Linking trait similarity to interspecific spatial associations in a moist tropical forest

Aim  Community assembly theory predicts that niche differentiation promotes the spatial clustering of functionally dissimilar species, whereas habitat filtering has the converse effect. We used these predictions to assess the relative effects of habitat filtering and niche differentiation on recruit community assembly over spatial (5- and 30-m neighbourhoods) and temporal (20-yr) scales in the Forest Dynamics Plot at Barro Colorado Island.  Location  Barro Colorado Island, Panama.  Methods  We integrated data on the spatial patterns of ≥1cm DBH (diameter at 1.3m above ground) recruits with data on seven functional traits for 64 species. First, we quantified the interspecific association patterns of all species pairs i and j using the K-function Kij(r) and the nearest-neighbour distribution function Dij(r). Second, for those pairs with significant spatial associations, we calculated an index of interspecific spatial association using the results of these two summary statistics. Finally, we examined the relationship between interspecific spatial association and trait similarity using simple and partial Mantel tests.  Results  In all censuses, almost one-half of species pairs had no spatial associations, but for pairs that were significantly spatially associated, positive relationships between trait similarity and spatial association occurred in 5-m and 30-m neighbourhoods, whereas significant negative relationships only appeared in 5-m neighbourhoods. This suggests that habitat filtering was more important than niche differentiation in assembling recruit communities at 5- and 30-m scales. Habitat filtering mainly acted upon traits related to topographic habitat preferences and dispersal mode, whereas spatial association was inversely related to similarity in terms of wood specific gravity and shade tolerance.  Conclusions  Our findings suggest that both stochastic and deterministic processes operate in species-rich ecological communities, but the role of habitat filtering and niche differentiation as determinants of community assembly vary over spatial and temporal scales. Species co-occurrence was driven by habitat filtering at small and large scales, but also by a combination of niche differentiation and weaker-competitor exclusion effects at small scales. Temporal variations in the importance of habitat filtering and niche differentiation could be related to the occurrence of disturbances such as tree falls. Our results emphasize the role of trait-based processes in plant community assembly