Consequences of Global change on the composition of boreal forest.

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Classical vegetation theory predicts that under a constant climate and site conditions determinist processes lead towards a unique steady state. However, several stable states have been reported for many ecosystems. The boreal forest is still a large relatively intact biome controlled by large disturbances such as fires and insect outbreaks. Both convergence towards a single steady state and multiple trajectories towards alternative stable steady states have been reported. Global change, either via climate warming or via human disturbance, may contribute to move a currently stable state into an alternative state, and consequently future forests may change dramatically following a single disturbance. Two species (aspen and black spruce), despite their very different impacts on ecosystem processes and dynamics, can share the same niche in a large part of the North American boreal forest. Factors controlling the historical dominance of one species over the other remains partially unknown, and a better understanding of past and current dynamics could help forecast future forest composition under the impact of global changes. My long term objective is to understand what factors control the distribution and dominance of these two species and understand the consequences of global change on the composition and dynamics of the boreal forest. During the next 5 years I propose to address this research objective by examining the following complementary questions: i) What was the historical proportion of mixedwood and black spruce dominance during the Holocene and how does it relate to changes in climate and fire regime? ii)What are the mechanisms favoring aspen and black spruce post-fire dominance? And iii)In the context of climate change what are the consequences on future successional trajectory of replacing black spruce by aspen?***These questions will be addressed by a series of research projects resulting in the training of 3 Ph.D, 2 M Sc., several undergraduated students and 1 post-doctoral fellow. These projects will imply training in paleoecology, field ecology, experimental design and modeling. This work will add to our basic knowledge on how forest composition is controlled by historical disturbances and how forest composition influences ecosystem processes and dynamics. It will also allow forecasting the impacts of global change on future forests and therefore could contribute to a more sustainable management strategy.**