A spatially explicit comparison of sapling distribution and seed fall of a threatened tropical tree species: Testing the Janzen-Connell hypothesis

Background/Question/Methods

Examining distribution patterns of tree species can help our understanding of why tropical forests have high diversity and guide conservation strategies. The Janzen – Connell hypothesis predicts that recruitment patterns of young life stages of tropical trees will be skewed away from reproductive trees and areas of high seed fall. To test the Janzen – Connell hypothesis, we compared the spatial distribution of saplings with patterns of conspecific seed fall of Manilkara maxima, a threatened and ecologically important tree species in the Atlantic forest of southern Bahia, Brazil, a global biodiversity hotspot. Five reproductively mature trees, > 25 m from the closest mature tree, from three study sites were selected to estimate seed fall. Seed fall was quantified over six months using 10 seed traps per tree. The GPS coordinates of reproductive trees, saplings, seed traps were recorded and mapped, and seed density was estimated using interpolation across the surface of the study sites and reclassified into seed density zones using ArcMap v.10. We tested if the observed sapling distribution was different from the expected distribution of saplings using a chi-square test of distributions, and assuming all seeds contribute to the sapling life stage and sapling density is proportional to seed density.

Results/Conclusions

The sapling distribution pattern of M. maxima, from the three study sites, supports the spatially explicit predictions of the Janzen – Connell hypothesis. The observed sapling densities were significantly different from the expected sapling densities. The areas with the lowest expected sapling densities had the highest observed sapling densities. The low sapling recruitment in high seed density zones suggests density dependent processes play a role in the recruitment pattern of this tropical tree. Potential mechanisms for this distribution pattern could be intraspecific or interspecific competition. Additionally, these data suggest that animal-mediated long distance seed dispersal is important for recruitment of M. maxima. Additionally, conservation of the animal seed disperser species, some of which are endangered, is important for the long-term sustainability of this threatened tree species.