Estimating combustion of large downed woody debris from residual white ash

Abstract

The production of residual white ash patches within wildfires represents near-complete combustion of the available fuel and releases a considerable quantity of gases to the atmosphere. These patches are generally produced from combustion of large downed woody debris (LDWD) such as fallen trees and snags. However, LDWD are generally ignored in calculations of fuel combusted within environments where surface fires dominate (e.g. southern African savannas). To assess the potential of fractional white ash cover as a remotely sensed measure of LDWD combustion, both the proportion of the surface covered by white ash and the combustion completeness required to produce white ash must be quantified. An aerial photograph of woodland savanna fires in north-western Zimbabwe was analysed to estimate the proportion of white ash cover within a typical satellite sensor pixel. The proportion loss on ignition (LOI) of wood samples from the study area was measured and combined with previous estimates of mean tree biomass. The proportion of white ash within the aerial photographs was 0.2% (± 0.06), which corresponded to an additional 67 320 kg ha⁻¹ of biomass combusted above that typically recorded as combusted from a surface fire in this environment (~7000 kg ha⁻¹). This analysis should be repeated in other savannas and forests, where pre-fire fuel loads and post-fire fractional white ash cover may be higher.