High frequency of positive interspecific interactions revealed by individual species–area relationships for tree species in a tropical evergreen forest

Background: The spatial structure of tree species diversity may identify the relative importance of positive, negative and neutral interactions among species and the processes that maintain high species richness in tropical forests. Aims: We tested the hypothesis that common species accumulate higher species richness at small spatial scales than expected from a null model of complete spatial randomness (CSR) and that the strength of this signal decays when tested against a null model reflects plot-wide environmental heterogeneity. Methods: We computed individual species–area relationships (ISARs) to examine the accumulation of tree species richness with distance around 18 abundant species on a fully mapped 2-ha plot in a tropical evergreen forest in Vietnam. Results: Thirteen species displayed ISARs suggesting that they accumulated a higher than expected species richness of trees when tested against a null model assuming CSR, but this fell to eight species after accounting for non-random species distributions using a heterogeneous Poisson null model. Only the pioneer species Macaranga denticulata showed lower than expected species richness in local neighbourhoods when tested against the heterogeneous Poisson null model. Conclusions: Environmental heterogeneity contributes to the distribution of species diversity at small spatial scales and must be accounted for using an appropriate null model when analysing point pattern data. The accumulation of species richness in the local neighbourhoods of eight species may reflect facilitation, species herd protection, habitat heterogeneity or overlap in frugivore diets. Low species richness in local neighbourhoods surrounding M. denticulata stems reflect its early colonisation of canopy gaps.