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

Abstract

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%, respectively. The estimated cost of the control programmes to prevent these losses would be about US$ 13.2, 9.9, 4.1 and 6.6 million for the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand catchments, respectively. Should the catchments be allowed to become fully invaded before control operations were started, then the costs would rise to US$ 86.5, 20.5, 278.0 and 11.1 million, respectively. The impacts and costs are significant and are comparable with those calculated independently for other South African catchments. Water is acknowledged to be a key constraint to economic growth in South Africa and there is considerable pressure for efficient and sustainable use of the limited water resources. The projected impacts would justify control programmes aimed at clearing alien invaders for water conservation. #

Ecological implications for assisted migration in Canadian forests

“…S.T. Blake) in the Florida Everglades (Turner et al 1998), Pinus species, Eucalyptus species, and others in South Africa (Richardson 1998, Le Maitre et al 2002, lodgepole pine in New Zealand (Ledgard 2001), and Scots pine in Nova Scotia and Ontario (Catling and Carbyn 2005). On the other hand, some intercontinental introductions have resulted in "naturalized" populations with less aggressive behaviour, e.g., Norway spruce in the forests of eastern North America (e.g., Stover and Marks 1998, Hunter and Mattice 2002, PDCNR 2011; sycamore (Acer pseudoplatanus L.) in England (Peterken 2001); or lodgepole pine in Scandinavia (Knight et al 2001).…”

“…1, this assessment is a decision-making step for the modes of AM with higher risk, portrayed by a key question: "Do the benefits of translocation outweigh the biological, social, and economic costs and constraints?" In contrast to smaller weed species, trees that become invasive following assisted migration practices are also likely to be more easily identified and controlled through traditional silvicultural methods (Ledgard 2001, Le Maitre et al 2002; among the trees and shrubs that have been introduced to Britain, it has been observed that extensive control programs have been able to eradicate Rhododendron ponticum L., despite its significant threat to ecosystems (Peterken 2001).…”

Measurements of canopy interception and transpiration of openly-grown eastern redcedar in central Okalhoma

“…Considerable research has been conducted assessing the impacts of invasive plant species on local and regional hydrology (Le Matire et al, 2011). Le Maitre et al (2002) studied the impact of invasive alien trees in the Sonderend, Keurbooms, Upper Wilge, and Sabie-Sand catchments in South Africa. At the time of the study (2002), 44, 54, 2, and 23%, respectively, of each catchment had been invaded by non-native trees resulting in 7.2, 22.1, 6.0, and 9.4% reductions in river flow.…”

Physiological mechanism for the reduction in soil water in poplar (Populus deltoides) plantations in Dongting Lake wetlands

“…The water-pumping effects of fast-growing tree plantations have been clarified in different ecosystems, such as in semiarid areas (Wilske et al 2009;Tan et al 2011) and wetlands (Le Maitre et al 2002;Hernández-Santana et al 2008;Migliavacca et al 2009). For example, poplar plantations have the potential to extract groundwater and to reduce the water table in wetland ecosystems (Migliavacca et al 2009).…”