Keith Smettem

Rising ground-waters and resultant salinity threaten agricultural land, conservation reserves and water resources in southern Australia. Although revegetation with woody plants is often considered as a strategy to restore catchment water balances, farm forestry has not been adopted in low-rainfall environments to the extent of that in high rainfall zones. Similarly, there is some conjecture that the proportion of revegetation needed to restore catchment water balances may be as high as 80%. The JVAP publication Trees, Water and Salt provides a set of guidelines for revegetation of farmland, however these have not been tested at the catchment scale in drier (<400 mm annual rainfall) environments that are representative of the wheat and wool-belt of much of southern Australia. Reforestation in these regions is often of limited scale, and thus at an inappropriate scale to assess catchment scale responses. This study measured the hydrologic response of an 80 ha catchment to partial reforestation, near Wickepin, Western Australia. This region, which has around 300 mm annual rainfall, has agriculture that comprises rotations of cropping and pastures. These trees were established using the procedures outlined in Trees, Water and Salt. An issue with dryland reforestation has been the lack of clear economic drivers. The emergence of markets for carbon sequestration and bioenergy from trees, in response to national climate change policies may increase the future rate of reforestation. Key issues include understanding the rates of both sequestration and biomass production in drier environments such as Wickepin and also how best to integrate reforestation with agricultural production. Of particular interest are the interaction of belts of trees with agriculture and the utilization of land that is poorly productive, such as that which has been affected by salinity.

This report presents the results of a field trial using phase farming with trees, a system designed to grow tree crops in short rotations of 3-5 years to intentionally deplete soil water as a buffer for agriculture in areas prone to dryland salinity. The trial showed that on suitable soils, a short tree phase will deplete soil water sufficiently to allow a subsequent return to agriculture for a 11-20 year rotation. Biomass production from the tree phase was also measured, with a view to emerging biomass energy and biofuel production systems. Some of the technical and policy issues that need resolution before this farming system can be introduced in southern Australia are discussed.