Mickaël Chauvet, Georges Kunstler, Jacques Roy, Xavier Morin
Most forests in Central Europe are composed of many tree species. How do so many species co-exist in the long-term? Using a simplified sequential view of species assembly, the abiotic conditions first select a set of species that can colonize a site, depending on their tolerances to local environmental constraints. Then, competition for light between established trees determines species that can coexist locally. In forest communities, small-scale disturbances like deaths of large trees create gaps in the canopy. Such gaps locally create new light conditions, and some tree species called pioneers can efficiently colonize these gaps immediately after their formation because of their ability to grow quickly in full light. Then, as the forest becomes darker because trees grow, some other tree species called late-successional species slowly colonize the gaps because they are shade-tolerant and are thus better competitors than pioneers. Forest communities are thus dynamic systems experiencing gaps over time. At the whole forest scale, however, it is not clear whether these gap dynamics lead to the coexistence of species with different characteristics, i.e. a mosaic of patches of various ages promoting the coexistence of both pioneers and late-successional species, or whether late-successional species mostly dominate forest communities because they are the best competitors on the long-term.
Here we used a model simulating gap dynamics in Central European forests to explore the relative importance of abiotic conditions and competition in determining species diversity, and also to test how gap dynamics drive tree species coexistence.
We found that abiotic conditions play an important role in structuring tree communities, especially in harsh conditions where it strongly reduces the number of species that can potentially establish in a particular site. Competition plays a crucial role in forest community structure both in favourable and harsh conditions, although its effects were more pronounced in favourable sites. Finally, despite gap dynamics, forest communities are mostly dominated by late-successional species that exclude pioneers when both type of species compete for light in the long-term. Finally, our model-based approach offers an alternative way to study the processes leading to tree species coexistence in forests, and complements empirical and experimental approaches.
Image caption: Image provided by authors.
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