Download Seed dispersal, spatial distribution and maintenance of tropical tree

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.
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Only important references. Please refer to the report for other authors cited.