by Carl Strang
A pair of papers in Science last year described global surveys of savannas and the ecological conditions that produce them. Their results were not surprising, but valuable in further documenting the ecology of these woodlands with their trees scattered more widely than in forests. They defined forests as woodlands with tree cover greater than 50-60%. Above 2500mm of annual rainfall, a region will be forested. Between 1000 and 2500mm, an area can be forest or savanna, with fire tipping the balance. In savannas, low tree cover promotes the spread of fire, and fire limits tree cover. Lengthy dry seasons and lower rainfall amounts tend to produce savanna.
When actual tree cover is measured, forests characteristically have 80%, and savannas 20%. There are few woodlands that measure close to the cutoffs separating these categories, supporting the idea that forest and savanna represent alternative stable states, or “attractors.” All of this is relevant to northeast Illinois. We fall in the range of rainfall where forest would cover the ground, but the rain is in the lower end of that range, and fire historically produced a prairie landscape with patches of savanna and, where rivers or topographic breaks produced little fire shadows, bits of forest. Here are the references:
Staver, A. Carla, Sally Archibald, and Simon A. Levin. 2011. The global extent and determinants of savanna and forest as alternative stable states. Science 334: 230-232.
Hirota, Marina, et al. 2011. Global resilience of tropical forest and savanna to critical transitions. Science 334:232-235.