Combustion in Natural Fires and Global Emissions Budgets

• Published in: Ecological applications 1998-05, Vol.8 (2), p.464-468
• Main Authors: McNaughton, S. J., Stronach, N. R. H., Georgiadis, N. J.
• Format: Article
• Language: English
• Subjects: Ashes; Biofuels; Biomass burning; biomass burning and global climate change; Combustion; combustion in natural fires; Fire ecology; Fire intensity; Forest fires; global emission budgets; Grassland fires; laboratory cf. natural combustion; N and C emissions; Pollutant emissions; Wildfire seasons
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/1051-0761(1998)008[0464:CINFAG]2.0.CO;2

Fires in tropical savannas are a principal source of emissions to the atmosphere, but there are few studies of retention in ash and residual plant mass following natural fires. Estimates of carbon and nitrogen emissions to the atmosphere from biomass burning have relied largely on indirect measures from laboratory incineration. Emissions from incinerated samples are then extrapolated to areally extensive field estimates of flammable plant biomass. In addition, both direct sampling of smoke plumes and comparison to ambient atmospheric samples, with combustion efficiency estimated by ratios of trace gases to carbon dioxide in plumes, are employed. To the extent that combustion in natural fires departs from laboratory burning or that assumed average standing crops are inaccurate, indirect estimates based on laboratory emissions will be in error. Similarly, the relation of emission ratios to natural fire intensity is poorly understood and will influence the accuracy of estimates based on plume sampling. Recently, data from the Southern African Fire-Atmosphere Research Initiative (SAFARI) has indicated that indirect methods can overestimate the proportion of biomass combusted in natural fires. However, how combustion is related to fire temperature is quantitatively unknown. Studies of combustion efficiency in natural fires, as the proportion of initial biomass actually volatilized, are rare, and we know of no estimates of mass and N retention in residual ash and partially combusted biomass that are related to fire temperature. We burned plots instrumented with thermocouples to measure fire temperature and analyzed biomass from adjacent clipped plots, residual post-fire plant residue, and ash to determine the proportion of biomass actually burned and N volatilized. Data from experimental savanna grassland fires under natural conditions in Serengeti National Park indicate that (1) the proportions of mass and N volatilized are substantially less than 100%, (2) most fuel loads are within a range where combustion and volatilization are highly dependent on mass burned and resultant fire intensity, (3) proportionally more N than mass is volatilized, particularly as fire intensity increases, but (4) much less mass and N are volatilized in natural fires than laboratory estimates suggest. Therefore, (5) obtaining accurate estimates of emissions from natural fires will be substantially more difficult than is reflected in current methodology.