Neighborhood interactions and functional trait differences in a hyperdiverse Amazonian forest
The highly diverse tropical forests have inspired many coexistence theories, although consensus on the ecological mechanisms shaping these communities is still largely lacking. As multiple processes can produce similar patterns of community composition, it remains difficult to implicate one particular mechanism from occurrence data alone. A promising way towards a more mechanistic understanding of the processes shaping communities is to quantify the linkage between individual performance and the functional composition of its local neighborhood. Combining demographic data from a lowland rainforest 50 ha plot in Ecuador with eight functional traits and phylogeny for ca. 1100 tropical tree species, we use spatially-explicit neighborhood models to quantify the relative importance of functional similarity and phylogenetic relatedness in predicting the effects of competitive interactions between tropical tree species on growth rate. For each species, we estimate model parameters using maximum likelihood and compare alternate models using the Aikaike Information Criterion corrected for small sample size.
The importance of competitive interactions on growth rate varies across tropical tree species. Among the 430 most abundant species in the plot, we find that 80% of species are insensitive to neighborhood effects, while 20% of species are strongly affected by the functional similarity of neighbors. The species best predicted by the models including neighborhood interactions had higher leaf thickness and maximum tree size but lower wood specific gravity than other species. They also tended to be more abundant in the plot and span a broader size range. Exploring the relationship between maximum growth rate and functional traits across tropical tree species, we show that maximum growth rate strongly correlates with several traits including specific leaf area, leaf nitrogen content, maximum tree size and wood specific gravity. These results suggest that the relative influence of competitive interactions on growth vary with species temperament. Combining demographic processes and functional traits, our study provides a better understanding of the ecological mechanisms underlying species coexistence in the hyper diverse tropical forests.