Neighbour-regulated mortality: the influence of positive and negative density dependence on tree populations in species-rich tropical forests

Density-dependent mortality has long been posited as a possible mechanism for the regulation of tropical forest tree density. Despite numerous experimental and phenomenological investigations, the extent to which such mechanisms operate in tropical forests remains unresolved because the demographical signature of density dependence has rarely been found in extensive investigations of established trees. This study used an individual-based demographical approach to investigate the role of conspecific and heterospecific neighbourhood crowding on tree mortality in a Panamanian and a Malayan tropical forest. More than 80% of the species investigated at each site were found to exhibit density-dependent mortality. Furthermore, most of these species showed patterns of mortality consistent with the Janzen-Connell hypothesis and the rarely explored hypothesis of species herd protection. This study presents some of the first evidence of species herd protection operating in tree communities.