The Janzen-Connell hypothesis is a leading explanation for the maintenance of tropical plant diversity. It postulates that specialized natural enemies maintain plant diversity by restricting recruitment at high densities near maternal trees. The persistence of such ecological processes is critical for tropical forest recovery in the wake of pervasive disturbances such as selective logging. In the dipterocarp-dominated rainforests of Southeast Asia, plant recruitment occurs primarily during episodic, community-wide, mast fruiting events. The most plausible explanation for mast fruiting is predator satiation. It has been hypothesized that predator satiation may override negative density-dependence in Southeast Asian rainforests since mast fruiting evolved to reduce mortalities at peak seed densities. However, it is unknown how logging would affect plant recruitment during a mast-fruiting year. We tested the effects of logging on density-dependent seed predation and seedling recruitment of an endemic and endangered tree Dryobalanops lanceolata (Dipterocarpaceae) during a recent mast-fruiting event in Sabah, Malaysian Borneo.
Results/Conclusions
We found that seed production in old growth forest was more than twice that in logged forest. Predator satiation prevailed over negative density-dependence old growth forest. The low density of seeds observed in logged forest was conducive for negative density-dependence to operate. Yet, we found that seedling survival increased with increasing plot-level seed density in logged forest, defying the density prediction of the Janzen-Connell hypothesis. Logging thus disrupted density-dependent seed predation. However, seed survival declined with experimental addition of seeds at densities higher than natural baselines, as expected under the Janzen-Connell hypothesis. This result strongly suggests that dipterocarp populations in logged forests are seed limited. Seedling survival in logged forest was similar to that in old growth. However, 93% of surviving seedlings in logged forest germinated at locations with canopy cover similar to that in old growth. Establishment limitation thus operated simultaneously with seed limitation to influence seedling recruitment. Stochastic escape from a nomadic large mammalian seed predator further facilitated localized survival in logged forest. Invertebrates and fungal pathogens were together the primary drivers of negative density-dependence in old growth. However, their contribution to seed predation was significantly lower in logged forest, while that of rodents was significantly higher. Our results suggest an anthropogenic shift in the operation of a critical ecological process in tropical forests. If generalizable to the wider plant community, our results have serious implications for the viability and recovery of logged forests in the biodiversity hotspots of Southeast Asia.