The role of island dynamics in the maintenance of biodiversity in an Alpine river system

1. The current concepts of large river systems have been advanced with limited empirical knowledge of natural systems, because most large rivers in Europe and USA have been more or less radically altered by engineering. For example virtually all large Alpine rivers were channelised during the 19th century. Without first hand knowledge of natural systems we lack baseline data to assess human impacts and to address restoration and conservation strategies. 2. In this project we are interested in the dynamics of vegetated islands on active floodplains and their role in maintaining biodiversity. The central hypothesis of the project is that these islands are a product of the interaction between the fluvial regime and the dominant plants, the Salicaceae. Through accumulation of sediments and woody debris which form islands these plants act as ecosystem engineers. These dynamic processes help to maintain a complex braided channel system which supports a high level of habitat diversity. From our preliminary studies we conclude that three aspects of the natural river system are particularly important for the maintenance of island dynamics: a natural disturbance regime, an unconstrained channel, and a sufficient supply of large woody debris. In addition sediment grain size and nutrient concentrations may have important effects as well. 3. The project investigates the following three overview hypotheses: (i) Five willow species (Salix alba, S. daphnoides, S. elaeagnos, S. purpurea, S. triandra) and Populus nigra are the key ecosystem engineers in the active zone of the River Tagliamento. We hypothesize differences in their habitat niches within the floodplain system, and a differential ability to influence island dynamics by vegetative (generative) regeneration and to withstand disturbance. (ii) Islands increase the diversity and heterogeneity of habitats at the reach scale and at the island/bar scale. Ecosystem expansion and contraction dictate the variability and connectivity of these habitats; islands create important refugia for aquatic invertebrates in dynamic natural systems. (iii) Islands, i.e. riparian ecotones within the active plain, function as sources, sinks and transformers of organic matter and nutrients. Ecosystem expansion and contraction facilitate the exchange of organic matter and nutrients across the floodplain.