Reproduction of dipterocarps during low intensity masting events in a Bornean rain forest

Question: The evolution of general flowering and mast fruiting of dipterocarps in tropical rain forest has been explained by different mechanisms. We studied whether the abundance of flowering conspecifics influences the recruitment success of tropical forest trees in Borneo.

Location: Sepilok Forest Reserve, Sabah, Malaysia.

Method: We examined the recruitment success of 17 species of Dipterocarpaceae in a 640-ha sample area over two flowering events in 2001-2002 using seed traps and quadrats. Seed predation experiments were used to investigate if post-dispersal seed survival was density-dependent.

Results: We found a negative relationship between the percentage of flowers abscised per individual and the number of flowering conspecifics and a positive relationship between the percent of flowering trees that produced viable seeds and the number of flowering conspecifics. However, we found no evidence of decreasing pre- and post-dispersal predation with increasing numbers of flowering conspecifics. High levels of flower abscission, and pre- and post-dispersal seed mortality, resulted in near-complete recruitment failure of most species, with only three species successfully recruiting in only one year. One of these, Parashorea tomentella, satiated seed predators over a large area, while Hopea beccariana and Shorea multiflora only recruited in small isolated clumps. Seed predation experiments suggest that post-dispersal seed survival was positively density-dependent in the short-term.

Conclusions: Increased density of flowering conspecifics may contribute to increased likelihood of successful cross-pollination during low intensity flowering events and, in some cases, to enhanced probability of short-term seed and seedling survival. Both processes may contribute to the evolution of mast fruiting and general flowering in Southeast Asian lowland dipterocarp forests.