Towards an understanding of tree diversity in Amazonian forests

Abstract

Amazonian forests harbor the highest biodiversity of all terrestrial ecosystems on Earth. The origin of this extraordinary biodiversity and its current distribution are recently becoming better understood. Still, our knowledge of the contribution of processes operating at different temporal and spatial scales to the tree diversity in Amazonia remains surprisingly limited. This thesis aims at deepening our understanding of tree diversity in Amazonian forests and the ecological mechanisms shaping its current pattern at a local and regional scale. By analyzing tree inventory plots established across Amazonia, we found that areas with high regional diversity coincide with areas of palaeo-climatic stability and long-term high ecosystem dynamics. As these two factors affect speciation and extinction, and shifts in species distribution, we propose that they are important drivers of the current regional tree diversity. Variation in tree diversity at the local scale is enormous but poorly explained by local disturbance. We suggest that the power of this explanation is low because local tree diversity is subject to various local processes, which are largely variable from one location to another. We investigated, in further detail, tree communities of white-sand and terra-firme forests, which have a relatively independent evolutionary history. We analyzed tree inventory plots established in white-sand and terra-firme forests in the upper Rio Negro. We found that tree communities of white-sand forests show a higher floristic similarity and lower diversity than those of terra-firme forests. Furthermore, we tested the hypothesis that a trade-off between seedling growth and herbivore defense drives habitat association. We found no evidence that this trade-off drives habitat association in these forests in the upper Rio Negro. This finding may be explained by the extreme low nutrient availability of the soils in this region, which may cause seedling mortality in their non-typical habitat and restrict seedling growth. Finally, we characterize the tree communities of white-sand and terra-firme forests in three Amazonian regions. We assessed how regional tree communities regulate the variation in local tree communities of white-sand and terra-firme forests. Furthermore, we assessed if habitat association is maintained across lower taxonomic levels. We found that local tree communities of white-sand forests in central Amazonia are more diverse than those occurring in the large white-sand areas of the Guianas and the upper Rio Negro. We found that most families and genera, which are significantly associated with either forest type, have the majority of their species associated with that particular forest type. This finding suggests that a large number of species have retained requirements for their typical habitat during speciation events. Taken as a whole, our findings indicate that tree diversity can only be understood if both long-term evolutionary and current ecological processes are taken into account. At long-term, the size of forest areas in Amazonia is expected to decline in response to a predicted drier climate. At short-term, ongoing deforestation reduces the size of forest areas. As deforestation occurs at such rapid rates, long-term species adaptations to new environmental conditions may be of minor importance for the future of Amazonian forests.