Dispersal limitation and fire feedbacks maintain mesic savannas in Madagascar.

Madagascar is regarded by some as one of the most degraded landscapes on Earth, with estimates suggesting that 90% of forests have been lost to indigenous Tavy farming. However, the extent of this degradation has been challenged: paleoecological data, phylogeographic analysis, and species richness indicate that pyrogenic savannas in central Madagascar predate human arrival, even though rainfall is sufficient to allow forest expansion into central Madagascar. These observations raise a question-if savannas in Madagascar are not anthropogenic, how then are they maintained in regions where the climate can support forest? Observation reveals that the savanna-forest boundary coincides with a dispersal barrier-the escarpment of the Central Plateau. Using a stepping-stone model, we show that in a limited dispersal landscape, a stable savanna-forest boundary can form because of fire-vegetation feedbacks. This phenomenon, referred to as range pinning, could explain why eastern lowland forests have not expanded into the mesic savannas of the Central Highlands. This work challenges the view that highland savannas in Madagascar are derived by human-lit fires and, more importantly, suggests that partial dispersal barriers and strong nonlinear feedbacks can pin biogeographical boundaries over a wide range of environmental conditions, providing a temporary buffer against climate change.

Dispersal Increases the Resilience of Tropical Savanna and Forest Distributions.

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, are also 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 determined not only 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 reproduce 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.

Post-fire reproduction of herbs at a savanna-gallery forest boundary in Distrito Federal, Brazil / Reprodução pós-fogo de espécies do estrato herbáceo em área de transição Cerrado-Mata de Galeria no Distrito Federal, Brasil

Abstract In Cerrado, studies of post-fire vegetation recovery show that some herbaceous species are able to flower shortly after fires. However, these were mainly short-term studies that focused on grasslands and savannas. Little is known about the effects of fire on ground layer of forests that border the savannas in Central Brazil. Thus, an accidental burning gave us the opportunity to describe the reproductive activity of the ground layer vegetation after a fire event along a savanna-forest boundary at the IBGE Ecological Reserve, Brasília, Brazil. During the 16-month of the inventory, we registered 170 herbaceous species flowering or fruiting, of which 52 species (31%) may have been influenced by fire that changed their times of reproduction. In the savanna plots reproduction peaked at the end of the rainy season. Of the total number of reproducing species, 90 species occurred only in the savanna and four in the forest. Five herbs were recorded in the forest, savanna and border environments. Late dry season fire probably lead the majority of herbaceous species to have their reproduction spread throughout the study time.

Resumo No Cerrado, estudos de recuperação da vegetação pós-fogo mostram que algumas espécies herbáceas são capazes de florescer logo após queimadas. No entanto, esses foram principalmente estudos de curto prazo, que se concentraram em áreas campestres e savânicas. Pouco se sabe sobre os efeitos do fogo no sub-bosque de matas que margeiam as savanas do Brasil Central. Assim, uma queima acidental nos deu a oportunidade de descrever a atividade reprodutiva pós-fogo do estrato herbáceo ao longo de ecótono cerrado-mata na Reserva Ecológica do IBGE, Brasília, Brasil. Durante os 16 meses do inventário, foram registrados 170 espécies herbáceas com flores ou frutos, das quais 52 espécies (31%) podem ter sido influenciadas pelo fogo, que alterou suas épocas reprodutivas. Nas parcelas de cerrado, a reprodução teve seu pico ao final da estação chuvosa. Do número total de espécies, 90 ocorreram exclusivamente no cerrado e quatro na floresta. Cinco espécies herbáceas foram registradas na floresta, cerrado e borda. O fogo ao final da estação seca pode ter levado a maioria das plantas a se reproduzir através do período de estudo.