Global controls on the distribution of savannas: impacts of discrete and continuous environmental factors

Background/Question/Methods

Research on savanna ecosystems has largely focused on tropical savannas to the exclusion of savannas in temperate and high latitude regions. We examine the possibility of generalizing to savannas outside of the tropics recent findings that show that tree cover in the tropics exhibits a bimodal distribution of either high or low tree cover, suggesting abrupt transitions to alternative states. Our analyses are based on tree cover percent estimates at moderate-resolution (500 m) between the equator and sub-polar regions derived from the Moderate Resolution Imaging Spectrometer (MODIS).  We estimate associated fire frequency estimates from MODIS, while deriving mean annual moisture and seasonality variables from rainfall and temperature layers provided by the WorldClim database. Though focused on savanna distribution, the methods we employ might be applied to consideration of species and community distributions whose controlling factors include a mixture of processes on scales from discrete-event to continuous.

Results/Conclusions

Fire acts as a discrete-event, discontinuous process at local to regional scales, with temperature being a continuous process at these scales. Moisture availability, though a discrete-event process on a fine spatio-temporal scale, is continuous on the broader space and time scales at which determination of global vegetation distributions are considered. We analyze the global distribution of savannas and their determinants with an objective of partitioning out how much continuous versus discrete factors influence tree cover percent. We create a unified empirical framework for savanna systems across the globe. Current views on controls of savanna distribution, based on tropical savannas, cannot readily be generalized using only mean annual precipitation. We present evidence that the seasonality of temperature and of moisture are strong predictors of the global tree cover distribution. Distribution of sub-polar savannas and forests are directly controlled by temperature including its indirect impact on fire disturbance. Temperate savannas mainly occur under the absence of fire or in situations with rare fires, with tree cover being limited by moisture availability. Tropical savannas are either associated with moisture limitation or, in moist conditions, maintained by frequent fires or other disturbances.