No abstract

Predicting species' responses to the combined effects of habitat and climate changes has become a major challenge in ecology and conservation biology. However, the effects of climatic and habitat gradients on species distributions have generally been considered separately. Here, we explore the relationships between the habitat and thermal dimensions of the ecological niche in European common birds. Using data from the French Breeding Bird Survey, a large-scale bird monitoring program, we correlated the habitat and thermal positions and breadths of 74 bird species, controlling for life history traits and phylogeny. We found that cold climate species tend to have niche positions in closed habitats, as expected by the conjunction of the biogeographic history of birds' habitats, and their current continent-scale gradients. We also report a positive correlation between thermal and habitat niche breadths, a pattern consistent with macroecological predictions concerning the processes shaping species' distributions. Our results suggest that the relationships between the climatic and habitat components of the niche have to be taken into account to understand and predict changes in species' distributions.

Altitudinal shifts in distribution were investigated in forest breeding birds, along two elevational gradients in the French northern and southern Alps, from counts repeated at exactly the same locations in the 1970s and the 2000s. Significant shifts were reported for eight of 24 species in the northern alpine site (five downwards, three upwards) and for two of the 17 species in the southern site (one downwards, one upwards). Apart from the Crested Tit Parus cristatus , which shifted significantly downwards at both sites, altitudinal shifts were not significantly correlated between sites. Bird communities did not shift their distribution upwards despite a 2.3 ° C increase in spring temperatures in the two study areas. These results suggest that bird distributions by altitude have not yet been affected by climatic warming, and that most specific elevational shifts are probably related to sitespecific factors.

-To date, very few studies have assessed the impact of summer droughts on forest biodiversity and ecosystem functioning. Decreased ecosystem productivity and increased mortality are general consequences of drought on biodiversity. Competitive species, species adapted to cold and wet conditions as well as species with low reproduction rates and/or limited mobility seem the more affected. However, species-specific effects are regulated by mechanisms allowing for resistance to drought. The short-term consequences of drought on biodiversity depend on species abilities to resist, and to recover after, drought, and on competitive interactions between species. Although the abundance of many species generally decreases during drought, some taxa may increase in number during drought or shortly after. The effects of recurrent droughts must be evaluated in the wider context of global climate and habitat change. Considering the predicted increase in drought frequency and intensity, interdisciplinary research initiatives on this issue are needed urgently. drought impact / biodiversity / forests / ecosystem functioning / global change Résumé -Impact de la sécheresse estivale sur la biodiversité forestière : que savons-nous ? On connaît mal les conséquences des sécheresses sur la biodiversité et le fonctionnement des écosystèmes forestiers. La productivité des écosystèmes semble diminuer et s'accompagne d'une mortalité accrue. Les espèces compétitives, liées à des habitats frais ou humides, avec un faible taux de reproduction et/ou une mobilité réduite semblent les plus sensibles à la sécheresse. La réponse spécifique dépend également des mécanismes de résistance à la sécheresse. Les conséquences à court terme dépendent de la capacité des espèces à résister et à se rétablir de la sécheresse, et de leurs interactions. Un nombre conséquent de taxons peuvent être plus abondants en période de sécheresse ou peu de temps après. Les effets de sécheresses récurrentes sont à replacer dans le cadre du changement climatique global et des modifications d'habitats. Sachant que la fréquence et l'intensité des sécheresses devraient croître dans l'avenir, il est urgent de lancer des recherches interdisciplinaires sur le sujet.sécheresse / biodiversité / forêts / fonctionnement de l'écosystème / changement global