Stable states and transitions in African savannas. Soil N availability is related to parent material and weathering rates that influence biological nitrogen fixation (P-limitation), import and export of N (herbivores, humans, fire). Tree recruitment is related to browsing and grazing intensity, seasonal weather variations and fire frequency/intensity.

Transition	Description	Example scenarios at continental to landscape scales (a) and at landscape to patch scales (b)
1	Fertile to infertile	a) Decline in soil fertility as soil weathers (P declines) during millennial time-scales b) Imposed high fire frequency with low grazing intensity reduces N at decade to century time scales
2	Infertile to fertile	a) Gradual build-up in N status when N inputs (e.g. industrial N deposition) exceed losses in fire and trace gas emissions. b) Nutrient redistribution/concentration in the landscape, e.g. herbivore-favored grazing lawns, old village sites, termitaria, etc.
3	Open to closed	a) and b) Replacement of wild herbivores by high and sustained populations of domestic cattle, especially with concurrent fire suppression
4	Closed to open	a) and b) Severe disturbance of tree population caused by, for example, fire after drought or pest/disease outbreak

Transitions between stable states in African savanna. Some transitions may occur under current conditions or can be induced experimentally, others are less easily observable/are hypothetical.