The distribution of savannas and forests in the tropics is thought to be controlled in large part by positive feedbacks between vegetation structure and fire. Open savannas promote frequent fires, which in turn kill trees and maintain savannas in an open state, while closed-canopy forests suppress fuels needed by fires. The strength of this feedback and the rates of transition between states must depend upon traits of trees, such as their ability to survive fire, their capacity to grow rapidly, and the amount of shade produced by a tree. These differences among species are seldom incorporated into regional or global scale models, though their effects may be large. We measured differences in growth rates, bark thickness, and leaf area allometry among 116 savanna and forest tree species in the Brazilian cerrado biome at two sites undergoing rapid forest encroachment. We compared growth rates among functional groups across a gradient from savanna to forest and, together with leaf area allometries, estimated the contribution of each group to the development of leaf area.
Results/Conclusions
We found that forest species grow faster than savanna species, especially in the open. While savanna trees appear to grow at the same rate as forest trees when suppressed under dense canopies, they suffer much greater mortality rates. Forest trees support greater leaf area, which allows them to more readily suppress grass growth and reduce fire severity for a given tree size. The differences in growth among functional types was greater than the influence of soils, demonstrating that the functional type of the trees present can have a dramatic impact on the dynamics of savanna-forest transitions. Furthermore, forest trees support greater leaf area per unit of basal area. The presence of forest species rapidly increases the rate of conversion of savannas to forests by rapidly producing stem basal area and greater canopy coverage. These differences between functional types are critical to modeling stand dynamics and forecasting future conditions across the Brazilian savanna biome.