The Amazon rain forest is the most diverse ecosystem on Earth, harbouring more than ten thousand tree species. In this project, I used ecological and molecular information to explore how ecological factors and historical events have determined the species distributions and population genetic structure of tree species and the phylogenetic diversity of tree communities in the Amazon rain forest.
Chapter 2 indicates that seasonally dry vegetation in northern South America represents a barrier to migration for Ficus insipida (Moraceae) and other wet-adapted Amazonian tree species as they have different plastid haplotypes restricted to Mesoamerica and Amazonia. Conversely, the ability of some pioneer species to survive seasonal drought may explain the weakly differentiated phylogeographic structure within these species, with some haplotypes occurring on both sides of this barrier. Chapter 3 explores whether patterns of population genetic structure in five widespread western Amazonian tree species are consistent with historical explanations. My results show that the genetic patterns among species are not entirely congruent suggesting that tropical rain forest species respond differently to long-term geological and climatic changes. Despite this, some tentative generalisations emerge, notably high genetic diversity and a strong geographic structure for plastid sequences suggesting long-term population stability across western Amazonia, and recent population expansions in the south-western Amazon. Chapter 4 uses 283 floristic inventories from the RAINFOR plot network to explore patterns of phylogenetic diversity across Amazonia. This study reveals that the species-rich communities of central Amazonia are dominated by phylogenetic close relatives compared to the equally species-rich communities of the north-west that tend to contain more distantly related species. Across Amazonia, an east-west gradient of the abundance of early divergent angiosperm clades was found, with the greatest percentage of tree species of Magnoliids and Monocots in the west. As these early diverging clades are also characteristic of pre-montane habitats, these results suggest that migration events from cooler environments at different geological times has played an important role in the assemblage of the most phylogenetically diverse communities in Amazonia.
The findings from these three chapters corroborate the notion that both ecological factors and historical events have been important in determining species distributions and the phylogenetic diversity of tropical tree communities in Amazonia. Regional differences in genetic structure among populations, and phylogenetic diversity among communities, should both be taken into account in forest conservation planning and management
