power requirement, have been used to compliment short-term eddy covariance
measurements (Clulow et al. 2012). One of the challenges in the quantification of
water use by invasive alien plants is that many of the measurement techniques only
provide ‘point-based’ estimates of transpiration or evapotranspiration.
Scintillometry overcomes the concerns relating to the ‘point-based’ and limited
‘footprint’ scale issue to an extent, as it provides a path averaged estimate of sensible
heat flux (Meijninger et al. 2002; Clulow et al. 2011, 2015).
15.4 Species and Stand-Level Studies
The impact of specific invasive alien tree species on streamflow has been assessed in
field studies, some of which covered extended periods (Table 15.1). These include
Acacia mearnsii (Dye and Jarmain 2004; Clulow et al. 2011; Everson et al. 2014),
Eucalyptus camaldulensis (River Red Gum) (Dzikiti et al. 2016), Pinus radiata
(Monterey Pine) (Dzikiti et al. 2013b), Prosopis species (Dzikiti et al. 2013a, 2017)
and Populus canescens (Grey poplars) (Ntshidi et al. 2018). In recent years some
studies have also focused on the impacts of the invasive alien plants on groundwater
[e.g. Prosopis (Dzikiti et al. 2013a)] although these measurements were also over
short periods.
In addition to invasive alien plants, some recent studies have also monitored the
water use patterns of co-occurring native vegetation in order quantify the incremental water use by the invasions over and above that of the native vegetation (Dzikiti
et al. 2016). Gush and Dye (2015) describe measurements of the water use (transpiration and ET) of a range of native tree species and forest types, and compare these
with the water use of introduced tree species. The results show that the water-use of
the introduced species is generally higher. Measurements of A. mearnsii growing
alongside an afro-temperate forest in the Eastern Cape found that its water-use was
about twice that of the native forest species (Scott-Shaw and Everson 2018). A
comparison of the water use of Prosopis species with the coexisting and structurally
similar native Vachellia karroo (Sweet Thorn) in an arid riparian environment in the
Northern Cape found that water use of comparably-sized trees was similar, but the
greater density, and more extensive invasions, of Prosopis resulted in much greater
impacts on groundwater per unit area (Dzikiti et al. 2017).
Overall, the findings of these studies are that trees invading riparian areas where
they have access to additional water, or areas where groundwater can be accessed by
their root systems, will transpire substantially more water than those in adjacent
dryland areas. The differences between dryland and riparian invasions vary but
range from 1.2 to 2 times as much water as the equivalent trees in dryland settings
(Le Maitre et al. 2015b, 2016). The Prosopis-Vachellia comparison found that
similarly-sized trees transpired similar quantities of water (Dzikiti et al. 2018), but
other comparisons of alien and native tree species in riparian settings have found the
water-use of the alien trees to be substantially greater (Everson et al. 2011; Gush
et al. 2015).
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
437
measurements (Clulow et al. 2012). One of the challenges in the quantification of
water use by invasive alien plants is that many of the measurement techniques only
provide ‘point-based’ estimates of transpiration or evapotranspiration.
Scintillometry overcomes the concerns relating to the ‘point-based’ and limited
‘footprint’ scale issue to an extent, as it provides a path averaged estimate of sensible
heat flux (Meijninger et al. 2002; Clulow et al. 2011, 2015).
15.4 Species and Stand-Level Studies
The impact of specific invasive alien tree species on streamflow has been assessed in
field studies, some of which covered extended periods (Table 15.1). These include
Acacia mearnsii (Dye and Jarmain 2004; Clulow et al. 2011; Everson et al. 2014),
Eucalyptus camaldulensis (River Red Gum) (Dzikiti et al. 2016), Pinus radiata
(Monterey Pine) (Dzikiti et al. 2013b), Prosopis species (Dzikiti et al. 2013a, 2017)
and Populus canescens (Grey poplars) (Ntshidi et al. 2018). In recent years some
studies have also focused on the impacts of the invasive alien plants on groundwater
[e.g. Prosopis (Dzikiti et al. 2013a)] although these measurements were also over
short periods.
In addition to invasive alien plants, some recent studies have also monitored the
water use patterns of co-occurring native vegetation in order quantify the incremental water use by the invasions over and above that of the native vegetation (Dzikiti
et al. 2016). Gush and Dye (2015) describe measurements of the water use (transpiration and ET) of a range of native tree species and forest types, and compare these
with the water use of introduced tree species. The results show that the water-use of
the introduced species is generally higher. Measurements of A. mearnsii growing
alongside an afro-temperate forest in the Eastern Cape found that its water-use was
about twice that of the native forest species (Scott-Shaw and Everson 2018). A
comparison of the water use of Prosopis species with the coexisting and structurally
similar native Vachellia karroo (Sweet Thorn) in an arid riparian environment in the
Northern Cape found that water use of comparably-sized trees was similar, but the
greater density, and more extensive invasions, of Prosopis resulted in much greater
impacts on groundwater per unit area (Dzikiti et al. 2017).
Overall, the findings of these studies are that trees invading riparian areas where
they have access to additional water, or areas where groundwater can be accessed by
their root systems, will transpire substantially more water than those in adjacent
dryland areas. The differences between dryland and riparian invasions vary but
range from 1.2 to 2 times as much water as the equivalent trees in dryland settings
(Le Maitre et al. 2015b, 2016). The Prosopis-Vachellia comparison found that
similarly-sized trees transpired similar quantities of water (Dzikiti et al. 2018), but
other comparisons of alien and native tree species in riparian settings have found the
water-use of the alien trees to be substantially greater (Everson et al. 2011; Gush
et al. 2015).
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
437