Abstract
(Englisch)
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The aims of ERMAS II is to evaluate the functional role of biodiversity in determining the sensitivity of river margin ecosystems to differences in environmental conditions. Two fundamental questions are being addressed : i. How does the sensitivity of river margin ecosystems to environmental changes vary along climatic, disturbance and successional gradients ? ii. How is the sensitivity of river margin ecosystems influenced by species richness and composition. ERMAS II is based on a set of 4 core protocols : i. Describe plant and macro-invertebrate diversity; ii. Establish the influence of plant diversity on litter decomposition; iii. Establish functional relationships between litter diversity and macro-invertebrate diversity; iv. Establish the influence of plant diversity on C and N dynamics in the soils of the river margin. The methods are various and concern: the monitoring of some key environmental variables such as the river discharge, the elevation relative to the water level records, the groundwater level and the climate; the plant assemblage, the biomass measurements and its C & N content; the invertebrate diversity, measured by means of pitfall traps and sweep netting or kick sampling technics; the leaf bags decomposition experiments, the soil analyses such as inorganic and dissolved organic nitrogen, soil respiration and denitrification, net nitrogen mineralization and nitrification, microbial N immobilisation.
Vegetation survey has been carried out during spring 96 and 97 on five river reaches: Aare river (' Altwässer der Aare und Zihl ' and ' Umiker Schachen - Stierenhölzli '), Orbe river (' Les Saignes de la Burtignière '), Doubs river (' La Récheresse '), Sarine river (' Rossens ' ). Thirty plots have been sampled on each reach for the plant diversity protocol. Data are now analysed with respect to three spatial scales: regional (i.e. European gradients), area (i.e. between reaches in each study area) and patch (i.e. between patches in each reach).
On the Aare river, all the other protocols have been carried out on five patches which have been chosen according to their vegetation, to their flooding regime and to their sediment grain-size characteristics. Invertebrate communities have different composition in frequently and rarely flooded patches. But there are no significant differences between sandy and silty soils. The main soil characteristics were analysed and N-cycle parameters were measured monthly over a period of 13 months (03.97 to 03.98). In all patches, soils are calcareous alluvial soils with hydromorphic horizons appearing at different depth. There is a large temporal and spatial variation of in situ denitrification within a patch. Denitrification rates were highest from July to August 1997 in all patches, corresponding to the flood period. The flooding affects the denitrification process by saturating the soils by overflow and principally by rising the water table. Moreover, the flood period coincide with the summer time when temperature is optimal for activity of denitrifying bacteria. The other prerequisites for denitrification such as available carbon and sufficient nitrate exist in these type of alluvial soils.
The core protocols are completed by some supplementary protocols in order to investigate i. C & N dynamics under 2 typical plant species (Alnus incana and Phragmites communis), ii. the dynamic of plant colonisation after an experimental perturbation (simulation of inundation with erosion), iii. Spatial distribution of vegetation communities and individual species with respect to soil heterogeneity and flooding regime.
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