A spatially explicit model of sapling growth in a tropical forest: does the identity of neighbours matter?

1 We quantified neighbourhood effects on sapling growth for 60 tree species in the 50-ha plot in Barro Colorado Island, Panama. Additionally, we tested whether target sapling growth responds to taxonomic or functional identity of neighbouring species by comparing four alternate models (that all neighbours have equivalent effects on the target; that conspecific and heterospecific neighbours have distinct effects; that heterospecific neighbours can be divided into confamilials and non-confamilials; and that they can be divided according to their response to light availability). 2 Over half of the species (34 out of 60) analysed were consistent with all neighbours having equivalent effects on the target. This may result from diffuse evolution allowing tolerance of a large number of neighbouring species or could be a statistical artefact of over-clumping species into large neighbour groups (e.g. heterospecific neighbours). 3 Other species supported models that differentiated between conspecific and heterospecific (n = 6) or between confamilial vs. non-confamilial (n = 5) neighbours and, in general, effects of neighbours were stronger if they were more closely related to the target. Where target species differentiated between neighbours from different light guilds (n = 15), effects were stronger if both belonged to the same guild (i.e. both gap requiring or both shade tolerant). 4 Despite the fact that the majority of species did not respond to the identity of neighbours, all differed in their response to the degree of crowding. Our results suggest that the response of target species to crowding, rather than individual species effects on targets, may be subject to selection. 5 Variation among species in response to crowding or to the identity of neighbouring species is likely to contribute to the maintenance of species diversity in tropical forests.