of chronic grazing and an absence of fire. In the Karoo biomes, livestock (mainly
sheep) have similarly facilitated invasion of dryland through dispersal of seed from
riparian areas.
In terms of impacts, Prosopis species have had a disproportionate impact on
riparian areas, which serve as key resource areas for livestock during the dry season.
About 30% of riparian areas have been invaded compared with 3.3% of dryland area
(van den Berg 2010), reducing their grazing capacity (Milton and Dean 2010). For
dryland rangeland, moderate levels of Prosopis invasion on a heavily grazed Nama
Karoo rangeland site (approximately 15% canopy cover) reduced grazing capacity
by 34% from 3.87 to 2.56 LSU per 100 ha (Ndhlovu et al. 2011). The negative
impacts of Prosopis invasion on native vegetation (especially grasses) become
apparent above a threshold of around 30% Prosopis cover (Ndhlovu et al. 2011,
2016). Shackleton et al. (2015) found that invasion by Prosopis reduced perennial
grass cover from above 15% where the basal area of Prosopis was below 2 m
2 /ha, to
0 where the Prosopis basal area was above 4.5 m
2 /ha. Similarly, the cover of native
perennial herbaceous plants decreased from above 20% to zero with similar
increases in Prosopis cover. In contrast, the cover of annual grasses or annual
herbaceous plants persisted at quite high levels of invasion.
16.5.3 Acacia mearnsii (Black Wattle)
Acacia mearnsii was introduced from Australia to provide tannins from bark and for
wood products. This tree species has extensively invaded the relatively humid parts
of South Africa, especially riparian areas, and it has occupied most of its suitable
climatic envelope (Rouget et al. 2004). It showed only an 8% increase in distribution
range between 2000 and 2016 (Henderson and Wilson 2017), but further increases in
density are possible. Grass cover decreases in a negative exponential manner with an
increase in wattle cover (Gwate et al. 2016). Yapi et al. (2018) investigated the
impacts of A. mearnsii invasion on rangeland condition, and rangeland recovery
after clearing. They found a 72% reduction in grazing capacity in densely invaded
sites, and this improved by 66% 5 years after clearing. In densely invaded sites, the
basal cover of grasses was reduced by up to 42%, resulting in a reduction in grazing
capacity of 75%, from 50 to 12.5 large stock units per 100 ha in uninvaded and
densely invaded sites, respectively. These results suggest that, if left uncontrolled,
wattle species can substantially reduce livestock carrying capacity within montane
grasslands in South Africa. Exclusion of fire, by intention or as a consequence of
chronic grazing that removes the grass fuels necessary for fires, promotes an increase
of wattle, whereas in grasslands, this species can be controlled using repeated
burning because it does not resprout. Fire-return intervals under repeated burning
in mesic grasslands are usually around 2 years. Although burning stimulates germination, seedlings would not develop into a reproductive plant because the fire-return
interval is too short. Acacia mearnsii possesses many traits typically associated with
an aggressive invader species including prolific seed set, seeds capable of remaining
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T. G. O’Connor and B. W. van Wilgen
sheep) have similarly facilitated invasion of dryland through dispersal of seed from
riparian areas.
In terms of impacts, Prosopis species have had a disproportionate impact on
riparian areas, which serve as key resource areas for livestock during the dry season.
About 30% of riparian areas have been invaded compared with 3.3% of dryland area
(van den Berg 2010), reducing their grazing capacity (Milton and Dean 2010). For
dryland rangeland, moderate levels of Prosopis invasion on a heavily grazed Nama
Karoo rangeland site (approximately 15% canopy cover) reduced grazing capacity
by 34% from 3.87 to 2.56 LSU per 100 ha (Ndhlovu et al. 2011). The negative
impacts of Prosopis invasion on native vegetation (especially grasses) become
apparent above a threshold of around 30% Prosopis cover (Ndhlovu et al. 2011,
2016). Shackleton et al. (2015) found that invasion by Prosopis reduced perennial
grass cover from above 15% where the basal area of Prosopis was below 2 m
2 /ha, to
0 where the Prosopis basal area was above 4.5 m
2 /ha. Similarly, the cover of native
perennial herbaceous plants decreased from above 20% to zero with similar
increases in Prosopis cover. In contrast, the cover of annual grasses or annual
herbaceous plants persisted at quite high levels of invasion.
16.5.3 Acacia mearnsii (Black Wattle)
Acacia mearnsii was introduced from Australia to provide tannins from bark and for
wood products. This tree species has extensively invaded the relatively humid parts
of South Africa, especially riparian areas, and it has occupied most of its suitable
climatic envelope (Rouget et al. 2004). It showed only an 8% increase in distribution
range between 2000 and 2016 (Henderson and Wilson 2017), but further increases in
density are possible. Grass cover decreases in a negative exponential manner with an
increase in wattle cover (Gwate et al. 2016). Yapi et al. (2018) investigated the
impacts of A. mearnsii invasion on rangeland condition, and rangeland recovery
after clearing. They found a 72% reduction in grazing capacity in densely invaded
sites, and this improved by 66% 5 years after clearing. In densely invaded sites, the
basal cover of grasses was reduced by up to 42%, resulting in a reduction in grazing
capacity of 75%, from 50 to 12.5 large stock units per 100 ha in uninvaded and
densely invaded sites, respectively. These results suggest that, if left uncontrolled,
wattle species can substantially reduce livestock carrying capacity within montane
grasslands in South Africa. Exclusion of fire, by intention or as a consequence of
chronic grazing that removes the grass fuels necessary for fires, promotes an increase
of wattle, whereas in grasslands, this species can be controlled using repeated
burning because it does not resprout. Fire-return intervals under repeated burning
in mesic grasslands are usually around 2 years. Although burning stimulates germination, seedlings would not develop into a reproductive plant because the fire-return
interval is too short. Acacia mearnsii possesses many traits typically associated with
an aggressive invader species including prolific seed set, seeds capable of remaining
478
T. G. O’Connor and B. W. van Wilgen