Keith Smettem

Salinization of land is a form of desertification; salinization of rivers is a global threat to biodiversity and compromises the ecosystem goods and services of rivers, wetlands, and lakes. Salinization is caused by flooding or inundation with saline waters, breaching of dykes, storm surges, tsunamis, or the drying of large inland water bodies. Salinization can result where irrigation waters are compromised by salinity. Salinity intersects with major global concerns, including food security, desertification, and biodiversity protection. Soil salinity results from an excess of salts in the soil that reduces plant growth and crop productivity and affects soil biological activity. Salinized soils impose an osmotic stress on plants, reducing water uptake and concentrating toxic level of sodium and chloride. Different plant species exhibit different degrees of salinity tolerance. Salinization removes arable land from production, causing abandonment globally of 0.3–1.5 million hectare year−1. With adequate drainage, salts can be leached and the soil recovered but where the water table remains near the surface, the salinity problem will remain. It may be possible to reverse the effects of salinization. A crucial consideration is whether the desired end point is stabilizing the soils against further change, or reversing the process and restoring soils to another state. Approaches include prevention, stabilization, active management, or land retirement or abandonment. Successful restoration of salinity at the landscape-scale relies on broadscale land-use change. This is problematic where the most profitable land-use is agriculture, thus there has therefore been considerable investigation of land-use systems that at least replicate the profitability of the current agricultural system. Recent approaches have explored how to make the higher water using farming systems acceptable by making the replacement plants profitable in their own right.

Salinization threatens up to 17 million hectares of Australian farmland, major fresh water resources, biodiversity and built infrastructure. In higher rainfall (>600 mm/year) areas of south-western Australia a market based approach has resulted in the reforestation of over 280,000 ha of farmland with Eucalyptus globulus plantations. This has had significant collateral environmental benefits in terms of reducing salinity in several watersheds. This model has not been replicated in the lower (300–600 mm/year) rainfall areas of this region, which is a global biodiversity hotspot. In this area, conventional forestry species have lower wood yields and longer rotations, compromising profitability, and reinforcing land-holder preference to maintain existing agricultural activities. Two complementary strategies are being used to restore landscape function across this drier region, through increased reforestation. The first is to shift from the paradigm of forestry comprising tall trees grown in relatively long rotations and producing timber to one based on the production of a range of biomass products (bioenergy, chemicals, sequestered carbon), and environmental services such as providing fresh water. As a consequence of breaking this paradigm, silvicultural practices such as stand densities and rotation length can also be redefined. The second strategy is to integrate these new systems into the existing dryland farming systems. Four broad approaches are being assessed viz. (a) belts of trees with farming maintained in inter-row alleys, (b) blocks of trees located on areas of water accumulation or of high recharge, (c) adjusting species selection to soil conditions, such as those that are shallow or saline, and (d) alternating short phases (3–5 years) of trees with farming. These systems offer the prospect of sequestering carbon, and producing wood or biofuels from farmland without displacing food production.

This chapter mainly examines the second option and describes the major aspects of producing short rotation pulpwood crops on farmland, from the perspective of the independent private grower who finances the trees, undertakes or supervises all the work and sells the pulpwood on harvest. The information is general in nature as many management practices are tailored to suit particular growers, sites and market situations. Establishing a commercial plantation is a significant investment and it is recommended that landholders seek specific additional advice before proceeding.

In this chapter we review some of the key aspects to consider when planning revegetation strategies that use trees to manage local and regional water balances. We commence by reviewing how trees use water and how they respond to environmental conditions, including drought, waterlogging and salinity. Understanding these responses are critical when selecting suitable species for managing the water balance in specific landscape locations. After providing definitions of typical ground water systems we introduce the principle of ecological optimality and use this to explore available design options to manage the water balance using trees in dryland catchments (typically, 300—600 mm mean annual rainfall). We conclude that the prospects for lowering watertables by revegetation with perennial vegetation would appear to be best in local groundwater systems, or where annual rates of groundwater inflow are considerably less than annual transpiration losses.

Plantations and farm forestry represent significant land uses in parts of SWWA, with climate variability and change being major considerations in managing the existing resource and future expansion plans. Impacts of climate change on natural forests are considered in the section dealing with biodiversity. Development work on carbon sinks is a direct response to changes in international climate change policy, particularly the Kyoto Protocol to the United Nations Framework Conve on Climate Change (U ntion NFCCC).

To date, the Australian government has attempted to use various legislative and policy initiatives to manage the spread of dryland salinity and protect the natural environment. Despite these initiatives, the area of land affected by dryland salinity continues to increase and may now be difficult to control using existing management capabilities. This paper evaluates three quite different approaches to dryland salinity management which have been attempted in the Kent River catchment, located in the southwest of Western Australia. The approaches are; regulatory, co-operative and market driven. The implementation and impact of each management approach are assessed within their respective historical, social and environmental contexts. The assessment reveals that existing regulatory and co-operative approaches have implementation problems and have not been effective in controlling dryland salinity at the catchment scale. It is concluded that although current policy evaluation methods are somewhat underdeveloped, it is possible to combine a number of evaluation approaches in order to gain insight into the advantages and disadvantages of economic and behavioural incentives to manage salinity problems.