Due to their conic shape and the reduction of area with increasing elevation, mountain ecosystems were early identified as potentially very sensitive to global warming. Moreover, mountain regions may experience unprecedented rates of warming during the next century, two or three times higher than records of the 20^th century. In this context, alpine floras are expected to be particularly threatened. Species distribution models (SDM) have become essential tools to predict climate threat on alpine plants. Yet, despite great progress from its origin, SDM still required improvement. Using the Western Swiss Alps as model region, we first developed and tested new predictors for SDM to improve current and future geographic projections of plant species in a mountain system. Since meso- and micro-topography are relevant to explain geographic patterns of plant species in alpine environments, we assessed the effect of scale on predictors and geographic projections of SDM. Last, we developed and used a dispersal model, based on a cellular automaton, to run dynamic simulations of plant migration under climate change in a mountain landscape. These simulations included realistic seed dispersal distances. All our results on predicted future distributions are also discussed in regard of vegetation changes monitored during the 20^th century.

Results/Conclusions

Overall, we show that, based on the most severe A1FI climate change scenario and realistic simulations of plant dispersal, species extinctions in the Western Swiss Alps could affect nearly one third (28.5%) of the 284 species modeled by 2100. With the less severe B1 scenario, only 4.6% of species are predicted to become extinct. However, even with B1, 54% (153 species) remain at risk of loosing more than 80% of their initial surface. Results from future simulations also show that heavy extinctions of alpine plants may start as early as 2040, and at the latest in 2080.Results of monitoring past vegetation changes suggests that plant species can react quickly to warmer conditions provided that inter-specific competition is low. However, in subalpine grasslands, inter-specific competition is probably important and limits establishment of incoming species. We also highlighted the importance of fine scale and regional assessments of climate change impact on mountain vegetation, using more direct predictor variables. Indeed, predictions at the continental scale may fail to predict local refugees or local extinctions, as well as loss of connectivity between local populations. On the other hand, migrations of low-elevation species to higher altitude may be difficult to predict at the local scale.