combined with their canopy structure and physiological traits, could explain how
changes in dominant plant characteristics could alter runoff. The concept was
derived from studies worldwide which have shown that certain kinds of changes
in the characteristics of the dominant plants in catchments led to changes in the
relationship between rainfall and runoff, indicating shifts in the water balance
(Bosch and Hewlett 1982; Zhang et al. 2001).
The limits concept can be applied to invasive plants to provide insights into the
likely impacts of invasions replacing natural vegetation in situations where the
available water is limited to that from rainfall (Le Maitre 2004; Le Maitre et al.
2015b) (Fig. 15.1). The first distinction is based on plant size, which is linked to
growth and associated traits. Trees are taller than grasses so their height exposes
them to more light and air movement (Jarvis and McNaughton 1986), and they also
tend to have deeper root systems (Canadell et al. 1996; Jackson et al. 1996) which
allows them to tap into more of the stored soil moisture than the grasses. So, trees
that are invading grasslands will typically cause a large increase in transpiration, and
a concomitant reduction in runoff. Where the grasses are seasonal rather than
evergreen, and the trees are evergreen, the difference will typically be the greatest.
Where the invading and the native trees are similar, the degree of the change in
runoff will depend more on differences in physiological traits. For example evergreen trees replacing seasonally deciduous ones are likely to have less impact than
seasonally deciduous trees replacing grasses. The shifts can also be more subtle but
still have substantial impacts, such as the replacement of native mixtures of annual
and perennial plants by invading alien grasses and then by invading thistles (Gerlach
2004).
The limits concept was implicit in models based on biomass and growth-form,
which were initially used to estimate the hydrological impacts of invasions as the
incremental reductions in streamflow (i.e. the change relative to the natural vegetation). The models grouped the invading species according to their growth forms,
linked this to models of biomass versus age for each growth form and, finally,
distinguished between riparian and upland invasions (Le Maitre et al. 1996, 2000;
van Wilgen et al. 1997). The early models estimated the reductions in runoff in
actual amounts (mm per year), which was acceptable in well-watered environments
but was problematic as it failed to allow for limitations on water availability to
plants, especially in more arid environments. These models were revised and
modified to a percentage reduction, again with adjustments for riparian environments (Dzvukamanja et al. 2005; Le Maitre et al. 2013, 2016).
Calder’s limits concept was developed for upland or non-riparian environments
where the amount of water available is determined primarily by rainfall and the
water-holding characteristics of the soil. This is not the case in riparian environments, or where there is shallow groundwater, which means that the plants have
access to more water than is supplied by the rainfall. Thus, invasive alien plants in
these areas can have substantially higher water-use than those in water-limited
upland environments, with the new limits being imposed by factors such as the
atmospheric demand or maximum transpiration rates (Le Maitre et al. 2015b). This is
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
435
changes in dominant plant characteristics could alter runoff. The concept was
derived from studies worldwide which have shown that certain kinds of changes
in the characteristics of the dominant plants in catchments led to changes in the
relationship between rainfall and runoff, indicating shifts in the water balance
(Bosch and Hewlett 1982; Zhang et al. 2001).
The limits concept can be applied to invasive plants to provide insights into the
likely impacts of invasions replacing natural vegetation in situations where the
available water is limited to that from rainfall (Le Maitre 2004; Le Maitre et al.
2015b) (Fig. 15.1). The first distinction is based on plant size, which is linked to
growth and associated traits. Trees are taller than grasses so their height exposes
them to more light and air movement (Jarvis and McNaughton 1986), and they also
tend to have deeper root systems (Canadell et al. 1996; Jackson et al. 1996) which
allows them to tap into more of the stored soil moisture than the grasses. So, trees
that are invading grasslands will typically cause a large increase in transpiration, and
a concomitant reduction in runoff. Where the grasses are seasonal rather than
evergreen, and the trees are evergreen, the difference will typically be the greatest.
Where the invading and the native trees are similar, the degree of the change in
runoff will depend more on differences in physiological traits. For example evergreen trees replacing seasonally deciduous ones are likely to have less impact than
seasonally deciduous trees replacing grasses. The shifts can also be more subtle but
still have substantial impacts, such as the replacement of native mixtures of annual
and perennial plants by invading alien grasses and then by invading thistles (Gerlach
2004).
The limits concept was implicit in models based on biomass and growth-form,
which were initially used to estimate the hydrological impacts of invasions as the
incremental reductions in streamflow (i.e. the change relative to the natural vegetation). The models grouped the invading species according to their growth forms,
linked this to models of biomass versus age for each growth form and, finally,
distinguished between riparian and upland invasions (Le Maitre et al. 1996, 2000;
van Wilgen et al. 1997). The early models estimated the reductions in runoff in
actual amounts (mm per year), which was acceptable in well-watered environments
but was problematic as it failed to allow for limitations on water availability to
plants, especially in more arid environments. These models were revised and
modified to a percentage reduction, again with adjustments for riparian environments (Dzvukamanja et al. 2005; Le Maitre et al. 2013, 2016).
Calder’s limits concept was developed for upland or non-riparian environments
where the amount of water available is determined primarily by rainfall and the
water-holding characteristics of the soil. This is not the case in riparian environments, or where there is shallow groundwater, which means that the plants have
access to more water than is supplied by the rainfall. Thus, invasive alien plants in
these areas can have substantially higher water-use than those in water-limited
upland environments, with the new limits being imposed by factors such as the
atmospheric demand or maximum transpiration rates (Le Maitre et al. 2015b). This is
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
435