The Drivers of Community Assembly for Canopy Trees in the Brazilian Amazon

The Amazon rainforest is hyperdiverse, supporting as many as 16,000 tree species. However, the processes governing the spatial structure of plant community composition are poorly understood. This is especially true of large canopy trees which have proven difficult to study owing to the small spatial scales of the current network of forest plots. This is a key knowledge gap given that canopy trees are the primary seed dispersers, competitors and carbon storers. In this thesis, I use landscape scale, contiguous forest inventories from the logging sector to address this. I first assess the capacity of neutral models to adequately reconstruct the observed patterns of beta-diversity. I demonstrate that stochastic dispersal and environmental filtering processes superimpose to drive compositional turnover. While dispersal processes explain the majority of compositional turnover, environmental filtering can operate at small spatial scales to dictate community composition. I then assess compositional turnover under the lens of niche conservatism. Phylogenetic diversity metrics revealed that environmental variables play a greater role in driving community composition of canopy trees than was previously detectable and that environmental factors can operate at scales as low as 1 ha to influence community composition. Lastly, I examined the key drivers of species aggregation patterns and asked whether species functional traits could explain the degree of aggregation. Surprisingly, canopy trees exhibited similar aggregation patterns to those of juveniles with respect to environmental associations and dispersal limitation. Further, I found that species dispersal syndrome and seed mass controlled aggregation patterns. Put together, my findings show that both environmental variables and dispersal limitation are key drivers of the spatial structure of large tropical canopy tree community composition and that species identity and functional traits play an important role. As we enter the Anthropocene, tropical forests face increasing threats of logging and land-use change. My results reveal key community assembly processes which should be taken into consideration when planning sustainable forest management and conservation measures.