Invasive alien trees and water resources in South Africa: case studies of the costs and benefits of management

Invasive alien plants are consumptive water-users, and may have reduced river flows in South Africa by about 6.7% according to a broad-scale study. An effective programme to bring the invasions under control would cost about US $ 92 million per year for the next 20 years. This paper reports on studies of four representative catchments (the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand) to assess the impacts and costs of invasions at a scale that is more relevant to managers. Several alien plant species have invaded the catchments. Non-riverine invasions are mainly Pinus and Hakea species in Sonderend and Keurbooms, eucalypts in the Upper Wilge, and pines and scramblers (e.g. Lantana camara) in the Sabie-Sand catchment. Riverine invasions are dominated by Acacia mearnsii and, to a lesser extent, A. dealbata, except in the Sabie-Sand and the lower Sonderend River where Eucalyptus species are important. About 44 % of the Sonderend, 54 % of the Keurbooms, 2 % of the Upper Wilge and 23 % of the Sabie-Sand catchments has been invaded to some degree. The corresponding reductions in the natural river flows attributed to these invasions are about 7.2, 22.1, 6.0 and 9.4%. If the invasions are not controlled they could potentially spread, and occupy 51, 77, 70%, respectively, of the first three catchments. At an annual expansion rate of 10–15 % this would take about 13, 26 and 63 years, respectively. The invadable areas in the Sabie-Sand catchment are already invaded so invasions will only increase in density. It would take about 26–30 years to reach 100 % canopy cover. The projected flow reductions for the four catchments would increase to 41.5, 95.5, 25.1 and 22.3%