marked impacts on the Fynbos Biome (DEA 2015). The projected increases in the
numbers of high and extreme fire danger days (Le Maitre et al. 2015a) could,
provided there is a suitable source of ignitions, favour fast-maturing invading species
and increase invasion rates and impacts in the Fynbos Biome. Fire return intervals in
moist grassland and savanna are already short, so changes in fire regimes may not
have a significant effect on invasion rates.
Another major driver is the increasing CO 2 concentration in the atmosphere
which favours woody plant densification (bush encroachment), especially when
combined with reduced fuel loads due to grazing (and thus reduced fire intensity)
(Bond and Midgley 2000, 2012; O’Connor et al. 2014). There is good evidence that
native woody plant densification is taking place throughout the Grassland and
Savanna Biomes (Davis-Reddy and Vincent 2017; Skowno et al. 2017), but whether
or not the same is happening with invading species in these biomes is not known.
Fynbos is associated with low soil nutrients which may limit the response of plants to
increased CO 2 . However, pines have mycorrhizae to harvest nutrients and Australian
Acacia species fix nitrogen, so these taxa may become even more aggressive
invaders (Richardson et al. 1994). Any gains in water-use efficiency may be offset
by their increased invasion rates.
There have not yet been any local studies of the impacts of current invasions
under current and future climates, but there are some indications based on the
projected changes in climate across South Africa. The most detailed assessment of
the implications of climate change for water resources and for their planning is the
long-term adaptation scenarios (DEA 2013b). The projected changes are for an
increase in runoff along the eastern side of the country and the central interior,
with decreases in the west and south-west, including much of the Western Cape. The
winter rainfall region, and much of the west, is predicted to experience a combination of increased evaporation and decreased rainfall, and faces the greatest water
security risks. Overall the variability of the rainfall will increase and the likelihood of
more intense and extended droughts, such as those being experienced across the
country at the time of writing, will increase. Given the indications that most of the
woody alien plant species, especially the trees species, use more water than the
natural vegetation and will, therefore, decrease water availability and water security,
it is critical that effective control is achieved.
15.8 Policy and Governance
Although the estimated impacts of invasive plants on mean annual runoff have been
a key motivation for the Working for Water Programme, no policy or legislative
measures have been developed to require their control specifically to limit impacts
on water. The legislation that does require invasive alien plant control is either based
on their impacts on agricultural resources (excluding water) (DoA 1983) or environmental resources which broadly include water (DEA 2013c). The National Water
Act (DWAF 1998) does restrict commercial afforestation, via the Stream Flow
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
447
numbers of high and extreme fire danger days (Le Maitre et al. 2015a) could,
provided there is a suitable source of ignitions, favour fast-maturing invading species
and increase invasion rates and impacts in the Fynbos Biome. Fire return intervals in
moist grassland and savanna are already short, so changes in fire regimes may not
have a significant effect on invasion rates.
Another major driver is the increasing CO 2 concentration in the atmosphere
which favours woody plant densification (bush encroachment), especially when
combined with reduced fuel loads due to grazing (and thus reduced fire intensity)
(Bond and Midgley 2000, 2012; O’Connor et al. 2014). There is good evidence that
native woody plant densification is taking place throughout the Grassland and
Savanna Biomes (Davis-Reddy and Vincent 2017; Skowno et al. 2017), but whether
or not the same is happening with invading species in these biomes is not known.
Fynbos is associated with low soil nutrients which may limit the response of plants to
increased CO 2 . However, pines have mycorrhizae to harvest nutrients and Australian
Acacia species fix nitrogen, so these taxa may become even more aggressive
invaders (Richardson et al. 1994). Any gains in water-use efficiency may be offset
by their increased invasion rates.
There have not yet been any local studies of the impacts of current invasions
under current and future climates, but there are some indications based on the
projected changes in climate across South Africa. The most detailed assessment of
the implications of climate change for water resources and for their planning is the
long-term adaptation scenarios (DEA 2013b). The projected changes are for an
increase in runoff along the eastern side of the country and the central interior,
with decreases in the west and south-west, including much of the Western Cape. The
winter rainfall region, and much of the west, is predicted to experience a combination of increased evaporation and decreased rainfall, and faces the greatest water
security risks. Overall the variability of the rainfall will increase and the likelihood of
more intense and extended droughts, such as those being experienced across the
country at the time of writing, will increase. Given the indications that most of the
woody alien plant species, especially the trees species, use more water than the
natural vegetation and will, therefore, decrease water availability and water security,
it is critical that effective control is achieved.
15.8 Policy and Governance
Although the estimated impacts of invasive plants on mean annual runoff have been
a key motivation for the Working for Water Programme, no policy or legislative
measures have been developed to require their control specifically to limit impacts
on water. The legislation that does require invasive alien plant control is either based
on their impacts on agricultural resources (excluding water) (DoA 1983) or environmental resources which broadly include water (DEA 2013c). The National Water
Act (DWAF 1998) does restrict commercial afforestation, via the Stream Flow
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
447