Linking resource- and disturbance-based models to explain tree-grass coexistence in savannas

Area of Science:

• Ecology
• Plant Ecology
• Ecohydrology

Background:

• Savannas host persistent C3 tree and C4 grass coexistence, with mechanisms debated.
• Existing quantitative models offer contrasting explanations for savanna persistence.

Purpose of the Study:

• To synthesize existing models and empirical data to explain tree-grass coexistence across savanna rainfall gradients.
• To identify the crucial mechanisms underlying stable coexistence in these ecosystems.

Main Methods:

• Integration of Walter's two-layer model with demographic bottleneck frameworks.
• Analysis of empirical data to support proposed coexistence mechanisms.

Main Results:

• No single existing model fully explains tree-grass coexistence.
• Functional rooting separation is identified as a necessary condition for coexistence.
• Asymmetric competition for soil moisture limits tree growth, creating demographic bottlenecks.

Conclusions:

• A synthesized model combining Walter's two-layer approach and demographic bottlenecks provides a robust explanation.
• Functional rooting separation is the central element for understanding savanna tree-grass coexistence.
• Ecohydrological constraints, fire, and herbivores prevent tree dominance.