The dynamics of grazed woodlands in southwest Queensland, Australia and their effect on greenhouse gas emissions

This study outlines the development of an approach to evaluate the sources, sinks, and magnitudes of greenhouse gas emissions from a grazed semiarid r angeland dominated by mulga (Acacia aneura) and how these emissions may be altered by changes in management. This paper describes the modification of an existing pasture production model (GRASP) to include a gas emission comp onent and a dynamic tree growth and population model. An exploratory study was completed to investigate the likely impact of changes in burning practi ces and stock management on emissions. This study indicates that there is a fundamental conflict between maintaining agricultural productivity and red ucing greenhouse gas emissions on a given unit of land. Greater agricultura l productivity is allied with the system being an emissions source while pr oduction declines and the system becomes a net emissions sink as mulga dens ity increases. Effective management for sheep production results in the sys tem acting as a net source ( similar to 60-200 kg CO2 equivalents/ha/year). The magnitude of the source depends on the management strategies used to m aintain the productivity of the system and is largely determined by startin g density and average density of the mulga over the simulation period. Prio r to European settlement, it is believed that the mulga lands were burnt al most annually. Simulations indicate that such a management approach results in the system acting as a small net sink with an average net absorption of greenhouse gases of 14 kg CO2 equivalents/ha/year through minimal growth o f mulga stands. In contrast, the suppression of fire and the introduction o f grazing results in thickening of mulga stands and the system can act as a significant net sink absorbing an average of 1000 kg CO2 equivalents/ha/ye ar. Although dense mulga will render the land largely useless for grazing, land in this region is relatively inexpensive and could possibly be develop ed as a cost-effective carbon offset for greenhouse gas emissions elsewhere . These results also provide support for the hypothesis that changes in lan d management, and particularly, suppression of fire is chiefly responsible for the observed increases in mulga density over the past century. (C) 2001 Elsevier Science Ltd. All rights reserved.