Soil seed banks and vegetal succession in the Southern Alps : effects of historic and ecologic factors

In a Southern Alps watershed, the authors have studied density, richness, and similarity of vegetation and soil seed banks, along a secondary succession, submitted or not to grazing. Stratified sampling, based on a geographic information system, allowed to take in account relevant historical and ecological parameters and to correlate them with seed bank and vegetation. Data show that (i) density and richness of the grassland and shrubland seed bank were higher in ungrazed as compared to grazed areas, (ii) similarity between seed bank and vegetation increased along the succession, before decreasing in old-growth forests, and (iii) seed banks of these forests were markedly different from those of younger forests. Bitmap correlation tests show that, whatever was the dynamic stage, edaphic variables were responsible for structuring soil seed banks. In this mountain area, vegetation emphasizes vegetative reproduction at the expense of long life seed bank establishment. Pastoralism thus appears as the only mean allowing maintenance of the actual biodiversity.