Water availability determines hydrological impact of tree belts in dryland cropping systems

Shallow groundwater can often develop when perennial ecosystems are replaced by annual agriculture. Returning trees to these landscapes is one option available to farmers to use more of the incident rainfall and stop the rising water table before it begins to affect production of the traditionally planted crops and pastures. This study examines the growth and water use of mallee (multi-stemmed) eucalypts in landscape positions with different water availability, integrated into an annual cropping and pasture farming system in the dryland agricultural zone of Western Australia. We found a ten-fold difference in biomass accumulation and a six-fold difference in rates of water use depending on water availability. Water use on a leaf area basis appeared to be independent of changes in water availability but transpiration efficiency in terms of grams of carbon fixed per litre of water transpired increased with increasing water availability. Thoughtful placement of trees in the landscape will minimize the area of land required to be planted to have the desired effect on the local hydrology, thus maximizing the amount of land on which the traditional annual crops and pastures can continue to be grown.

► Presence of a fresh water table resulted in a ten-fold increase in growth.
► Rates of water use increased 6-fold with increased water availability.
► Transpiration efficiency appeared to increase with increasing water availability.
► Accessible groundwater may increase the lateral extent of root development.