Tropical forests harbour remarkable density and diversity of woody plants, but how different processes interact to enable so many species to co-exist despite intense competition for resources is yet unresolved.
Traditionally, tropical diversity has been studied by establishing field inventory plots, but this is difficult and labour-intensive task, which limits the area that can be surveyed. In our latest paper, we use remote observation approaches to map how plant diversity changes over an entire landscape where three distinct forest types have previously been identified. We find that the species composition is spatially autocorrelated – nearby forest patches support similar plant communities while distant patches are more different. This spatial autocorrelation in community composition spans over kilometres – much farther than what is recorded from field surveys.
When the three forest types were examined separately, we detected different pattens of spatial autocorrelation in each of them. One process that may explain those different pattens is environmental niche filtering – differences in the topography and soil composition of the soil types may select for different species, creating the patterns we observe. To test if this is the case, we combined the landscape diversity maps with a fine scale reconstruction of the topography of the landscape and found that environmental niches have varying contribution to the diversity of each soil type.
However, differences in the underlying environment were insufficient to explain the high levels of change in diversity from one forest patch to the next. Another process that may structure the pattens we observed is limited seed dispersal: distant patches may be different not because their environments are distinct, but because trees are unable to disperse their seeds far away. We created a set of models that were parametrised to operate around environmental constraints and we found that once the contribution of environmental niches is accounted for, the residual variation in spatial autocorrelation can be reconstructed as dispersal limitation over the entire landscape.
Previous studies have been unable to partition the contribution of environmental niches and limited dispersal as they only looked at the small spatial scales surveyed on the ground. By considering larger spatial scale, we were able to show that niche processes structure tropical diversity over a range of scales while limited dispersal acts locally to enhance species richness in tropical landscapes.