Widespread forest-savanna coexistence but limited bistability at a landscape scale in Central Africa

Tropical forest and savanna frequently coexist under the same climatic conditions, which has led to the hypothesis that they could represent alternative ecosystem states, stabilized by internal feedbacks. An implication of this hypothesis is that forest and savanna may be bistable and exhibit tipping behavior in response to changing conditions. However, we pose that the local presence of forest and savanna within coexistence landscapes is not sufficient evidence that these are alternative stable states at larger ecosystem scales. Therefore, we explore forest-savanna coexistence and bistability at landscape scale in Central Africa. Using remote sensing data on tree cover, we classify 10 x 10 km landscapes as homogeneous forest, homogeneous savanna, or coexistence, and analyze the factors driving their distributions. We find that the precipitation ranges for which homogeneous forest and savanna occur have only limited overlap, and that this overlap can largely be explained by other external drivers, such as seasonality, soil sand content, and elevation. Conversely, local coexistence of forest and savanna under the same climatic and edaphic conditions is common within landscapes. In these coexistence landscapes, however,  the spatial configuration of tree cover can often be predicted based on topographic variables, indicating that the apparent bistability of forest and savanna is likely caused by local redistribution of resources, rather than internal feedbacks. Considering the limited evidence found for forest and savanna as true alternative ecosystem states, particularly at landscape scale, we conclude that the likelihood of tipping between both states may be lower than previously thought. Intermediate coexistence states, facilitated by topographic heterogeneity, may lead to more gradual and reversible transitions between tropical forest and savanna, increasing the resilience of these ecosystems to changing drivers and disturbances.