Tropical forests are diverse, with hundreds of tree species commonly co-existing within just a few hectares. Understanding how such diversity evolved and can be maintained are questions that have occupied evolutionary ecologists for decades. This talk presents findings from a study exploring the link between demography and phylogeny to assess whether divergence of tropical forest trees was accompanied by partitioning of demographic rates.
We fit novel growth and survival models to 132 tree species from a 50 ha permanent forest plot located on Barro Colorado Island, Panama. Our survival model captures the changing probability of survival through ontogeny while preventing estimates of survival in large trees being influenced by the skewed size distributions in the majority of species. We model growth with multiple distributions to capture the intraspecific variation in growth rates corresponding to asymmetric access to resources.
Using a highly-resolved community phylogeny based on DNA barcodes we tested whether demographic rates were phylogenetically conserved. We explored whether life-history trade-offs (e.g. between growth and survival rates) persisted once phylogenetic relationships were accounted for. Finally, we explored whether commonly measured functional traits mapped to the `demographic modes' found by clustering species based on a PCA of growth and survival parameters.
We found no phylogenetic signal in demographic rates, i.e. closely related species were not more similar than distantly related species. This was true when we tested parameters from vital rates models separately (although size-dependent juvenile survival was an exception) and when we tested life-history strategies using axes from a PCA. Phylogenetic independent contrasts revealed that observed life-history trade-offs, specifically the negative relationship between growth and survival, were significant even after shared evolutionary history was accounted for. Finally, we found no relationship between commonly measured functional traits (wood specific gravity, leaf mass area, maximum height and seed mass) and either phylogeny or demographic rates.
Our results suggest that the divergences leading to the current levels of biodiversity in tropical forest trees were accompanied by diversification of demographic rates, to the extent that closely related species can occupy different `demographic niches’. However, despite the diversity in demographic rates, trade-offs between growth and survival remain conserved across species illustrating that constraints on resource acquisition and allocation are common across species. The lack of relationship between functional traits and demographic rates suggests we need to re-evaluate whether these traits are in fact good proxies for the evolutionary strategies to which they are often assigned.