However, invasions (including those spreading from plantations) can also
increase fuel loads and fire intensity and severity, especially where there are coarse
dead fuels on the soil surface, potentially leading to severe erosion and flooding
downstream (Scott 1993; van Wilgen and Scott 2001; Le Maitre et al. 2014). Dense
and closed stands of eucalypts and wattles suppress ground-layer vegetation, altering
infiltration and reducing soil stabilisation, leading to increases in erosion, potentially
decreasing water quality and increasing sedimentation of dams (van der Waal et al.
2012) and reducing grazing capacity (Yapi 2013).
Although long-term catchment studies have demonstrated the impacts of plantations on mean annual runoff, the major invading tree species are the same as those in
the plantations, and reach comparable stand densities and stature, the Department of
Water Affairs and Sanitation has not acknowledged that the impacts of invasions can
be as significant as plantations. A good example is the impacts of invasions on the
yields from water supply systems (Table 15.2). These impacts have not been
explicitly taken into account in supply augmentation plans, except for Berg River
Dam and De Hoop Dam where control measures were included in the construction
budgets (Preston et al. 2018). The growing body of evidence supporting reductions
in yields should inform any future decisions about water supply systems as well as
motivating for effective invasive alien plant control measures as part of the
maintenance.
Acknowledgements We thank the Natural Resource Management Programmes and the Working
for water Programme for funding and supporting much of the research that has been reviewed for
this chapter.
References
Allen R, Pereira L, Raes D, Smith M (1998) Crop evapotranspiration: guidelines for computing
crop water requirements. FAO Irrigation and Drainage Paper No. 56, FAO, Rome
Bailey AK, Pitman WV (2015) WR2012 Study User’s Guide Version 1. Water Resources of
South Africa 2012 Study (WR2012). Report K5/2143/1, Water Research Commission, Pretoria
Bennett BM, Kruger FJ (2014) Forestry and water conservation in South Africa: history, science
and policy. ANU E Press, The Australian National University, Canberra. https://doi.org/10.
22459/FWCSA.11.2015
Blignaut J, Mander M, Schulze R et al (2010) Restoring and managing natural capital towards
fostering economic development: evidence from the Drakensberg, South Africa. Ecol Econ
69:1313–1323. https://doi.org/10.1016/j.ecolecon.2010.01.007
Bond WJ, Midgley GF (2000) A proposed CO 2 -controlled mechanism of woody plant invasion in
grasslands and savannas. Glob Chang Biol 6:865–869. https://doi.org/10.1046/j.1365-2486.
2000.00365.x
Bond WJ, Midgley GF (2012) Carbon dioxide and the uneasy interactions of trees and savannah
grasses. Philos Trans R Soc Lond Ser B Biol Sci 367:601–612. https://doi.org/10.1098/rstb.
2011.0182
Bosch JM, Hewlett JD (1982) A review of catchment experiments to determine the effect of
vegetation changes on water yield and evapotranspiration. J Hydrol 55:3–23. https://doi.org/
10.1016/0022-1694(82)90117-2
450
D. C. Le Maitre et al.
increase fuel loads and fire intensity and severity, especially where there are coarse
dead fuels on the soil surface, potentially leading to severe erosion and flooding
downstream (Scott 1993; van Wilgen and Scott 2001; Le Maitre et al. 2014). Dense
and closed stands of eucalypts and wattles suppress ground-layer vegetation, altering
infiltration and reducing soil stabilisation, leading to increases in erosion, potentially
decreasing water quality and increasing sedimentation of dams (van der Waal et al.
2012) and reducing grazing capacity (Yapi 2013).
Although long-term catchment studies have demonstrated the impacts of plantations on mean annual runoff, the major invading tree species are the same as those in
the plantations, and reach comparable stand densities and stature, the Department of
Water Affairs and Sanitation has not acknowledged that the impacts of invasions can
be as significant as plantations. A good example is the impacts of invasions on the
yields from water supply systems (Table 15.2). These impacts have not been
explicitly taken into account in supply augmentation plans, except for Berg River
Dam and De Hoop Dam where control measures were included in the construction
budgets (Preston et al. 2018). The growing body of evidence supporting reductions
in yields should inform any future decisions about water supply systems as well as
motivating for effective invasive alien plant control measures as part of the
maintenance.
Acknowledgements We thank the Natural Resource Management Programmes and the Working
for water Programme for funding and supporting much of the research that has been reviewed for
this chapter.
References
Allen R, Pereira L, Raes D, Smith M (1998) Crop evapotranspiration: guidelines for computing
crop water requirements. FAO Irrigation and Drainage Paper No. 56, FAO, Rome
Bailey AK, Pitman WV (2015) WR2012 Study User’s Guide Version 1. Water Resources of
South Africa 2012 Study (WR2012). Report K5/2143/1, Water Research Commission, Pretoria
Bennett BM, Kruger FJ (2014) Forestry and water conservation in South Africa: history, science
and policy. ANU E Press, The Australian National University, Canberra. https://doi.org/10.
22459/FWCSA.11.2015
Blignaut J, Mander M, Schulze R et al (2010) Restoring and managing natural capital towards
fostering economic development: evidence from the Drakensberg, South Africa. Ecol Econ
69:1313–1323. https://doi.org/10.1016/j.ecolecon.2010.01.007
Bond WJ, Midgley GF (2000) A proposed CO 2 -controlled mechanism of woody plant invasion in
grasslands and savannas. Glob Chang Biol 6:865–869. https://doi.org/10.1046/j.1365-2486.
2000.00365.x
Bond WJ, Midgley GF (2012) Carbon dioxide and the uneasy interactions of trees and savannah
grasses. Philos Trans R Soc Lond Ser B Biol Sci 367:601–612. https://doi.org/10.1098/rstb.
2011.0182
Bosch JM, Hewlett JD (1982) A review of catchment experiments to determine the effect of
vegetation changes on water yield and evapotranspiration. J Hydrol 55:3–23. https://doi.org/
10.1016/0022-1694(82)90117-2
450
D. C. Le Maitre et al.