A review of changing perspectives on Eucalyptus water-use in South Africa

Abstract

South Africa is poor in natural forests and therefore heavily dependent on forest plantations for timber. These were first established in 1875, and currently cover in excess of 1,257,000ha, of which 39.1% comprise Eucalyptus spp. They are concentrated in limited areas of high rainfall which are critically important source areas for rivers. Originally covered by seasonally dormant grasslands and Fynbos (Macchia shrub), their replacement by deep rooted evergreen trees has increased evapotranspiration, and reduced catchment water yields.Concern over impacts of forest plantations on catchment water yields in South Africa were already being expressed by 1915. Research on this subject started in 1937 and continues today. Methodologies have changed over the years, providing increasingly realistic estimates of the hydrological impacts of forest plantations, and changing perceptions of their water-use. Regulations were introduced by Government in 1972 to curb expansion of forest plantations in catchments where water supply is fully committed. A new Water Act in 1998 laid the foundation for a more equitable water allocation system based on the principle of integrated water resource management. Among other adopted principles, it recognizes that water resources can only be effectively managed within larger catchments, that local interested and affected parties should participate in the management of the water resource, and that water must be allocated equitably and used beneficially in the public interest. This has shifted the research effort towards measurement and modeling of ET at large catchment scale, as well as linking water-use to the economic benefits obtained from forests and crops. A clearer and more balanced picture of the efficiency of water-use by forest plantations and other major crop species is now emerging, but further studies are required.The forestry industry showed considerable foresight in initiating research into the water-use efficiency (WUE) of Eucalyptus clones in the 1990s. These investigations need to continue, not only to search for genotypes that limit the current hydrological impacts of forest plantations, but also to explore options for sustainable management of forest plantations into a future with potentially significant climate change. Environmental stresses in plantation trees may increase in response to such factors as a reduction in rainfall, changes in rainfall intensity, changes in rainfall distribution and increased vapor pressure deficits. WUE is likely to decrease in response to greater physiological stress, and clones may be available to minimize production and economic losses under such a scenario.