Tree distribution pattern and fate of juveniles in a lowland tropical rain forest – implications for regeneration and maintenance of species diversity

Abstract

Three analyses of species diversity in a lowland dipterocarp forest were conducted to examine whether the nature of forest community dynamics are determined by density-dependent recruitment and mortality of saplings with a data set obtained in a 50 ha plot in Pasoh Forest Reserve. The first analysis examined whether sapling density varied as a function of distance from the nearest conspecific adult. The second analysis assessed the relationship between the spatial distribution patterns of saplings and adult trees. A third analysis examined sapling recruitment and mortality based on data from 2 censuses, taken in 1985 and 1990. Four hundred forty-four species (each with more than 100 individuals) out of the total of 814 species recorded in the plot, were chosen for the analyses. Of these selected species, 56 species showed significant reduction in sapling densities close to the conspecific adults. Within this group, 11 species were in the emergent layer (29.0% of the total species in this layer), 17 were in the canopy layer (10.5%), 18 were in the understory layer (11.3%), and 10 were in treelet and shrub layer (11.8%). In contrast, the sapling densities of 53 species decreased with increasing distance from conspecific adults; 2 of these species were in the emergent layer (5.2% of the total species in this layer), 14 were in the canopy layer (8.6%), 21 were in the understory layer (13.2%), and 16 were in the treelet and shrub layer (18.8%). The saplings of 35 of the 444 total selected species were clumped, while adults were regularly or randomly distributed. Of the remaining species, in 183 species (41.2%), the distributions of both adults and saplings were clumped. Thus, these 2 analyses do not support the prediction that most of the species of lowland tropical forests fail to produce new adults in their vicinity and as a result of this, adult trees are more regularly distributed than their conspecific juveniles (Janzen 1970). In the third analysis, the recruitment of saplings of species in the emergent and canopy layers increased significantly and in proportion with mortality, suggesting that the dominant species suffer higher mortality than do less common species. This trend is not so apparent in the understory, and the treelet and shrub layers. The results imply that a dynamic equilibrium process, which prevents competitive exclusion and maintains space for minor species, may be active among the species in the upper layers (particularly the emergent layer); however, such a dynamic equilibrium condition is not due exclusively to the reduced recruitment of saplings near conspecific adults, and the dynamic equilibrium condition is not prevalent among the lower story species.