The environmental triangle of the Cerrado Domain: Ecological factors driving shifts in tree species composition between forests and savannas

1. The Cerrado Domain of central Brazil houses the largest extent of savanna in the Neotropics, but despite its simple characterization as a giant savanna, it contains considerable vegetation heterogeneity that is poorly understood. 2. We aimed to determine how vegetation types in the Cerrado diverge in their tree species composition and what role ecological factors play in driving compositional patterns. 3. We used a dataset of 1,165 tree species inventories spread across the Cerrado Domain, which come from six vegetation types that have a substantial arboreal component: woody savannas, dystrophic cerradao, mesotrophic cerradao, seasonally dry tropical forests, semideciduous forests and evergreen forests. We found three extremes in terms of tree species composition, with clear underlying ecological drivers, which leads us to propose a ternary model, the Cerrado Vegetation Triangle, to characterize woody vegetation in the Cerrado. At one extreme, we found that semideciduous and evergreen forests are indistinguishable floristically and are found in areas with high water availability. At another extreme lie seasonally dry tropical forests which are found on more fertile soils. At the third extreme, we found that all types of savanna, and dystrophic cerradao, are highly similar in tree species composition and are commonly found in areas of poor soils and high flammability. Mesotrophic cerradao is transitional in tree species composition between savannas and seasonally dry tropical forest. 4. The lack of variation in tree species composition attributed to climatic variables indicates that within homogeneous macroclimatic zones, many types of forest and savanna co-exist due to complex mosaics of local substrate heterogeneity and fire history. 5. Synthesis. Our findings highlight the complexity of forest-savanna transitions in the Cerrado Domain, with relevance for understanding the future of Cerrado vegetation under environmental change. If nitrogen deposition is extensive, some savannas may be more likely to transition to mesotrophic cerradao or even seasonally dry tropical forest, whereas if water availability increases these same savannas may transition to semideciduous or evergreen forest. Our Cerrado Vegetation Triangle model offers a simple conceptual tool to frame discussions of conservation and management.