Providing more informative projections of climate change impact on plant distribution in a mountain environment

Summary 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 systems may experience unprecedented rates of warming during the next century, two or three times higher than that records of the 20th century. In this context, species distribution models (SDM) have become important tools for rapid assessment of the impact of accelerated land use and climate change on the distribution plant species. In my study, I developed and tested new predictor variables for species distribution models (SDM), specific to current and future geographic projections of plant species in a mountain system, using the Western Swiss Alps as model region. Since meso- and micro-topography are relevant to explain geographic patterns of plant species in mountain environments, I assessed the effect of scale on predictor variables and geographic projections of SDM. I also developed a methodological framework of space-for-time evaluation to test the robustness of SDM when projected in a future changing climate. Finally, I used a cellular automaton to run dynamic simulations of plant migration under climate change in a mountain landscape, including realistic distance of seed dispersal. Results of future projections for the 21st century were also discussed in perspective of vegetation changes monitored during the 20th century. Overall, I showed in this study that, based on the most severe A1 climate change scenario and realistic dispersal 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 61 scenario, only 4.6% of species are predicted to become extinct. However, even with B1, 54% (153 species) may still loose more than 80% of their initial surface. Results of monitoring of past vegetation changes suggested that plant species can react quickly to the warmer conditions as far as competition is low However, in subalpine grasslands, competition of already present species is probably important and limit establishment of newly arrived species. Results from future simulations also showed that heavy extinctions of alpine plants may start already in 2040, but the latest in 2080. My study 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. Resume La forme conique des montagnes ainsi que la diminution de surface dans les hautes altitudes sont reconnues pour exposer plus sensiblement les ecosystemes de montagne au rechauffement global. En outre, les systemes de montagne seront sans doute soumis durant le 21eme siecle a un rechauffement deux a trois fois plus rapide que celui mesure durant le 20eme siecle. Dans ce contexte, les modeles predictifs de distribution geographique de la vegetation se sont imposes comme des outils puissants pour de rapides evaluations de l'impact des changements climatiques et de la transformation du paysage par l'homme sur la vegetation. Dans mon etude, j'ai developpe de nouvelles variables predictives pour les modeles de distribution, specifiques a la projection geographique presente et future des plantes dans un systeme de montagne, en utilisant les Prealpes vaudoises comme zone d'echantillonnage. La meso- et la microtopographie etant particulierement adaptees pour expliquer les patrons de distribution geographique des plantes dans un environnement montagneux, j'ai teste les effets d'echelle sur les variables predictives et sur les projections des modeles de distribution. J'ai aussi developpe un cadre methodologique pour tester la robustesse potentielle des modeles lors de projections pour le futur. Finalement, j'ai utilise un automate cellulaire pour simuler de maniere dynamique la migration future des plantes dans le paysage et dans quatre scenarios de changement climatique pour le 21eme siecle. J'ai integre dans ces simulations des mecanismes et des distances plus realistes de dispersion de graines. J'ai pu montrer, avec les simulations les plus realistes, que pres du tiers des 284 especes considerees (28.5%) pourraient etre menacees d'extinction en 2100 dans le cas du plus severe scenario de changement climatique A1. Pour le moins severe des scenarios B1, seulement 4.6% des especes sont menacees d'extinctions, mais 54% (153 especes) risquent de perdre plus 80% de leur habitat initial. Les resultats de monitoring des changements de vegetation dans le passe montrent que les plantes peuvent reagir rapidement au rechauffement climatique si la competition est faible. Dans les prairies subalpines, les especes deja presentes limitent certainement l'arrivee de nouvelles especes par effet de competition. Les resultats de simulation pour le futur predisent le debut d'extinctions massives dans les Prealpes a partir de 2040, au plus tard en 2080. Mon travail demontre aussi l'importance d'etudes regionales a echelle fine pour evaluer l'impact des changements climatiques sur la vegetation, en integrant des variables plus directes. En effet, les etudes a echelle continentale ne tiennent pas compte des micro-refuges, des extinctions locales ni des pertes de connectivite entre populations locales. Malgre cela, la migration des plantes de basses altitudes reste difficile a predire a l'echelle locale sans modelisation plus globale.