CANCELLED - Change in plant biodiversity along a latitudinal gradient: Insights from functional diversity

Background/Question/Methods

Macroecological patterns of species diversity have long ago interested ecologists. In particular, the variation in diversity along latitudinal gradients has been widely discussed, but the underlying mechanisms remain unclear. One reason is the lack of consideration for the community level which is however probably the most relevant level of organization to explain species coexistence. At the community level, the maintenance of species coexistence is primarily regulated by interspecific differences that can be assessed by the differences between species’ traits like traits related to resource capture and reproductive strategy. Macroecological focus on community-level trait diversity is still lacking, probably due to the lack of available trait data. Such data are now available thanks to worldwide initiatives. Here we ask whether the within-community trait diversity does vary in a predictable way along a latitudinal gradient and whether it provides insights into taxonomic diversity patterns. As part of the BIEN initiative that aims at gathering herbarium and plot data as well as species traits across the New World, we calculated the moments of the trait distribution within 600 tree communities distributed along a wide latitudinal gradient. We studied four plant traits: specific leaf area, plant height, seed mass and wood density.

Results/Conclusions

As expected, we found higher taxonomic diversity in the tropics. Trait diversity displayed a weak hump-shaped relationship along the latitudinal gradient and did not correlate with taxonomic diversity. However trait-by-trait analysis was of great interest to explain macroecological patterns of species coexistence. Seed mass distribution was uniform in the tropics while more peaky in temperate zones, suggesting a higher level of reproductive divergence in tropical areas. On the opposite, specific leaf area was more evenly-distributed in temperate zones, underlying a higher level of differentiation in terms of resource capture. Altogether this macroecological trait-based approach to community assembly highlights contrasting mechanisms that regulate the maintenance of species coexistence along latitudinal gradients and the important role of reproductive differentiation as a promoter of higher diversity in the tropics.