Quantifying the relative importance of greenhouse gas emissions from current and future savanna land use change across northern Australia
Authors: Bristow M. , Hutley L. B. , Beringer J. , Livesley S. J. , Edwards A. C. , Arndt S. K. 
Published: May 2016

Abstract. Clearing and burning of tropical savanna leads to globally significant emissions of greenhouse gases (GHG) although there is large uncertainty relating to the magnitude of this flux. Australia’s tropical savannas occupy over 25 % of the continental land mass and have a potential to significant influence the national greenhouse gas budget, particularly because they are the focus of likely agricultural expansion. To investigate the role of deforestation on GHG emissions, a paired site approach was used. The CO2 exchange was measured from two tropical savanna woodland sites, one that was cleared, and a second analogue site that remained uncleared for a 22 month observation period. At both sites, net ecosystem exchange (NEE) was measured using the eddy covariance (EC) method. Observations at the cleared site was continuous before, during and after the clearing event, providing high resolution data that tracked CO2 emissions through multiple phases of land use change. At the cleared site, post-clearing debris was allowed to cure for 6 months and was subsequently burnt, followed by extensive soil preparation for cropping. Emissions were estimated from the debris fire by quantify the on-site biomass prior to clearing and applying savanna-specific emissions factors to estimate a fire-derived GHG emission. This was added to net CO2 fluxes as measured by the eddy covariance tower giving a total GHG emission of 154 Mg CO2-e ha−1 from a savanna woodland with a total fuel load (above- and below- ground woody debris, course woody debris, litter plus C4 grass fuel) of 40.9 Mg C ha−1. This emission was dominated by debris combustion from the fire event, which was 83 % of the total emission, the remained from soil emissions and decay of debris during the curing period prior to burning. Soil disturbance from ploughing and site preparation for cropping was responsible for almost 10 % of the total emission. Fluxes at the uncleared site were tracked using an additional flux tower for 668 days and over this period, cumulative NEE was −2.1 Mg C ha−1, a net carbon sink. Estimated emissions for this savanna type were then upscaled to provide estimates of the magnitude of emissions from any future deforestation. At current rates of deforestation, savanna burning is as significant a source of GHG emissions as deforestation, with fire emissions occurring every year across this savanna biome. However, expanded deforestation could exceed fire emissions and a clearing scenario was examined which suggested clearing over and above current rates could add up to 5 % to Australia’s national GHG account, depending on the annual rate of deforestation. This bottom-up study provides data that can reduce uncertainty associated with land use change for this extensive tropical ecosystem and provide an assessment of the relative magnitude of GHG emissions from savanna burning and deforestation as well as informing northern land use decision making processes.