Fuel biomass and combustion factors associated with fires in savanna ecosystems of South Africa and Zambia

Abstract

Fires are dominant factors in shaping the structure and composition of vegetation in African savanna ecosystems. Emissions such as CO2, NOx, CH4, and other compounds originating from these fires are suspected to contribute substantially to changes in global biogeochemical processes. Limited quantitative data exist detailing characteristics of biomass, burning conditions, and the postfire environment in African savannas. Fourteen test sites, differentiated by distinct burn frequency histories and land‐use patterns, were established and burned during August and September 1992 in savanna parklands of South Africa and savanna woodlands of Zambia. Vegetation physiognomy, available fuel loads, the levels of biomass consumed by fire, environmental conditions, and fire behavior are described. In the South African sites, total aboveground fuel loads ranged from 2218 to 5492 kg ha−1where fire return intervals were 1–4 years and exceeded 7000 kg ha−1at a site subjected to 38 years of fire exclusion. However, fireline intensity was only 1419 kW m−1at the fire exclusion site, while ranging from 480 to 6130 kW m−1among the frequent fire sites. In Zambia, total aboveground fuel loads ranged from 3164 kg ha−1in a hydromorphic grassland to 7343 kg ha−1in a fallow shifting cultivation site. Dormant grass and litter constituted 70–98% of the total fuel load among all sites. Although downed woody debris was a relatively minor fuel component at most sites, it constituted 43–57% of the total fuel load in the fire exclusion and shifting cultivation sites. Fire line intensity ranged between 1734 and 4061 kW m−1among all Zambian sites. Mean grass consumption generally exceeded 95%, while downed woody debris consumption ranged from 3 to 73% at all sites. In tropical savannas and savanna woodlands of southern Africa, differences in environmental conditions, land‐ use patterns, and fire regimes influence vegetation characteristics and thus influence fire behavior and biomass consumption.