Résumé
The current decline of agro-pastoral activities in the northern Alps modifies the balance between human-induced factors and environmental constraints of natural type which was source of a characteristic landscape in mountain zones for decades. We studied that modification by using the example of the Alnus viridis (Chaix) D.C. expansion. This shrub is favoured by the drop of human activities in the subalpine belt and produces a dense foliage cover which could be a threat for the maintenance of biodiversity. The work was divided in two points : expansion mechanisms and influence on the biodiversity of herbaceous semi-natural ecosystems. Results show that biodiversity at the specific level is considerably modified during growing A. viridis cover, although the effect is qualitatively different according to the taxonomic group, the diversity index, and the spatial level studied. A. viridis expansion seems to be favoured by residuals of pastoral activities, mainly presence of bare soils, and its persistence to the detriment of arboreal species is explained by a modification in the reproduction strategy in dense thickets, i.e. from sexual to vegetative mode, which allows it to regenerate. Consequently, the A. viridis expansion on subalpine grasslands is a major event concerning the subalpine ecosystems functioning, its ecological value in terms of biodiversity depends on the conservation targets taken into account.