Comparison of sapling demography of four dipterocarp species with different seed-dispersal strategies

To determine the relative roles of seed dispersal in regeneration, we compared the spatial distribution and demography of four co-occurring dipterocarp species with different seed dispersal strategies: two long-distance seed dispersers with winged seeds (Dipterocarpus globosus and Shorea beccariana) and two short-distance dispersers with wingless seeds (D. tempehes and S. laxa). The spatial distribution and dynamics of saplings of the four species were investigated to test whether seed dispersal confers benefits with respect to either increasing the availability of safe sites or avoiding high mortality near conspecific adults. The median distance between a sapling and the nearest conspecific adult increased with sapling size in the long-distance dispersers. The saplings of short-distance dispersers were largely aggregated near conspecific adults, and the median distance did not change with sapling size. Light- and topography-dependent mortality and density/distance-dependent mortality were observed in some size classes of saplings of both long-distance dispersers. However, the mortality of short-distance dispersers did not show clear density dependence, partly because of the low mortality. We concluded that the functions of long-distance seed dispersal, i.e., escape or colonization, differ with size in D. globosus and S. beccariana, while the short-distance seed dispersers D. tempehes and S. laxa have other mechanisms to avoid mortality.