Water-use dynamics of alien plant invaded riparian forests in South Africa.

Abstract

In South Africa the invasion of riparian forests by alien trees has the potential to affect the country’s limited water resources. It is difficult for government initiatives, such as the Working for Water (WfW) alien clearing programmes, to justify alien tree removal and implement rehabilitation unless a known hydrological benefit can be seen. Consequently water-use within riparian forests at three climate diverse research catchments within South Africa were monitored using the heat ratio method. The use of the Soil Water Assessment Tool (SWAT) and a Sequential Uncertainty Fitting (SUFI-2) algorithm allowed for the auto-calibration of the SWAT model at each research site using measured water-use data. Within the winter rainfall region of the Western Cape the alien stand used nearly six times more water per unit area than the indigenous stand annually. The combined accumulated daily sap flow over a two year period for three Vepris lanceolata and three Acacia mearnsii trees was 36 000 L and 55 700 L respectively, clearly demonstrating the higher water-use of the alien A. mearnsii trees. In contrast, the water-use of alien species within the summer rainfall region of KwaZulu-Natal was double that of the indigenous species. The accumulated seasonal water-use was the least in the indigenous Searsia (~2 100 L), moderate in the indigenous Maytenus (~7 100 L) and high in the alien A. mearnsii (~15 900 L) trees. The spatial distribution of water-use within northern Zululand showed that the commercial forestry areas were the dominant water-users in the catchment. These findings indicate that there would be a hydrological gain if the alien species are removed from riparian forests and rehabilitated back to their natural state. The use of the SWAT model provided substantial insights into the spatial distribution of total evaporation (ET) throughout the selected catchment areas and is a suitable hydrological model for examining the impacts of different land-uses in catchments in South Africa. Given the quantified hydrological benefit, indigenous trees should be promoted for use in rehabilitation programmes where the natural vegetation is or was forests. This is especially relevant in light of South Africa’s limited water resources.