Distance and density dependence in two native Bornean dipterocarp species

The Janzen-Connell hypothesis proposes that density and distance-dependent mortality generated by specialist natural enemies prevent competitive dominance. Much literature on Janzen-Connell mechanisms comes from the neotropics, and evidence of the role of distance and density-dependence is still relatively sparse. We tested the predictions of the Janzen-Connell hypothesis in a South-East Asian system dominated by mast fruiting species. We hypothesized that seedling survival would decrease with distance and density, seedling growth would increase, and herbivory would decrease, according to the predictions of the Janzen-Connell hypothesis. Experiments were conducted to determine the strength of the Janzen-Connell mechanism by manipulating the density and identity of tree species as a function of the distance from parent trees. Survival of conspecific seedlings was reduced near adult trees of one species, but not another. High densities of seedlings decreased the growth of conspecific seedlings of both species. In both species, herbivory rates decreased with distance in low-density areas. This study indicates that dipterocarp species experienced weak Janzen-Connell effects of distance and density dependence at the growth stage studied. Future studies in this system might focus on earlier life-history stages such as seeds and small seedlings, as well as studying mortality during mast-seeding events.

Automated summary

The Janzen-Connell hypothesis suggests that specialist natural enemies restrict competitive dominance through density and distance-dependent mortality. However, empirical evidence for the role of distance and density-dependence in the hypothesis is still limited. This study conducted experiments in a South-East Asian system dominated by mast fruiting species and found that while survival of conspecific seedlings was reduced near certain adult trees, high densities of seedlings inhibited the growth of conspecific seedlings in both species. Additionally, herbivory rates decreased with distance in low-density areas. These findings suggest that the Janzen-Connell effects of distance and density dependence were relatively weak in this system during the growth stage studied.