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

Abstract

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.

Spatial and temporal tree colonization in a midstream sediment bar and the mechanisms governing tree mortality during a flood event

“…Riverine landscapes that have been subject to a moderate duration of inundation accumulate fine sediment, even at low-inundation depths (Steiger et al, 1998;Gurnell and Petts, 2002;Richards et al, 2002;Asaeda and Rajapakse, 2008), but this accumulation may be highly affected by local morphology (Gurnell and Petts, 2002;Gilvear and Willby, 2006;Asaeda and Rajapakse, 2008). The accumulated sediment contains high levels of moisture and nutrients, making these areas more conducive to plant colonization.…”

Channel migration inferred from aerial photographs, its timing and environmental consequences as responses to floods: A case study of the meandering Topľa River, Slovak Carpathians

“…The accuracy and precision of spatial data, methods for calculating lateral movement of the channel and the selection of consistent rules for identifying landform dynamics and delimiting the features of interest, significantly affect geomorphological interpretations (Zanoni et al, 2008). Not only do historical data serve to identify lateral migration (Hooke, 1979;Pišút, 2002;Gilvear and Bryant, 2003;Michalková et al, 2011), but they also contribute to the study of changes of channel and riparian zone patterns (Bryant and Gilvear, 1999;Gilvear and Willby, 2006). Floods and their erosional power affect the overall ecological diversity in the riparian landscape by eroding channel landforms or vegetation in the riparian zone, by depositing sediments and seed dissemination (Ward et al, 2002;Petts, 2002, 2006;Corenblit et al, 2007;Opperman et al, 2010;Corenblit et al, 2010;Erskine et al, 2012).…”

Riparian vegetation as indicator of channel adjustments and environmental conditions: the case of the Panaro River (Northern Italy)

“…While some of these studies stress the dominant effect of hydrogeomorphological processes on plant communities (e.g. Piégay et al 2000;Gilvear and Willby 2006), others highlight the active role of vegetation in influencing river channel dynamics (e.g. Tal et al 2004;Corenblit et al 2007;Gurnell et al 2012).…”