Résumé
Global change is expected to increase savanna woody encroachment as well as fire spreading into forest. Forest-savanna ecotones are the frontier of these processes and can thus either mitigate or enhance the effects of global change. However, the ecology of the forest-savanna ecotone is poorly understood. In this study, we determined whether a distinct ecotonal tree community existed between forest and savanna. We then evaluated whether the ecotonal tree community was more likely to facilitate fire spreading into the forest, woody encroachment of the savanna or the stabilisation of both forest and savanna parts of the landscape.
We sampled 28 vegetation transects across forest-savanna ecotones in a central African forest-savanna mosaic. We collected data on the size and species of all established (basal diameter >3 cm) trees in each transect. Split moving window dissimilarity analysis detected the location of borders delineating savanna, ecotone and forest tree communities. We assessed whether the ecotonal tree community was likely to facilitate fire spreading into the forest by burning experimental fires and evaluating shade and grass biomass along the transects. To decide whether the ecotone was likely to facilitate woody encroachment of the savanna, we evaluated if ecotonal tree species were forest pioneers.
A compositionally distinct and spatially extensive ecotonal tree community existed between forest and savanna. The ecotonal tree community did not promote fire spreading into forest and instead acted as a fire buffer, shading out flammable grass biomass from the understorey and protecting the forest from 95% of savanna fires. The ecotone helped stabilise the forest-savanna mosaic by allowing the fire-dependant savanna to burn without exposing the fire-sensitive forest to lethal temperatures.
The ecotonal tree community was comprised of many forest pioneer species that will promote woody encroachment in the savanna, especially if fire frequency is decreased.
Synthesis. A distinct fire-buffering ecotonal tree community in this forest-savanna mosaic landscape illustrated that savanna fires are unlikely to compromise forest integrity. Conversely, suppression of fire in this landscape will likely lead to loss of savanna as the ecotone becomes the frontier of woody encroachment. Regular burning is essential for the preservation of this forest-savanna mosaic.