Invasive alien plants and water resources in South Africa: advances in understanding and predictive ability since 2004 and research challenges
David C. Le Maitre1,2, Mark B Gush1,Sebinasi Dzikiti1
1Natural Resources and the Environment, CSIR, Stellenbosch
2Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University
A key motivation for initiating of the Working for Water programme in 1995 was predictions that invasive alien plants would use significant amounts of water and that clearing invasions would protect water resources. These predictions were based on the results of hydrological experiments on the impacts of commercial tree plantations on water resources and the estimates of the biomass present in invaded areas. During the past 20 years there has been significant progress in the understanding of biophysical processes and the factors that control vegetation and plant water-use, as well as datasets that enable comparisons between indigenous and alien plant species. These include the development, and ongoing improvement in, methods of measuring transpiration and evaporation at levels from the individual plants to stands of plants using sap flow and micro-meteorological techniques. The development and of remote-sensing-based methods of estimating evaporation opens up new approaches to scaling-up invasive tree water use from the site or small catchment level measurements to large catchments and even regions of South Africa. Challenges remain, including adequate data some major invaders, and robust methods for modelling the impacts of invasions on water yields. We require more long-term catchment-based studies of invasions and their control on the water balance. Knowledge of their impacts has not been effectively integrated into catchment management practices and water resource planning, or into funding for catchment management. These knowledge gaps and failures in translating knowledge into action should be given a high priority in research and in knowledge transfer.