Fire in neotripical savannas

Abstract

This chapter reviews the main characteristics of tropical savannas and their interactions with fire, a main factor involved in the dynamics of these ecosystems. Burning in savannas generally takes place by the end of the dry season, favored by the accumulation of dry biomass in the herbaceous layer. Several factors affect fire characteristics and behavior, such as quantity, quality and composition of the combustive matter, wind direction and velocity, relative humidity of air, and season of burning. Although fire is a mortality factor, common savanna plant species seem to be adapted to burning. Adaptations include underground reserves, protected meristems, re-sprout capacity, clonal multiplication, growth seasonality, thick cork layer, and sclerophyllous leaves. Deleterious effects of fire are greater in the younger or smaller size classes in the population. On the other hand, the passage of fire restores light and nutrients on the topsoil, that together with the return of rains create better conditions for seasonal growth. Under fire exclusion herbs escape from the direct mortality effects of burning but exclusion promotes shading and nutrient depletion which are harmful for the renewal of herbaceous growth. Species differ in their responses to fire regime, therefore the fire regime affects species composition. The fire regime is also important in terms of the physiognomy of the savanna. Frequent burning causes qualitative and quantitative changes in plant community structure and composition promoting open physiognomies with fewer woody elements. Conversely, fire exclusion has been shown to increase tree density in tropical savannas transforming open savannas in woodlands. In modern times, most fires are set by humans. However, natural fires by electric discharges during storms do occur although less frequently than human induced fires. Although fire has been used in savanna management since pre-Columbian times, it remains a complex issue that needs further research.