Global change impacts on native pastures in south-east Queensland, Australia

Increases in atmospheric concentrations of greenhouse gases such as carbon dioxide (CO₂) are likely to impact on grazing industries through direct effects on plant growth and through possible changes in climate. Assessment of the likely direction and magnitude of these impact requires development of appropriate modelling capacities linked with experimental work. This paper documents the adaption of an existing soil-pasture-livestock model, GRASP, to simulate system responses to changes in CO₂. The adapted model is then used to compare these responses under current climate and CO₂ conditions with four possible future scenarios: (1) doubled CO₂; (2) doubled CO₂ and increased temperature; (3) as in the previous scenario but with a drier climate; and (4) as in (2) but with a wetter climate. These studies suggest that CO₂ changes alone are likely to have beneficial effects, with increased pasture growth, increased and less variable liveweight gain, and increased ground cover. However, subsoil drainage is likely to increase. Growth responses to CO₂ are likely to be greater in drier years than in wetter years partly due to nitrogen limitations in the soils of the region. Increases in temperature in combination with CO₂ further increased animal production due to the increased number of growing days in the cooler months. The increased rainfall scenario had few additional positive effects but further increased subsoil drainage. In contrast, the drier scenario had reduced plant and animal production when compared with current conditions even though seasonal transpiration efficiency was increased by 20% due to increased CO₂.