Disturbance Regimes in Savannas

Savannas are ecosystems in which a continuous or near-continuous grass layer coexists with scattered trees and shrubs. Their structure is maintained not by complete stability, but by a recurring disturbance regime involving fire, herbivory and climatic variability. A regime includes the frequency, intensity, season and spatial extent of disturbances, rather than merely one event.

Control of the tree-grass balance

Disturbances prevent either woody plants or grasses from gaining complete dominance. Their effects interact with rainfall, soils and plant traits to maintain tree-grass coexistence.

• Periodic fire removes dry grass biomass and repeatedly damages many woody seedlings and saplings, while established trees and resprouting grasses may survive.
• Grazers reduce grass biomass, while browsers can suppress woody recruitment; selective feeding also changes competition among plant species.
• Seasonal drought restricts plant growth and causes episodic mortality, helping preserve the characteristic open canopy.

Creation of ecological heterogeneity

Disturbance is uneven across a landscape, producing a shifting mosaic of recently burned, heavily grazed and relatively undisturbed patches. This spatial heterogeneity supports species requiring different vegetation heights, cover and food resources.

• Herbivores create short-grass patches, pathways and dung-rich nutrient hotspots.
• Patchy burning leaves refuges from which plants and animals can recolonise.
• Alternating disturbance and recovery maintain vegetation of different ages and structures.

Renewal, nutrient cycling and limits

Fire converts part of the standing biomass into ash, while herbivores redistribute nutrients through dung and urine. Disturbance also opens space and light for germination and vegetative regrowth, sustaining productivity and renewal.

• The ecological outcome depends on the regime: excessive frequency or intensity can degrade soil and vegetation, while prolonged suppression can permit woody thickening and fuel accumulation.
• Restoration should therefore recover context-appropriate variability and feedbacks, rather than impose uniform burning, grazing or complete disturbance exclusion.