Modern climate change is reshaping species distributions, particularly on slow shifting organisms such as trees. Forests composition is therefore expected to change in the coming decades, which will alter ecosystem functions ...
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Modern climate change is reshaping species distributions, particularly on slow shifting organisms such as trees. Forests composition is therefore expected to change in the coming decades, which will alter ecosystem functions and biodiversity, with negative ecological and societal consequences for the planet.Tree distribution depends on several demographic traits such as recruitment, growth and mortality that interact across large climatic gradients. Yet, mortality is rising in all forested biomes in the world. In Europe for instance, forest mortality increases towards the climatic trailing edge of the species ranges as a response to drought. These high mortality rates are usually related to a lack of recruitment, which may induce vegetation shifts, but also opening new opportunities for the establishment of exotic invasive species. As demographic trait responses to climate vary across and within species, understanding trait interactions along large climatic gradients is crucial to better predict the impact of climate change on forest productivity, composition and range-shift dynamics.In this work I analyzed tree mortality and recruitment patterns of twenty of the most common native species and two exotic species in European forests and their triggered drivers. To this aim, I used data of 2 million trees from 153 892 plots measured in the National Forest Inventories from France, Spain, Germany, Belgium (Wallonia), Sweden and Finland.In the first chapter, I analyzed tree mortality and showed that the highest mortality occurrence happens in the climatic trailing edge, driven by drought, whereas the intensity of mortality is triggered by competition, drought and high temperatures and was uniformly scattered across species ranges. In addition, the occurrence of mortality was the highest in the trailing edge of temperate species and the lowest in the leading edge for half of the Mediterranean species.In the second chapter I analyzed tree recruitment, showing that for most species, there are no differences in recruitment across species ranges. Recruitment was strongly limited by competition and often depended on age, or growth rate of the plot. Surprisingly, the role of drought in tree recruitment only was evident in interaction with tree competition.In the third chapter, I assessed the invasiveness of two exotic invasive species, Quercus rubra and Robinia pseudoacacia. My results showed that both species are able to recruit new individuals under all other species canopies, to become dominant at the expanse of many trees species and suggested that they are both expanding their ranges northwards and southwards, in part because they are relatively less sensitive to drought than the other species.All together, my results highlight that trees sensitivity to current climate change is trait-dependent and differs across species ranges. The southern part of the species ranges can be shaped by drought-induced mortality, while recruitment is much less affected by drought. This different sensitivity to climate of tree mortality and recruitment suggests that recruitment could counteract the negative effects of climate change to a certain extent and that forests might be more resilient than what was previously thought. Yet, the exotic species expansion is less affected by the surrounding environment than Mediterranean and temperate species and could benefit from climate warming. Hence, the potential help of recruitment for in-situ species range persistence, and the management strategies which could help forests to mitigate future climate change remains to be explored.
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