Consequences of green alder expansion on vegetation changes and arthropod communities removal in the northern French Alps

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 pa stures 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 th e biodiversity changes induced during alder expansion by taxonomic and func tional traits, using two representative taxa, i.e. vegetation and arthropod s. The results show that the alder expansion strongly affects the 0.5-1m ve getation layer and the plant species richness as well as the biomass and co mposition of the arthropods active on the soil surface. Hymenoptera, Orthop tera and Coleoptera, which compose the major part of the athropod biomass, decline significantly as the green alder canopy grows. Such changes in spec ific and functional diversity may have strong effects on the ecosystem func tioning and particularly on the maintenance of the habitat of several endan gered species such as the black grouse (Tetrao tetrix L.). The impact of gr een alder on biodiversity must be taken into account as well as its functio nal role in the ecosystem before leading any management against its expansi on. (C) 2001 Elsevier Science B.V. All rights reserved.