighted the relative importance of elevation, grain size, moisture content and infiltration and trapping of fines in controlling plant species composition. After standardizing sampling effort the habitat mosaic was found to support on average 1.36 times more species than an equivalent sample of any one habitat. In terms of biodiversity and river management, our results emphasize the importance of sustaining fluvial processes that preserve the habitat mosaic in order to conserve the characteristic biota of gravel bar complexes and river channel islands. Copyright (c) 2006 John Wiley & Sons, Ltd.