Tree seed dispersal strategies affect resilience of savanna-forest boundary ecosystems

Anthropogenic disturbances to ecosystems on the boundary between tropical savannas and forests may lead to changes in vegetation cover that are difficult to reverse. Given a sufficiently large disturbance, feedback mechanisms between forest tree cover and fire frequencies could trigger a transition between forest and savanna. Tree cover suppresses grass fires that maintain savanna, and thus tropical forest recruitment drives forest resilience against critical transitions. Tree seed dispersal strategies may be important determinants of spatial patterns of forest recruitment. However, their role in shaping forest resilience is not well understood. We therefore investigate the influence of tree seed dispersal strategies on tropical forest resilience. To this end, we introduce a novel individual-based model of Seed Dispersal in Savanna-Forest ecosystems (SDSF), simulating the interaction between tree dispersal strategies, spatial patterns of tree cover, and fire percolation. We find that seed dispersal by birds induces lower recruitment rates and more heterogeneous seed deposition patterns than dispersal by wind. In addition, forest recruitment rates and resilience are more sensitive to spatial pattern morphology when dispersed by birds than by wind. This effect is more pronounced for coarser spatial patterns containing larger forest patches that are spaced further apart. Our findings demonstrate for the first time that tree seed dispersal strategies interact with spatial patterns of tree cover and fire percolation, affecting tropical forest resilience against tipping toward a savanna state. Thus, efforts to understand the impact of global change on critical transitions in savanna-forest boundaries should account for the different effects of seed dispersal strategies.