Analogies between mineral sediment and vegetative particle dynamics in fluvial systems

Riparian vegetation fragments and propagules form part of the organic sediment load of rivers. This review paper draws analogies between the transfer of these vegetative particles and that of mineral sediment within river systems. Whole plants, vegetation fragments and propagules are delivered to rivers by physical processes but the timing of seed release and the morphological characteristics of the plants moderate delivery. The transport and deposition of plant material is partly controlled by flow patterns but is also influenced by the buoyancy and morphology of the plant material. Once deposited, the vegetative fragments, propagules and the plants that develop from them can have significant local effects on flow hydraulics and sediment erodibility, producing complex assemblages of physical features. Aggregate vegetative particles are identified as being particularly important in accelerating vegetation colonisation of exposed river sediments, driving landform development and supporting the rapid development of a diverse cover of plants. The geomorphological significance of interactions between vegetative particles and fluvial processes is illustrated at different spatial scales. At the catchment scale, ribbons of mineral and vegetative particles are transferred downstream with inputs continuously supplied throughout the river's course. Individual particles experience phases of mobilisation, transport and deposition. As a result, along larger rivers, down-river changes are typically seen in the size and type of mineral sediment and vegetative particles, and also the species composition of the vegetative particles. Coupled with downstream changes in hydroclimatological conditions within a river's active zone, marked downstream changes in interactions between fluvial processes, vegetation propagules and landforms can result. At the patch scale individual vegetative particles (particularly aggregate particles) deposited on exposed riverine sediments create local hydraulic complexity that induces the scour and deposition of sediment and further plant propagules, resulting in a suite of distinctive landforms which may support many plant species. Within individual reaches, patch-scale interactions between mineral sediment and vegetative particles are moderated or sieved by the form of the reach and its flow patterns, so that the integration of patch-scale effects can support the development of different landforms in different locations. (c) 2006 Elsevier B.V. All rights reserved.