Water and Australia's Forests

The total area of public forests managed primarily for protection grew by more than 8% in the five years to 2005-06, to over 30 million hectares (20% of the total forest area). This area includes forests in public nature conservation reserves (such as national parks), areas in multiple-use public forests in which harvesting and roading are not permitted, and more than four million hectares of forest managed specifically for water supply. In most of these forests, timber harvesting and grazing are prohibited and infrastructure development is minimal. Recreation may be permitted, although some forests are completely closed to the public.

All multiple-use public forests are subject to codes of practice designed, among other purposes, to reduce soil erosion, minimise damage to soil physical properties and to maintain water quantity and quality. For example, the areas in managed forests most susceptible to erosion, such as log extraction tracks, log landings and logging access roads, must be rehabilitated after harvesting to specified standards. Most private forests are subject to similar codes of practice.

Fire is perhaps the biggest threat to the quality of water flowing from forests. In 2003, a bushfire burnt more than 840 km of protection forest in the main water catchments of the Australian Capital Territory. Subsequent intense rainstorms washed massive amounts - an estimated 27 years worth - of sediment and burnt organic and mineral material from stream banks and hill slopes into streams. The Bendora, Corin and Cotter reservoirs, which form the Territory's main water supply, became so polluted that water authorities closed them temporarily.

Increased erosion is not the only way bushfires affect water supply. In most native forests it also stimulates vigorous forest regrowth, which can reduce water yields in affected catchments for decades.

Salinity

Dryland salinity is a widespread and growing problem in Australia. It occurs predominately on cleared agricultural land and also affects adjacent forests. A contributing factor is the previous widespread clearing of deep-rooted native tree species, which caused watertables to rise and to mobilise salt in the soil profile. The resultant waterlogging and surface salinity can reduce biodiversity and agricultural and forestry productivity, damage infrastructure, and jeopardise the supply of potable water to urban areas.

Most states have salinity monitoring programs. Rehabilitation and salinity management are often a joint effort between government and communities and involve various strategies such as tree planting, the regeneration and improved management of native forests and other vegetation, and strategic water use and redirection, including pumping.

In 2000, the Council of Australian Governments endorsed a National Action Plan for Salinity and Water Quality. This action plan included a joint commitment between the Australian, state and territory governments to provide $1.4 billion over seven years for regional solutions to salinity and water-quality problems. In addition, the Australian Government committed $1.3 billion to repair and conserve the natural environment and ensure the sustainable use of the nation's natural resources. As of 30 June 2005, governments had approved $858 million of regionally focused investment through these programs. Of this, about $80 million had been spent on activities with a focus on native vegetation, including the development of almost 1,420 conservation covenants and agreements, and vegetation improvement and revegetation covering over 180,000 hectares. Other major new initiatives in tree planting and the implementation of increased controls on land clearing in salinity-risk areas are also under way.

Plantation water use

Plantations can have positive environmental effects by lowering saline watertables, but some communities have become concerned that they can also reduce water availability for other uses, such as irrigated agriculture. Plantation development is one of the land-use changes covered by the Intergovernmental Agreement on a National Water Initiative, which provides a national framework for considering the impacts of activities that may intercept water. Research is also being conducted into the effects of plantation management on water quality and quantity. For example, the results of research into the effects of forecast plantation expansion in the Upper Murray basin are shown in the table below.

Forest plantations in Australia occupy only a small percentage of the catchments in which they occur. Because rainfall and hydrological factors are highly variable, it is difficult to measure the impact of plantations on water yields in small catchments if the plantations occupy less than 15%-20% of the catchment (this threshold is lower in larger catchments). In most catchments, the plantation proportion is much less than 15%.

Further reading

Bari M and Ruprecht J (2003). Water Yield Response to Land Use Change in South-western Western Australia Salinity and Land Use Impact Series, Report No SLUI 31. Department of Environment, Perth.

Benyon R, Theiveyanathan S and Doody T (2006). Impacts of tree plantations on groundwater in Southeastern Australia. Australian Journal of Botany 54:181-192.

Davey S, Baker P, Frakes I and Mullen I (2006). Opportunities for Commercial Environmental Forestry in Australia Bureau of Rural Sciences, Canberra.

McIntosh P and Laffan M (2005). Soil erodibility and erosion hazard: extending these cornerstone conservation concepts to headwater streams in the forest estate in Tasmania. Forest Ecology and Management 220:128-139.

MIG (2008). Criterion 4 - Conservation and maintenance of soil and water resources, State of the Forests Report 2008. Montreal Process Implementation Group for Australia, Canberra.

Parsons M, Frakes I and Gerrand A (2007). Plantations and Water Use Science for Decision Makers Series, Bureau of Rural Sciences, Canberra.

Prosser I, (2008). Forestry and water. Proceedings, Outlook Conference. Australian Bureau of Agricultural and Resource Economics, Canberra.

Webb A, Jarrett B and Turner L (2007). Effects of plantation forest harvesting on water quality and quantity: Canobolas State Forest, NSW. In: Proceedings of the 5th Australian Stream Management Conference. Australian Rivers: Making a Difference Wilson A, Deehan R, Watts R, Page K, Bownan K and Curtis A (eds), Charles Sturt University, Thurgoona.