Present species richness of plant communities in alpine stream corridors in relation to historical river management

Since the early 19th century, particularly in the Alps, stream corridors have been affected by two major disturbances: diking/channelization which cut off large areas of floodplain and the infilling of by-pass channels which has raised the level of alluvial deposits by siltation. While these engineering works have favoured farming, some areas remained uncultivated. These are: (1) well-silted basins which became waterlogged a:Rer the water table was raised (sedimentation within the diked channel), (2) well-silted basins used to produce firewood, and (3) unsilted or insufficiently silted basins which still show evidence of landforms characterizing the ancient braided pattern. Anthropogenically provoked disturbances such as flooding/silting followed by agricultural practices such as clearing, mowing and grazing are responsible for the establishment of relatively young fens (< 150 years old) exhibiting a higher plant diversity than unsilted areas. A comparison was made between sites in the Isere valley and similar but more ancient habitats in the Rhone valley. This showed the role of flooding/silting operations in promoting a rapid reinstatement of biological richness in alluvial wetlands. However, it was evident that the more newly created sites were generally much poorer in species than the older sites. Conservation biologists interested in preserving species and plant communities in stream corridors of the Alps therefore need to incorporate a dynamic perspective of biological systems that includes the overriding impact of prior land-use and historical river management. It is suggested that old flooding techniques can be used again in order to preserve or enhance present biodiversity. (C) 1998 Elsevier Science Ltd. All rights reserved.