THE ROLE OF SHELTER IN AUSTRALIA FOR PROTECTING SOILS, PLANTS AND LIVESTOCK

The purpose of this review is to examine the current knowledge of the role of trees in providing shelter for pastures, crops, and livestock, for controlling erosion of soils and improving productivity and sustainability of agricultural production in Australia - and the extent to which this knowledge has been applied. Land degradation - tree loss and associated soil salinity, water and wind erosion, soil acidification, soil structural decline arid nutrient degradation - is evidence that our primary production systems are not sustainable. We have sought increased production without proper consideration of the ecological context of that system. About half of Victoria's crop and pasture lands are affected or at risk, and in Western Australia about 25% of the cleared agricultural land is wind-eroded and 60% is potentially susceptible, salinity affects 0.43 m ha and half of the divertible surface water is affected by salinity. Similar problems occur in other States. At least 43 m ha or 13% of our rangelands are seriously degraded by wind erosion caused by overgrazing, often coinciding with drought or a run of drier years. 'Minimum tillage' and stubble management for erosion control in cropping has been a major extension and research activity in Australian agriculture. Severe weather, combined with imperfect adoption of appropriate grazing and crop management systems, shows the weakness of complete reliance on these methods of erosion control. An effective system must accommodate the impact of extreme events, which are the most damaging. However, the complementary use of windbreaks to reduce soil erosion is rare, and their establishment has not been promoted, despite the wide-spread adoption of this technology by other countries. In the cropping and higher rainfall grazing areas, the systematic planting of 10% of the land in a net of shelterbelts/timberbelts/clusters could achieve a 50% windspeed reduction; this would substantially improve livestock and pasture production in the short and long-term. Wind erosion could be dramatically reduced and crop production probably increased by the use of windbreaks. Wheat and oat yield at Rutherglen (Victoria), and lupin yield at Esperance (Western Australia), were increased in the sheltered zone by 22% and 47%, and 30%. respectively. In semi-arid and dry temperate areas, planting of 5% of the land to shelter could reduce windspeed by 30-50% and soil loss by up to 80%. This planting would also contribute substantially to achieving other objectives of sustainable agriculture. Agroforestry -particularly timberbelts applications - will be important in the long-term strategy for achieving revegetation. If some of the trees yield a marketable product then the adoption of the system will be more readily achieved. In the arid (pastoral) areas there is an urgent need to promote the ethic that preservation and improvement of the perennial grass and shrub vegetation is critical for die protection of the soil and maintenance of land capability. Control of animal grazing remains die sole means of preventing erosion in much of this zone. While satellite imagery allows us to assess the condition of leasehold lands, we have failed to achieve stocking policies that will halt the degradation of our rangelands.