Consequences of green alder expansion on vegetation changes and arthropod communities removal in the northern French Alps Fabien Anthelme a,* , Jean-Luc Grossi a , Jean-Jacques Brun a , Lydie Didier b a UMR Ecosyste Ámes et Changements Environnementaux, Cemagref Grenoble, 2 rue de la papeterie BP 76, F-38042 Saint Martin d'He Áres cedex, France b UMR Ecosyste Ámes et Changements Environnementaux, Universite  Joseph Fourier, BP 53X, 38041 Grenoble cedex, France Abstract 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 signi®cantly as the green alder canopy grows. Such changes in speci®c 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. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Alnus viridis; Arthropods; Biodiversity; Functional descriptors; Subalpine; Vegetation dynamics 1. Introduction Green alder (Alnus viridis (Chaix) D.C.) is an early successional woody species. It is a major component of the subalpine vegetation in the northern French Alps. Its current distribution is mainly a consequence of agricultural management: the systematic and large scale deforestation of the last centuries created wide open grasslands favorable to the shrub invasion after pastoral disuse. Thus, green alder is spreading from established stands such as avalanche corridors, where it is highly competitive due to wet conditions and the strong disturbance level (Richard, 1990), towards abandoned pastures. Since the land-use changes initiated by the development of industry in the middle of the 19th century and the development of tourism in the middle of the 20th century, the green alder shrub- lands have grown rapidly, and cover now 5±10% of the total area between 1600 and 2200 m a.s.l. in the northern French Alps (Richard, 1990). The modi®cations induced in ecological systems by the green alder expansion are related to the impact of Forest Ecology and Management 145 (2001) 57±65 * Corresponding author. Tel.: 33-4-76-76-27-27; fax: 33-4-76-51-38-08. E-mail address: fabien.anthelme@grenoble.cemagref.fr (F. Anthelme). 0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-1127(00)00574-0