Carbon balance of a eucalypt open forest savanna of Northern Australia
Creators: name: Chen, Xiaoyong
Publisher: name: Charles Darwin University
Year: 2002

description: Tropical savannas, a vegetation system comprising both trees and grasses in varying proportions, cover more than 10% of the global land surface and are a significant biome. These plant communities are largely distributed in the developing world and are subjected to high levels of population and development pressure. Australian tropical savannas are not subjected to such land use pressures, although this is slowly changing. They are and extensive ecosystem, occupying approximately 25% of the continent in the wet-dry tropics of northern Australia, a region that is subjected to a strongly seasonal, monsoonal climate. Over their entire range in northern Australia, these ecosystems receive annual rainfalls between 500 mm to 1900 mm. Given their extensive distribution, it is likely that tropical savannas contribute significantly to the continental-scale carbon balance of Australia. This thesis investigates the carbon balance of the Eucalyptus miniata/Eucalyptus tetrodonta dominated open-forest savannas, the dominant ecosystem of the tropical savannas above 1200 mm rainfall isohyet in northern Australia. Study sites were located near Darwin (130° 45'E, 12° 30'E), Northern Territory and receive approximately 1600 mm rainfall annually. The study concentrated on the seasonal and annual carbon budget for these savannas, with carbon stocks, sequestration rates and sink strength of this ecosystem examined. Gross Primary Productivity (GPP), Net Primary Productivity (NPP) and Net Ecosystem Productivity (NEP) were estimated. In addition, the potential long-term savanna carbon sink strength, the Net Biome Productivity (NBP) was estimated from NEP values and losses of carbon via non-respiratory pathways, mainly fire. The total carbon (C) stock of this savanna ecosystem was 202 ±52 t C ha-1, with approximately 80% stored below-ground and 20% above-ground. Soil organic carbon content (SOC) for 0-1 m depth was 151 ± 33 t C ha-1, accounting for about 75% of the total C content of the ecosystem. Standing biomass was 48 ± 18 t C ha-1, 35% of which was found in the root component and 65% in above-ground live tree components. Annual GPP was 20 t C ha-1 y-1 • NPP wa$ estimated at 10 t C ha-1 , 50% of GPP. Above-ground net primary productivity (ANPP) was low at 3 t C ha-1 and productivity was dominated by below-ground net primary productivity (BNPP), which accounted for 70% of total NPP. Below-ground productivity was dominated by fine-root production (NPPfr), which was 6 t C ha-1 , of which 5 t C ha-1 (80%) occurred during the wet season. This high below-ground C allocation was offset by high rates of soil carbon effiux (Fcs, root respiration plus soil heterotrophic respiration) which was 14 t C ha-1 y-1 , with 70% of this flux occurring during the wet season. These data suggest that the mesic savannas are a relatively strong C sink on an annual basis, with an NEP of 2.8 t C ha-1 • However during the dry season, this ecosystem was a C source, with NEP becoming negative at - 0.4 t C ha-1 • This estimate of productivity does not include the impacts of fire, which were not specifically examined in this study. However, using literature values, the non-respiratory loss of C via vegetation burning was estimated in order to calculate the true savanna sink strength, NBP (NEP minus carbon losses due to disturbance agents, mainly fire). Fire could potentially consume more than 50% of sequestered C, with NBP estimated at 1.3 t C ha-1 .