Résumé
In recent decades, many studies have documented local declines of orchid species due to environmental changes. However, few data are available about the changes of orchid species distributions at large spatial and temporal scales. We aimed to characterize the recent dynamics of orchid distributions in Western Europe, in regard to (i) climate and land use changes and (ii) species’ ecology (i.e. biogeographic and open/closed habitat preferences). We analyzed data from a large-scale survey of 134 orchid species over 20 years in France, Belgium and Luxembourg. We designed null models to assess the nature and significance of recorded changes within each administrative unit and for each orchid species. We found sharp decline of most orchids in the northern part of the study area (i.e. northern France, Belgium and Luxembourg) resulting in an overall shift to the Mediterranean area in the south. We found more frequent decline of heliophilous species than of sciaphilous species, possibly due to the open habitat loss in relation to strong urbanization and changes in agricultural practices in the northern part of the study area. No significant differences in appearance or disappearance were detected between Mediterranean and Euro-Siberian species, thus suggesting that climate change is not a primary constraint of distribution changes over the period considered. Conversely, land use changes seem to affect in a higher extent the distribution of western European orchids. Observed distribution reduction of orchid species in the southern Mediterranean area will exacerbate the already existing contrast in species richness and related conservation priorities between the north and south of the study area