Prediction and scale in savanna ecosystems

Savannas are highly variable systems, and predicting variation, especially in the tree layer, represents a major unresolved challenge for forecasting biosphere responses to global change. Prediction to date has focused on disentangling interactions between resource limitation and chronic disturbances to identify what determines local savanna vegetation heterogeneity. By focusing at too fine a scale, this approach overlooks: sample size limitation arising from sparse tree distributions; stochasticity in demographic and environmental processes that is preserved as heterogeneity among tree populations with slow dynamics; and spatial self-organization. Renewed focus on large (1-50ha) permanent plots and on spatial patterns of tree-layer variability at even larger landscape spatial scales (>= 1000s of ha) promises to resolve these limitations, consistent with the goal of predicting large-scale biosphere responses to global change.