American Society of Naturalists

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“Dispersal increases the resilience of tropical savanna and forest distributions”

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Nikunj Goel, Vishwesha Guttal, Simon A. Levin, and A. Carla Staver (May 2020)

Dispersal may allow recovery of tropical savanna and forest biomes, but it may be slow

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High elevation savanna-forest mosaics, commonly referred to as Shola Grasslands, in human-dominated regions of Western Ghats, Karnataka, India.<br />(Credit: Nikunj Goel)
High elevation savanna-forest mosaics, commonly referred to as Shola Grasslands, in human-dominated regions of Western Ghats, Karnataka, India.
(Credit: Nikunj Goel)

Anthropogenic carbon emissions are changing the global climate at an unprecedented rate. Although it is well accepted that these climatic changes will result in large-scale biome shifts, it is unclear whether these shifts are reversible. Historically, biogeographers have argued that biomes match perfectly to their prevailing climate, such that biomes continuously track spatially shifting climatic envelopes. Based on this historical view, biome shifts should be reversible, provided climatic changes are reversed. However, recent theoretical and empirical advances suggest that, in tropical savanna and forest ecosystems, fires can maintain multiple biomes states in intermediate rainfall conditions (the bistable region). If the rainfall changes beyond this bistable region, tropical ecosystems will undergo large-scale irreversible biome shifts. “Both of these modeling frameworks, however, ignore the fact that savanna and forest interact spatially at their ecotone through seed dispersal, which may facilitate biome recovery as seen in many abandoned agricultural plots in tropical forests,” said the lead author Nikunj Goel, Master’s student at Yale University in Department of Ecology and Evolutionary Biology (now a Ph.D. candidate at The University of Texas at Austin).

In a new paper in The American Naturalist, Nikunj and his collaborators from Yale University, Princeton University, and Indian Institute of Science present a spatially explicit reaction-diffusion model to examine the role of dispersal in determining distribution and recovery of tropical biomes. Their model suggests that the savanna-forest boundary is not only determined by climate but also by continental-scale source-sink dynamics that are mediated by the geometrical shape of rainfall contours. The researchers find this non-intuitive prediction is consistent with biome patterns in sub-Saharan Africa. Intriguingly, the diffusion models predict that biome shifts due to global change might be reversible unless the remnant biome patches are too small.

The spatial position of the savanna-forest boundary (brown line) is not solely determined by precipitation but also depends on the ‘roundness’ of the precipitation contour (grey line). When precipitation contour is curved, the boundary deviates from its bioclimatic limit, PM. (Credit: Nikunj Goel)
The spatial position of the savanna-forest boundary (brown line) is not solely determined by precipitation but also depends on the ‘roundness’ of the precipitation contour (grey line). When precipitation contour is curved, the boundary deviates from its bioclimatic limit, PM. (Credit: Nikunj Goel)

Abstract

Global change may induce changes in savanna and forest distributions, but the dynamics of these changes remain unclear. Classical biome theory suggests that climate is predictive of biome distributions, such that shifts will be continuous and reversible. This view, however, cannot explain the overlap in the climatic ranges of tropical biomes, which some argue may result from fire-vegetation feedbacks, maintaining savanna and forest as bistable states. Under this view, biome shifts are argued to be discontinuous and irreversible. Mean-field bistable models, however, too, are limited as they cannot reproduce the spatial aggregation of biomes. Here, we suggest that both models ignore spatial processes, such as dispersal, which may be important when savanna and forest abut. We examine the contributions of dispersal to determining biome distributions using a 2D reaction-diffusion model, comparing results qualitatively to empirical savanna and forest distributions in sub-Saharan Africa. We find that the diffusion model resolves both the aforementioned limitations of biome models. First, local dispersive spatial interactions, with an underlying precipitation gradient, can reproduce the spatial aggregation of biomes with a stable savanna-forest boundary. Second, the boundary is not only determined by the amount of precipitation but also by the geometrical shape of the precipitation contours. These geometrical effects arise from continental-scale source-sink dynamics, which reproduces the mismatch between biome and climate. Dynamically, the spatial model predicts that dispersal may increase the resilience of tropical biome in response to global change: the boundary continuously tracks climate, recovering following disturbances, unless the remnant biome patches are too small.