A plant functional approach to the prediction of changes in Australian rangeland vegetation under grazing and fire

Grazing by domestic livestock and changed fire regimes by humans have caused major changes in the productivity and composition of rangelands in Australia and other continents. Of particular concern are the commonly observed loss of perennial forage species and the increasing abundance of woody plants. Grazing and fire-induced changes are difficult to predict from current process knowledge and are often too costly or time-consuming to investigate experimentally. We describe the development and use of ARENA, a new simulation model. A plant functional approach is used in which the relative growth rate, competitive ability and life cycle of the plant types are mainly defined by the plant's morphology and allocation pattern, plus its water-use efficiency and nitrogen concentration. The soil and plant types can be parameterized to a large extent with information from the literature, facilitating application in a broad range of dryland environments. The model has been tested for two soils and pasture communities in the seasonally dry tropics of the Victoria River District, N Australia. Predictions of pasture production and perennial grass fraction under undisturbed conditions agreed with observations in field exclosures. Predictions of maximum tree density also coincided with observations along a rainfall gradient. Simulation experiments were conducted to explore the effect of different stocking levels and fire management regimes on pasture productivity and composition. Responses of pastures on red loam and grey clay soils were generally consistent with regional field experience, but the model did not reproduce the expected changes in the abundance of woody plants.