Prospects for Managing Salinity in Southern Australia Using Trees on Farmland

Abstract

Trees are typically planted in salt-affected agricultural landscapes of southern Australia for environmental and economic benefit. Environmental benefits include land reclamation and containing groundwater rise, reducing salt loads in streams, carbon sequestration, and enhanced biodiversity. Most efforts have concentrated on managing dryland (rainfed) catchments where recharge (water intake) and discharge (water outflow) locations are usually well defined. Trees are typically planted in agroforestry, compact plantation, or shelterbelt configurations mostly in recharge areas or upslope of discharge areas and, less commonly, to stabilize discharge areas. In saline discharge areas, the amount and concentration of salt to which tree roots may be exposed varies with landscape position, salt load in the soil regolith, management practices, the extent of lateral subsurface flow of water, and the degree to which saline soil water and groundwater are used by trees. Tree survival and growth are progressively decreased at higher soil and groundwater salinities, with the extent of this reduction depending on tree species’ (genotype) tolerance to salinity and associated stresses, such as waterlogging and sodicity. Several Australian native tree species, including Eucalyptus camaldulensis, E. occidentalis, and E. spathulata perform consistently well on saline soils. The use of selected eucalypt hybrid clones may enhance prospects for more economically productive farm forests on saline land. Saline water tables are more likely to be lowered by planting trees in saline areas if trees are planted at an appropriate scale and can reduce recharge and/or use groundwater directly. Opportunities to lower the water table beneath a plantation are greater if lateral flows from surrounding areas are relatively small. However, if the aim is to maximize the use of groundwater by trees for improved tree growth and environmental outcomes, then lateral flow from the surroundings could be an advantage. Case studies are provided from several regions in southern Australia from field experiments and process-based modeling to illustrate growth and water use responses to salinity, opportunities for maintaining water quality of rivers and streams in salinized catchments, and for both utilising and stabilizing salt-affected land in dryland and irrigated situations.