Impact of savanna burning on surface properties and feedbacks to local and regional climate

Tropical savanna ecosystems account for 11.5% of the global landscape (Scholes and Hall 1996). Up to 75% of this landscape burns annually (Hao et al. 1990) and 50% of all biomass burning in tropical regions originates from savannas (Hao and Liu 1994). The wet-dry tropics of northern Australia feature extensive tracts of savanna vegetation which occupy approximately 2 million km². This area is equivalent to 12% of the world's tropical savanna estate, making this savanna biome of global significance. Fire is arguably the greatest natural and anthropogenic environmental disturbance in this region. Vast tracts are burnt each year by pastoralists, aboriginal landholders and conservation managers (Russell-Smith et al. 2000; Williams et al. 2002). Fire in Australian savannas, results in a scorched canopy that dramatically reduces the green Leaf Area Index (LAI) and blackens the soil. These surface changes are likely to result in altered energy partitioning (enhanced sensible heat flux) and shifts in albedo. In addition, the aerodynamic and biological properties of the ecosystem may change, affecting surface-atmosphere coupling. For example, a loss of canopy leaf area due to fire could reduce canopy photosynthesis and evapotranspiration, greatly influencing post-fire fluxes of water and carbon. We measured radiative, energy and carbon exchanges over unburned and burned open forest savanna at Howard Springs, Darwin, Australia. Fire affected the radiative balance immediately following fire through the consumption of the grass-dominated understorey and blackening of the surface. Albedo was halved following fire (0.12 to 0.06). A moderate intensity fire resulted in a comprehensive canopy scorch and almost complete leaf drop in the weeks following fire. The shutdown of most leaves within the canopy reduced transpiration and altered energy partitioning. Leaf death and shedding also resulted in a cessation of ecosystem carbon uptake and the savanna turned from a sink to a source of carbon to the atmosphere because of the continued ecosystem respiration. Post-fire, the Bowen ratio increased greatly due to large increases in sensible heat fluxes. These changes in surface energy exchange following fire, when applied at the landscape scale, may have impacts on larger scale climate. At the local scale, enhanced sensible heat fluxes over patches of burnt landscape could generate mesoscale circulation systems (Knowles 1993). Variations in atmospheric heating rates above burnt and unburnt savanna and associated horizontal pressure gradients will produce atmospheric motion at a range of scales. In order to examine these processes we performed a sensitivity analysis using a global climate model (CSIRO Conformal-Cubic Atmospheric Model (C-CAM)). This model uses a stretched grid, so that the model has a resolution of about 60 km over Australia, and far field nudging provided by the NCEP/NCAR Reanalysis. In the first phase of the study, changes of the vegetation properties by fires are implemented in the C-CAM land surface model by modifying key vegetation parameters such as albedo, leaf area index and roughness length, accounting for the vegetation succession after the fires, based on observational data and plant physiological information. In a sensitivity study, results from a 21-year model run (1979 to 1999) with 5-years spin up time (1974 to 1978) are compared to simulations without any fires. Observational data are used to assess the suitability of the approach and the model. The impact on the atmosphere is analysed by response metrics for each year, i.e. Australian Monsoon Index, location and central pressure of Cloncurry and Pilbara heat lows, ITCZ properties over Australia and precipitation parameters for special areas. Initial results show a generalised increase in convection associated with burned areas that leads to high precipitation (Gorgen, et al. 2006). There are also indications of circulation changes through a strengthening of NW flow. As such, these results suggest that local-to-regional scale circulation changes associated with burning may modify patterns of precipitation and could potentially affect the strength of the Australian monsoon as suggested by Beringer et al. (2003).