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−1 where fire return intervals were 1–4 years and exceeded 7000 kg ha−1 at a site subjected to 38 years of fire exclusion. However, fireline intensity was only 1419 kW m−1 at the fire exclusion site, while ranging from 480 to 6130 kW m−1 among the frequent fire sites. In Zambia, total aboveground fuel loads ranged from 3164 kg ha−1 in a hydromorphic grassland to 7343 kg ha−1 in 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−1 among 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.

Interannual variability in global biomass burning emissions from 1997 to 2004

“…We estimated fuel wood collection and herbivory as in van der Werf et al (2003). The main result of including these two processes was a decrease in fuel loads in savanna ecosystems, in better agreement with measured fuel loads (e.g., Shea et al, 1996;Hoffa et al, 1999). Within tropical forest ecosystems, aboveground biomass levels from the model were broadly consistent with published estimates.…”

“…Several generalities about CC have emerged from studies that have measured CC's in a wide range of vegetation types (e.g., Shea et al, 1996;Hoffa et al, 1999;Carvalho et al, 2001). First, CC of fine fuels is usually very high, up to 1 (complete combustion) for dry surface litter.…”

“…CC has been measured in the field for various biome and fuel types (e.g., Carvalho et al, 1995;Shea et al, 1996;Hoffa et al, 1999), and varies over the course of the fire season with more complete combustion at the end when fuels have had more time to dry out, as shown by Hoffa et al (1999) for savanna ecosystems. In general, fine and dry fuels burn more completely than coarse and wet fuels, although a paucity of data makes it challenging to quantitatively link CC with climate in global models.…”

“…We also allowed CC to vary from month to month to simulate the effects of seasonal changes in fuel moisture content (Shea et al, 1996). We set minimum and maximum values for each fuel type (Table 3), and used moisture conditions to scale between these values.…”

Evolution of human-driven fire regimes in Africa

“…Lightning is largely absent in the dry season -being associated with convective wet season thunderstorms and more rarely with dry storms just before the first rains. Thus, a lightning-driven fire regime probably had seasonal fire distributions different from modern ones, with major effects on the size, intensity and atmospheric emissions of fires (17,18).…”