Quantifying the relative importance of greenhouse gas emissions from current and future savanna land use change across northern Australia

Abstract

Abstract. The clearing and burning of tropical savanna leads to globally significant emissions of greenhouse gases (GHGs); however there is large uncertainty relating to the magnitude of this flux. Australia's tropical savannas occupy the northern quarter of the continent, a region of increasing interest for further exploitation of land and water resources. Land use decisions across this vast biome have the potential to influence the national greenhouse gas budget. To better quantify emissions from savanna deforestation and investigate the impact of deforestation on national GHG emissions, we undertook a paired site measurement campaign where emissions were quantified from two tropical savanna woodland sites; one that was deforested and prepared for agricultural land use and a second analogue site that remained uncleared for the duration of a 22-month campaign. At both sites, net ecosystem exchange of CO 2 was measured using the eddy covariance method. Observations at the deforested site were continuous before, during and after the clearing event, providing high-resolution data that tracked CO 2 emissions through nine phases of land use change. At the deforested site, post-clearing debris was allowed to cure for 6 months and was subsequently burnt, followed by extensive soil preparation for cropping.During the debris burning, fluxes of CO 2 as measured by the eddy covariance tower were excluded. For this phase, emissions were estimated by quantifying on-site biomass prior to deforestation and applying savanna-specific emission factors to estimate a fire-derived GHG emission that included both CO 2 and non-CO 2 gases. The total fuel mass that was consumed during the debris burning was 40.9 Mg C ha −1 and included above-and below-ground woody biomass, course woody debris, twigs, leaf litter and C 4 grass fuels. Emissions from the burning were added to the net CO 2 fluxes as measured by the eddy covariance tower for other postdeforestation phases to provide a total GHG emission from this land use change.The total emission from this savanna woodland was 148.3 Mg CO 2 -e ha −1 with the debris burning responsible for 121.9 Mg CO 2 -e ha −1 or 82 % of the total emission. The remaining emission was attributed to CO 2 efflux from soil disturbance during site preparation for agriculture (10 % of the total emission) and decay of debris during the curing period prior to burning (8 %). Over the same period, fluxes at the uncleared savanna woodland site were measured using a second flux tower and over the 22-month observation period, cumulative net ecosystem exchange (NEE) was a net carbon sink of −2.1 Mg C ha −1 , or −7.7 Mg CO 2 -e ha −1 .Estimated emissions for this savanna type were then extrapolated to a regional-scale to (1) provide estimates of the magnitude of GHG emissions from any future deforestation and (2) compare them with GHG emissions from prescribed savanna burning that occurs across the northern Australian savanna every year. Emissions from current rate of annual savanna deforestation across northern Austral...

TERN, Australia’s land observatory: addressing the global challenge of forecasting ecosystem responses to climate variability and change

“…Fire is responsible for the majority of productivity losses in the northern savanna (63%), whereas large weather events such as cyclones contribute very little to the long-term net biome carbon budget (Hutley et al 2013). Burning is furthermore largely responsible for greenhouse gas emissions from savannas and consequential greenhouse gas forcing of climate (Bristow et al 2016), amongst a cascade of indirect feedbacks between climate and ecosystems which are mediated through local atmospheric dynamics (figure 2; Beringer et al 2015). The outcome of this work is that it has contributed to a better understanding of tropical savanna functioning globally, where similarities of savanna structure have been found to conceal large differences across continents amongst vegetation, climate and fire dynamics (Lehmann et al 2014).…”

Seasonal, interannual and decadal drivers of tree and grass productivity in an Australian tropical savanna

“…However, plant respiration consumes approximately half of GPP, while heterotrophic respiration contributes to further carbon release to the atmosphere (Bonan, 2008;Chapin, Matson, & Vitousek, 2011). Over longer timescales, disturbances such as grazing, land cover change (Bristow et al, 2016;Hutley et al, 2013) and fire Bond & Keeley, 2005;Bowman et al, 2010;Shi, Matsunaga, Saito, Yamaguchi, & Chen, 2015;Van Der Werf et al, 2010) return a portion of the sequestered carbon from GPP back to the atmosphere. Taking these factors into account, savanna ecosystems are still an important terrestrial sink of atmospheric carbon (0.5-2.0 Gt C/year globally; Grace et al (2006); Scurlock and Hall (1998)) and explain a large portion of interannual variability in the global land carbon sink (Ahlstr€ om et al, 2015;Poulter et al, 2014).…”

Preface: OzFlux: a network for the study of ecosystem carbon and water dynamics across Australia and New Zealand

“…Likewise, dry temperate eucalypt forests can be a very large sink for carbon Leuning et al, 2005), although these highly productive ecosystems can become a carbon source due to the direct and indirect effects of climate variability (van Gorsel et al, 2013). Bristow et al (2016) show in this special issue that clearing of woody vegetation from the tropical savanna results in carbon emissions double the size of that reported for savanna burning. Rainfall is an important driver of photosynthesis in Australian ecosystems, and fluctuations of rainfall and productivity in Australia have been associated with multiple climate indices Rogers and Beringer, 2017).…”

“…This special issue reflects the breadth of scientific research to which Ray made significant contributions, including (i) methodological aspects of observations and their interpretation Isaac et al, 2017;McHugh et al, 2017); (ii) upscaling ecosystem-scale measurements to regional and larger scales using remote sensing and physical modelling Laubach et al, 2016;Moore et al, 2016b;Restrepo-Coupe et al, 2016;Trudinger et al, 2016;Whitley et al, 2016); and (iii) analysis of carbon, water and energy cycles in response to land use and climate change, extreme weather, and fire (Bristow et al, 2016;Hinko-Najera et al, 2017;Hunt et al, 2016;Moore et al, 2016aMoore et al, , 2017Fest et al, 2017). Ray's approach to science was to always capture process understanding and integrate this into models that could be used to deliver predictions and solutions.…”