Facilitation in drylands: Modeling a neglected driver of savanna dynamics

• We created an ecohydrological ODE model of savannas with grasses and trees.
• Grasses facilitate (shading, infiltration) trees with increasing aridity.
• Savanna biome boundaries are extended through grass facilitation.
• Facilitation as a key community-level process in savannas requires more attention.

Our current understanding regarding the functioning of the savanna ecosystem describes savannas as either competition- or disturbance-dependent. Within this generalized view, the role and importance of facilitation have been mostly neglected. This study presents a mathematical model of savannas with coupled soil moisture–vegetation dynamics, which includes interspecific competition and environmental disturbance. We find that there exist environmental and climatic conditions where grass facilitation toward trees plays an important role in supporting tree cover and by extension preserving the savanna biome. We, therefore, argue that our theoretical results in combination with the first empirical studies on the subject should stimulate further research into the role of facilitation in the savanna ecosystem, particularly when analyzing the impact of past and projected climatic changes on it.