Summary1 Spatial patterns of two codominant dioecious tree species, Podocarpus nagi and Neolitsea aciculata, were analysed at Mt. Mikasa, Nara City, Japan. Podocarpus nagi has a higher shade tolerance but a narrower seed dispersal range than N. aciculata. We examined the eects of dioecy on regeneration and coexistence of the two species. 2 Seeds of P. nagi are dispersed by gravity around female trees. Young plants of P. nagi were clumped and showed signi®cant attraction to large female trees and signi®cant repulsion from large male trees. Dioecy therefore aected the spatial heterogeneity of plant density in the P. nagi population. 3 Seeds of N. aciculata are widely dispersed by birds, and young plants of N. aciculata therefore showed no signi®cant attraction to female trees. This wide dispersal of seeds moderated the eects of dioecy on the spatial pattern in N. aciculata. 4 Large N. aciculata trees were clumped and showed signi®cant attraction to large male P. nagi trees, suggesting that growth of N. aciculata is facilitated where young P. nagi plants are uncommon and competition is therefore less intense. 5 One eect of dioecy may be to produce a population structure for P. nagi that promotes its coexistence with N. aciculata.

The effects of El Niño-induced droughts on dipterocarp forests must be quantified to evaluate the implications of future global climatic changes for the tropical forests of Southeast Asia. We studied the mortality of trees ! 1 cm in diameter in a lowland dipterocarp forest in Borneo before, during, and after the 1997/1998 El Niño drought. The annual mortality rates were 1.30, 1.75, and 1.66 percent/yr for the pre-drought, drought, and post-drought periods, respectively. The effect of drought was tree size-dependent being greater for larger trees. Modified logistic regression analysis revealed a significant interaction effect between species' habitat association and edaphic condition on mortality rates in all periods. For species associated with wet habitat, drought effect was greater in dry conditions than in wet conditions, in both the drought and post-drought periods. The mortality rates of dry-habitat species were less affected by the drought both in dry and wet conditions. A similar pattern was also found in common Dipterocarpaceae species; mortality rates increased more in species associated with wet-habitat in the drought and post-drought periods. Species and families with higher mortality in the pre-drought period tended to experience greater mortality increases during the drought and post-drought periods. These results suggest that changes in drought regimes alter the species composition and spatial distribution of dipterocarp forests.

We analyzed sex ratio, growth rates, and spacing among individuals of Podocarpus nagi, a dioecious tree, on Mt. Mikasa, Nara City, Japan. The sex ratio of reproductive trees ‡ 5 cm in stem diameter at breast height (dbh, 130 cm above ground level) was significantly male-biased. The sex ratio was male-biased in the < 20 cm and ‡ 50 cm size classes, while it did not depart from 1:1 in the 20 £ dbh < 50 cm class. Growth rate varied with tree size in males but not in females. The precocity and vigor of males suggests that differences in reproductive costs between sexes induce the biased sex ratio. Random labeling tests on the positions of reproductive trees showed that in the < 30 cm class, males and females were distributed randomly and independently from each other. In the ‡ 30 cm class, males were significantly clumped, whereas females were randomly distributed. Males and females showed significant repulsion, i.e., a spatial segregation of sexes. Both intra-and intersexual effects on the growth rate of crowding by neighbors were significant for females, but not for males. Maximum competitive interference was observed at a distance of 5 m, which corresponded approximately to the radius of clumps of large males and to the significant repulsive distance between large males and females. These results suggest that sexual differences in sensitivity to local crowding are related to the formation of gender-dependent spatial patterns. Formation of female-repulsive male clumps and a male-biased sex ratio may intensify the decreased probability of regeneration near males, as suggested by the limited seed-dispersal range of this species, thereby promoting coexistence with other species.

Fine-scale spatial genetic structure is increasingly recognized as an important factor in the studies of tropical forest trees as it influences genetic diversity of local populations. The biologic mechanisms that generate fine-scale spatial genetic structure are not fully understood. We studied fine-scale spatial genetic structure in ten coexisting dipterocarp tree species in a Bornean rain forest using microsatellite markers. Six of the ten species showed statistically significant fine-scale spatial genetic structure. Fine-scale spatial genetic structure was stronger at smaller spatial scales ( 100 m) than at larger spatial scales (> 100 m) for each species. Multiple regression analysis suggested that seed dispersal distance was important at the smaller spatial scale. At the larger scale (> 100 m) and over the entire sample range (0-1000 m), pollinators and spatial distribution of adult trees were more important determinants of fine-scale spatial genetic structure. Fine-scale spatial genetic structure was stronger in species pollinated by less mobile small beetles than in species pollinated by the more mobile giant honeybee (Apis dorsata). It was also stronger in species where adult tree distributions were more clumped. The hypothesized mechanisms underlying the negative correlation between clump size and fine-scale spatial genetic structure were a large overlap among seed shadows and genetic drift within clumped species.

The lowland tropical forests of Southeast Asia are dominated by a single family of canopy and emergent trees, the Dipterocarpaceae. The seeds of dipterocarps are gravity or gyration dispersed. Short distance and limited seed dispersal via these mechanisms result in the aggregation of related individuals and strong fine-scale spatial genetic structure (FSGS). In logged and fragmented forests, where gene flow may be disrupted, tree species with strong FSGS are predicted to exhibit increased inbreeding,