摘要：Subalpine grasslands in the western Alps are currently facing major environmental changes induced by pastoral disuse. In such a context, the rapid spread of a shrub ( Alnus viridis [Chaix] DC.) is expected to threaten biodiversity. The aim of this study was to assess the impact of its cover on vascular plant diversity. Data collected at several local levels (108 plots) in a representative area detected a linear decrease in species richness across an increasing A. viridis (green alder) cover gradient, whereas alder-stand β-diversity at the community level was particularly singular. We used constrained ordination methods based on Principal Components Analysis to partial out substantial variance explained by spatial structure of samples. The resulting partial Redundancy Analysis accounted for 62.3% of the initial inertia, and the environmental effect by A. viridis cover was identified as a major cause of variance in the whole species ordination (16.1%). The expansion of A. viridis on subalpine grasslands causes considerable environmental changes which have mostly a negative effect on the conservation of vascular plant diversity in the inner part of the western Alps. However, A. viridis appears to contribute to the diversity of the subalpine belt by inducing a peculiar floristic composition.
