Résumé
Green alder is a widespread shrub species in the Alps. The intense grazing practices of the last centuries relegated it to avalanche tracks on wet and steep slopes. However, it is currently colonizing abandoned meadows and pastures on relatively wet and drained soils at the subalpine and the montane belts, creating a dense shrub cover. The aim of this study is to assess the biodiversity changes induced during alder expansion by taxonomic and functional traits, using two representative taxa, i.e. vegetation and arthropods. The results show that the alder expansion strongly affects the 0.5–1
m vegetation layer and the plant species richness as well as the biomass and composition of the arthropods active on the soil surface.
Hymenoptera,
Orthoptera and
Coleoptera, which compose the major part of the arthropod biomass, decline significantly as the green alder canopy grows. Such changes in specific and functional diversity may have strong effects on the ecosystem functioning and particularly on the maintenance of the habitat of several endangered species such as the black grouse (
Tetrao tetrix L.). The impact of green alder on biodiversity must be taken into account as well as its functional role in the ecosystem before leading any management against its expansion.