PhD in functional ecology and biogeography
Drivers of sugar maple regeneration along an elevational gradient
The main goal is to determine the biotic and abiotic factors controlling the the elevational distribution of sugar maple. The studied elevational gradient located in Parc naturel du Mont Mégantic (Québec) covers the ecotone between temperate and boreal forests. Hence, the abundance of sugar maple adult trees decreases with elevation. We observed an higher sugar maple regeneration at its upper elevational limit (and so at the edge of its distribution) than at the core of its distribution towards lower altitudes.
The increase of abundance of seedlings toward higher altitudes is expected in response to climate change. Indeed, new habitats become favorable for their establishment because of the increase of temperature. However, the detected low regeneration at lower altitudes at the core of current sugar maple distribution where adult trees are the most abundant is surprising.
Mont St Joseph in Parc national du Mont Mégantic (Québec)- Autumn 2014
Responses of tree species to global change: biogeographic and ecophysiological approaches
Tree distribution could be highly affected by climate change. Results of paleogeographic studies showed that tree distribution ranges have already shifted with past climate changes. These data are currently used to model the evolution of species distribution in response to global warming. However, the ecological context in which species have to cope with climate change is very different from the past one: the current increase of temperature occurs faster than previous warming episodes and the areas likely to be colonized are covered by various ecosystems (forests, agricultural surfaces, urban areas). So will tree species be able to cope with the current global change? Will they be able to migrate to find more favourable conditions or to survive to drier environmental conditions?
Firstly, the analysis of historical data (French National Forest Office and Spanish National Forest Inventories) allowed determining colonization and extirpation events, and quantifying migration rates of tree species populations situated at the edges or the core of their distribution range. We evidenced that Q. ilex has substantially colonized new areas at its northern margin during the last 130 years, confirming the model trends. However, the colonization rates of Q. ilex are much lower than the shift of its bioclimate predicted by bioclimatic models. Species located at their rear edge showed higher upward shifts than other species located at the core of their range. To conclude, our results showed that global change have already impacted tree distribution in spite of a time-lag between forest species migration responses and their bioclimate shift.
Water stress is the main factor explaining tree dieback when water is limited and so particularly at the warm limit of tree species distribution range. Therefore, we studied drought resistance and its mechanisms in angiosperm tree species. Our results showed that embolism threshold of 90% leads to irreversible damage and tree death by dehydration. This threshold is considerably higher than the observed in Conifers. The study of hydraulic functioning of co-occurring oaks showed that the survival of Q. roburcould be threatened in the context of increasing drought in the Atlantic forests because of its functioning at high levels of embolism. On the contrary, Q. ilex presented negligible levels of embolism in the same study area.
The migration rates form primordial empirical data that give us information about tree effective migration abilities. They could be integrated within vegetation distribution models as well as embolism thresholds leading to tree mortality.