state and private land soon began raising concerns about the drying-up of streams
and rivers downstream of the planted areas, concerns which were opposed by the
foresters who believed the plantations were not the cause (Bennett and Kruger 2014).
These concerns increased following a succession of severe droughts in the 1920s,
and testimonies given to Commissions investigating the causes and consequences of
the droughts. At the fourth British Empire Forestry Conference in South Africa, a
Committee on Forest Influences was established and recommended that a long-term
research programme be initiated to determine the impacts of afforestation on water
supplies (Wicht 1949; Bennett and Kruger 2014; van Wilgen et al. 2016). This
research programme provided unequivocal evidence that tree plantations do reduce
runoff relative to the natural vegetation they have replaced, a finding supported by
numerous catchment-level studies across the world (van Lill et al. 1980; Bosch and
Hewlett 1982; van Wyk 1987; Scott et al. 2004; Farley et al. 2005).
Many of these tree species began to invade the adjacent natural vegetation, a
tendency which was initially praised, but later led to concern about their potential
impacts on river flows as well as biodiversity (Kruger 1977; van Wilgen et al. 1992,
2016). Although the initial forebodings were raised about tree invasions, they soon
extended to the hydrological impacts of species with other growth forms, especially
woody plants (Versfeld and van Wilgen 1986). These issues were raised in various
forums and were also used to motivate for the first assessment of the hydrological
impacts of invasions, which found that they could have a significant impact on Cape
Town’s water security (Le Maitre et al. 1996). The information from this research,
together with other findings, was used to motivate for the establishment of the
Working for Water Programme in 1995 (van Wilgen et al. 1998). The programme
supported ongoing research into the hydrological impacts of invasions, leading to the
first national assessment of their impacts which found that invasions were reducing
the naturalised mean annual runoff across South Africa by about 3.30 billion m
3
(6.7% of the country’s mean annual runoff) (Le Maitre et al. 2000). It also supported
the first assessment of the impacts of Acacia mearnsii (Black Wattle) (Dye et al.
2001; Dye and Jarmain 2004), short-term gains in stream flows following clearing
(Dye and Poulter 1995; Prinsloo and Scott 1999), and the first overview of their
hydrological impacts (Görgens and van Wilgen 2004).
Many other species with different growth forms were also introduced for various
purposes (e.g. fodder, hedges, and horticulture), and invading plant species in
South Africa now include the full range from herbaceous annuals and perennials,
succulents like cacti, scramblers, shrubs and large trees (Richardson et al. 1997;
Henderson 2007). This diversity of invading plant species and growth forms poses a
significant challenge to researchers and managers seeking to understand and quantify the hydrological impacts they could have, because there are too many species to
investigate individually. We discuss below how this challenge is being addressed,
before reviewing the findings of the studies of invasive alien plant impacts to date.
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
433
and rivers downstream of the planted areas, concerns which were opposed by the
foresters who believed the plantations were not the cause (Bennett and Kruger 2014).
These concerns increased following a succession of severe droughts in the 1920s,
and testimonies given to Commissions investigating the causes and consequences of
the droughts. At the fourth British Empire Forestry Conference in South Africa, a
Committee on Forest Influences was established and recommended that a long-term
research programme be initiated to determine the impacts of afforestation on water
supplies (Wicht 1949; Bennett and Kruger 2014; van Wilgen et al. 2016). This
research programme provided unequivocal evidence that tree plantations do reduce
runoff relative to the natural vegetation they have replaced, a finding supported by
numerous catchment-level studies across the world (van Lill et al. 1980; Bosch and
Hewlett 1982; van Wyk 1987; Scott et al. 2004; Farley et al. 2005).
Many of these tree species began to invade the adjacent natural vegetation, a
tendency which was initially praised, but later led to concern about their potential
impacts on river flows as well as biodiversity (Kruger 1977; van Wilgen et al. 1992,
2016). Although the initial forebodings were raised about tree invasions, they soon
extended to the hydrological impacts of species with other growth forms, especially
woody plants (Versfeld and van Wilgen 1986). These issues were raised in various
forums and were also used to motivate for the first assessment of the hydrological
impacts of invasions, which found that they could have a significant impact on Cape
Town’s water security (Le Maitre et al. 1996). The information from this research,
together with other findings, was used to motivate for the establishment of the
Working for Water Programme in 1995 (van Wilgen et al. 1998). The programme
supported ongoing research into the hydrological impacts of invasions, leading to the
first national assessment of their impacts which found that invasions were reducing
the naturalised mean annual runoff across South Africa by about 3.30 billion m
3
(6.7% of the country’s mean annual runoff) (Le Maitre et al. 2000). It also supported
the first assessment of the impacts of Acacia mearnsii (Black Wattle) (Dye et al.
2001; Dye and Jarmain 2004), short-term gains in stream flows following clearing
(Dye and Poulter 1995; Prinsloo and Scott 1999), and the first overview of their
hydrological impacts (Görgens and van Wilgen 2004).
Many other species with different growth forms were also introduced for various
purposes (e.g. fodder, hedges, and horticulture), and invading plant species in
South Africa now include the full range from herbaceous annuals and perennials,
succulents like cacti, scramblers, shrubs and large trees (Richardson et al. 1997;
Henderson 2007). This diversity of invading plant species and growth forms poses a
significant challenge to researchers and managers seeking to understand and quantify the hydrological impacts they could have, because there are too many species to
investigate individually. We discuss below how this challenge is being addressed,
before reviewing the findings of the studies of invasive alien plant impacts to date.
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
433