Tree reproduction in the Serengeti: The role of resources and disturbances across environmental gradients

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Deusdedith M. Rugemalila and T. Michael Anderson, Biology, Wake Forest University, Winston-Salem, NC, Ricardo M. Holdo, Odum School of Ecology, University of Georgia, Athens, GA

Background/Question/Methods
Savanna ecosystems are spatially heterogeneous, consisting of patchy fire-tolerant trees, with a continuous layer of C4 grasses. Tree mortality filters in savannas affect different life-history stages, disproportionately disrupting transition from seed to seedling and ultimately influencing tree community assembly. Tree reproduction is a critical phase in tree life-history, and its variability comprises a significant consequence for seedbank dynamics, recruitment, and population viability. The role of environmental heterogeneity on seed production patterns is not well understood in savannas because the focus has been on seedling survival processes at the post-germination stages and survival differences between species or functional groups.
We assessed how resources (rainfall), disturbance (fire and herbivory), and tree size influence tree reproduction probabilities in dominant Acacia (= Vachellia) tree species in the Serengeti National Park, Tanzania, Acacia robusta, and A. tortilis over three years. In the Serengeti, A. robusta trees dominate the mesic areas in the north and A. tortilis, the dry southern area. We used multiple logistic regression modeling for our statistical analysis.
Results/Conclusions
Tree reproduction probabilities varied between species and years. Tree reproduction probability was negatively associated with browsers' disturbances in both species but positively associated with grazers. Rainfall was essential for reproduction probability in A. tortilis but not in A. robusta with fire frequency. Tree size variably influenced tree reproduction in opposing fashion with A. tortilis trees having lower chances of reproducing when small but higher chances when large. Our study shows that resources and disturbances play variable roles in influencing tree reproduction in savannas tree community.