Savanna Fire Management
Currently, an average of 370,000 km² of Australia’s 1.9 million km² sparsely settled northern savannas are burnt annually, mostly by relatively intense and extensive late dry season wildfires.
Fire is a dominant conservation and land management issue for the Australian tropical savannas. Contributions from savanna fires to Australia's National Greenhouse Gas Inventory (NGGI) amount to approximately 1-3% annually, depending on the magnitude of respective fire seasons. Recent assessment of the extent of burning derived from satellite imagery shows that, over the period 1997-2004, an average of approximately 370,000 km² (19%) of the 1.9 million km² tropical savannas was burnt annually, mostly under severe late dry season (August - November) conditions. Reliable monsoonal (summer) rainfall and sustained high temperatures drive periods of rapid plant growth alternating with intense seasonal droughts of more than six months when no rain falls, evaporation rates are high and grasses cure rapidly. This regular annual cycle of several months of rapid fuel production, followed quickly by an equivalent or longer period of dry, often windy conditions that favour ignitions and fire spread, make the grassy savannas extraordinarily fire prone.
Savanna Burning and Greenhouse Gas Emissions
Carbon is removed from the atmosphere in rapidly growing grasses in the wet season and then much of it is released rapidly when oxidised by burning in the dry season. For the purposes of carbon accounting, the resultant annual pulse in emissions of carbon dioxide (CO2) from burning is treated as being entirely re-captured in the next cycle of rapid growth. Savanna burning is assumed to cause no net CO2 imbalance over timescales relevant to behaviour of the atmosphere. However, some other potent greenhouse gases (GHGs) produced in biomass burning, including methane (CH4) and nitrous oxide (N2O), have persistent effects, because equivalent volumes of these greenhouse-effective gases are not so directly returned to plants or otherwise removed rapidly from the atmosphere.
Fire Intensity is dependant on season
Fire intensities vary markedly with season, being relatively mild and spatially patchy early in the dry season, when air temperatures are relatively lower and fuels retain some moisture. Little coarser woody fuel is consumed in most early dry season fires. But fires burn more fiercely in the late dry season, when ambient air temperatures are higher and moisture levels lower. As a consequence, more coarse fuels are consumed. Large, smouldering stems produce much higher volumes of CH4 and N2O than do rapidly combusted fine, mostly grassy fuels. Studies in the region have shown that emissions of these potent greenhouse gases are relatively greater in late dry season fires than in early fires.
Fire frequency in the tropical savannas 1997- 2010, with fire mapping derived from coarse resolution (~1 km2 pixels) AVHRR imagery. Tropical savannas region north of line, derived from IBRA regionalisation.
Timing of strategic burning
This seasonal variation in amounts and types of GHG production with timing of fire is critical, because it provides options to intervene in fire regimes in greenhouse-relevant ways. Strategic early burning helps reduce the total area and total biomass burnt, as well as the proportion of woody fuels burned, by reducing the incidence, rate of spread and extent of late dry season wildfires. Savanna fire abatement projects seek to reduce GHG emissions to the atmosphere over a specified time period, rather than to increase capture and storage (sequestration) outside the atmosphere. Partial decoupling of emissions and carbon storage occurs because fuel that is not consumed quickly by fire is removed by chiefly invertebrate consumers whose metabolic products are taken up by soils instead of being immediately released to the atmosphere. For further reading please download PDF: Frequent fires reduce tree growth in northern Australian savannas: implications for tree demography and carbon sequestration by Brett P. Murphy, Jeremy Russell-Smith and Linda C. Prior.
It is important to appreciate that a savanna fire abatement project differs substantially and essentially from forestry-style sequestration projects established under different provisions of the Kyoto Protocol. No sequestration is involved; rather, accredited abatement projects operate against a pre-project baseline. Emissions abatement may be achieved annually against that baseline, both through reduction in the overall area (hence amount of fuels burnt), and also by shifting the intensity / seasonality of burning (also reducing amount of fuels burnt) through the undertaking of strategic management practices (e.g. prescribed burning of strategic firebreaks; prescribed burning earlier in the year to implement more patchy, more low intensity fires).