a concern because there are extensive riparian invasions, particularly in the grasslands and savannas (Le Maitre et al. 2000; van Wilgen et al. 2007).
The dominant vegetation types across the higher rainfall areas of South Africa are
savanna and grassland, dominated by shallow-rooted plants with low leaf-areas,
many of which are dormant in the dry season. The strategic surface water source
areas (SWSAs) of South Africa are high-lying, high-rainfall mountainous areas
dominated by grasslands and shrubs, notably fynbos, with isolated patches of native
forest (WWF-SA 2013; Le Maitre et al. 2018). Important invaders, particularly in the
SWSAs, are the eucalypts, pines and Black Wattle (Acacia mearnsii) used by the
commercial forest industry, which covers approximately 1.2 m ha (Godsmark 2017),
or 13% of these high rainfall environments, and is an important seed source that fuels
invasions (Rouget et al. 2003).
These tree species are characteristically deep-rooted, evergreen, with high leaf
area and canopy cover (High impact, Fig. 15.1), often differing fundamentally from
the natural vegetation they replace, both as individual plants, and when forming
stands. The primary impact is an increase in evapotranspiration (ET) relative to the
“Baseline” ET of the natural vegetation, resulting in a reduction in streamflow which
varies between species (see Sect. 15.3). In the case of groundwater, invasions lead to
a reduced recharge and, where the groundwater is within the rooting depth, to direct
exploitation of the groundwater (Le Maitre et al. 1999, 2015b). In well-managed
plantations their impacts on river flows can be anticipated and sustainably managed.
However, when these species invade, particularly in riparian areas, or in areas where
they use more water than the original vegetation (e.g. grassland or fynbos), they can
have a significant impact on river flows.
15.3 Modelling Versus Measuring Water Resource Impacts
A number of methods are available to quantify the water resource impacts of
invasive alien plants. These may be broadly divided into measurement techniques
and modelling approaches, which can occur at the plant and catchment levels.
Catchment scale measurements have used paired catchment studies (involving forest
plantations) and soil water balance assessments, while lysimetry can be used for
more localised assessments. Sap flow measurement (using techniques such as stem
heat balance, heat pulse velocity, or continuously heated probes) are often used to
estimate the transpiration of individual plants (Burgess et al. 2001). The ET component can be quantified using micrometeorological and surface energy balance
systems (e.g. Bowen ratio, eddy covariance, surface renewal) (Jarmain et al. 2008).
These methods measure ET around a flux tower and the flux footprint depends on the
height of the sensors above the canopies as well as the prevailing wind speed and
direction. Window periods of measurement with sensitive and high maintenance
equipment, for approximately 2 weeks at a time during different seasons, has been
used recent times (Dye et al. 2008; Jarmain et al. 2008; Clulow et al. 2012, 2013,
2015). Long-term surface renewal systems, which are cheaper and have a lower
436
D. C. Le Maitre et al.
The dominant vegetation types across the higher rainfall areas of South Africa are
savanna and grassland, dominated by shallow-rooted plants with low leaf-areas,
many of which are dormant in the dry season. The strategic surface water source
areas (SWSAs) of South Africa are high-lying, high-rainfall mountainous areas
dominated by grasslands and shrubs, notably fynbos, with isolated patches of native
forest (WWF-SA 2013; Le Maitre et al. 2018). Important invaders, particularly in the
SWSAs, are the eucalypts, pines and Black Wattle (Acacia mearnsii) used by the
commercial forest industry, which covers approximately 1.2 m ha (Godsmark 2017),
or 13% of these high rainfall environments, and is an important seed source that fuels
invasions (Rouget et al. 2003).
These tree species are characteristically deep-rooted, evergreen, with high leaf
area and canopy cover (High impact, Fig. 15.1), often differing fundamentally from
the natural vegetation they replace, both as individual plants, and when forming
stands. The primary impact is an increase in evapotranspiration (ET) relative to the
“Baseline” ET of the natural vegetation, resulting in a reduction in streamflow which
varies between species (see Sect. 15.3). In the case of groundwater, invasions lead to
a reduced recharge and, where the groundwater is within the rooting depth, to direct
exploitation of the groundwater (Le Maitre et al. 1999, 2015b). In well-managed
plantations their impacts on river flows can be anticipated and sustainably managed.
However, when these species invade, particularly in riparian areas, or in areas where
they use more water than the original vegetation (e.g. grassland or fynbos), they can
have a significant impact on river flows.
15.3 Modelling Versus Measuring Water Resource Impacts
A number of methods are available to quantify the water resource impacts of
invasive alien plants. These may be broadly divided into measurement techniques
and modelling approaches, which can occur at the plant and catchment levels.
Catchment scale measurements have used paired catchment studies (involving forest
plantations) and soil water balance assessments, while lysimetry can be used for
more localised assessments. Sap flow measurement (using techniques such as stem
heat balance, heat pulse velocity, or continuously heated probes) are often used to
estimate the transpiration of individual plants (Burgess et al. 2001). The ET component can be quantified using micrometeorological and surface energy balance
systems (e.g. Bowen ratio, eddy covariance, surface renewal) (Jarmain et al. 2008).
These methods measure ET around a flux tower and the flux footprint depends on the
height of the sensors above the canopies as well as the prevailing wind speed and
direction. Window periods of measurement with sensitive and high maintenance
equipment, for approximately 2 weeks at a time during different seasons, has been
used recent times (Dye et al. 2008; Jarmain et al. 2008; Clulow et al. 2012, 2013,
2015). Long-term surface renewal systems, which are cheaper and have a lower
436
D. C. Le Maitre et al.