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Seed dispersal, spatial distribution and maintenance of tropical tree diversity in a fragmented forest Z.X. Hu1 and Edward Webb2 Department of Biological Sciences, The National University of Singapore Blk S2, 14 Science Dr 4, Singapore 117543. ABSTRACT Spatial patterns of seed dispersal regulated by its limitations are crucial in sustaining diversity in a tropical forest landscape. Using data from a fully mapped 2ha primary coastal hill dipterocarp forest plot in Bukit Timah, the abundances for different trees species were investigated and their dispersion patterns were evaluated based on dispersal morphologies. The generalized hypothesis that species dispersed by biotic mechanisms are thinly spaced and evenly distributed did not hold. Overall they are significantly less clustered at small spatial scales(<30m) compared to abioticdependent species. This relationship remains true for some of the most abundant trees across different life stages. Interrupted seed dispersal capability can be disastrous for the integrity of the floristic composition over the long term and this study attempts to review the predicament facing trees in this fauna-depleted reserve. INTRODUCTION Seed dispersal is essential for sessile autotrophs viability as it allows diaspores to escape high mortality conditions near the parents, to maintain genetic diversity and metapopulation persistence. However there appears to be a dearth of studies on how different seed dispersal modes affect the spatial pattern of tropical trees in Bukit Timah and whether fragmentation is accelerating species extinction. Many theoretical studies had nevertheless suggested that animal-dispersed species are spatially less aggregated than mechanical-dispersed species but there are a few exceptions. Therefore I propose the hypothesis that seed dispersal has a pronounced effect on the spatial distribution of trees. MATERIALS AND METHODS The study was conducted in the Bukit Timah Nature Reserve and 103 out of a repertoire of 342 species in the CTFS plot was selected for spatial pattern analysis. Dispersal mode was assigned according to fruit morphologies and seed traits based on field survey and other literature sources. Using spatial analysis software(SpPack), the relative neighborhood density formulated by Condit et al.(2000) was generated for the examined species. In addition, spatial patterns at different growth stages and local statistics(Getis & Franklin 1987) were also inspected for some species to investigate their influence on seed dispersal. 1. Student 2. Supervisor RESULTS General pattern Animal-dispersed species are less clustered than their counterparts over all scales. The highest aggregation index was recorded for Shorea ochrophloia whereas Garcinia eugeniaefolia had the lowest clustering intensity at Ω5-10(relative neighbourhood density at 10m). At distances of more than 30m, most species tend toward spatial randomness. Notwithstandingly, every species are generally more aggregated than either hyperdispersed or randomized and have different degree of aggregation and spatial pattern at various distances. Relative abundance and floristic composition Streblus elongatus account for the most number of stems in the plot and there are more diaspores that are animal-dispersed than mechanically-dispersed. Nevertheless, both have the same proportion of rare species. Furthermore no significant correlation was found between degree of aggregation and relative abundance of the species. Behavior within a patch and growth stages pattern Most mechanically-dispersed species are considerably clumped locally compared to animaldispersed trees. Generally, population shift from tight to loose organization from the juvenile to adult stages for the investigated subjects. DISCUSSION Seed dispersal mode and spatial pattern Species dispersed by abiotic mechanisms exhibit greater spatial aggregation caused by local dispersal thus implicitly suggesting that secondary dispersal by animals is unlikely to play a major role. Besides being randomly dispersed by animal, the diaspores of some animal-dispersed species can also be dispersed non-randomly by directed dispersal agents. Furthermore I also found that other covariates like tree stature do not have any significant effect in influencing the pattern generated by different seed dispersal syndromes. Maintaining tree diversity Though the Janzen-Connell effects (Janzen 1970; Connell 1971) and competitive exclusion(Tilman 1994) are possible mechanisms maintaining local diversity, the study however suggested that dispersal limitation is the main factor restricting recruitment and promoting large scale diversity in this forest. Implications of fragmentation and conclusion Fragments engender decaying of ecosystem functions and uncouple the equilibrium promoting tree diversity in the forest. As a consequence, animal-dispersed families may fall into the vortex faster than wind-dispersed families. Unless drastic measures are implemented, the loss of seed dispersion capabilities will ultimately cause the extinction of many species of trees. References* Condit R., Ashton P.S., Baker P., Bunyavejchewin S., Gunatilleke S., Gunatilleke N., Hubbell S.P., Foster R.B., Itoh A., Lafrankie J.V., Lee H.S., Losos E., Manokaran N., Sukumar R. & Yamakura T.(2000) Spatial Patterns in the Distribution of Tropical Tree Species. Science 288: 1414-1418 Connell J.(1971) On the role of natural enemies in preventing competitive exclusion in some marine animals and rain forest trees. In: Den Boer P. & Gradwell G. (Eds.).Wagenigen, Center for Agricultural Publishing and Documentation. 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