fynbos ecosystems than alien pines (Mostert et al. 2017). Acacias shade out native
species quicker than do pines. Their legacy effects include large soil-stored seed
banks, increased leaf litter and biomass accumulation, increased pressure from
folivorous insects and phylopathogenic fungi and changes to soil chemistry (Maoela
et al. 2016; Nsikani et al. 2017; Strydom et al. 2012; Yelenik et al. 2004), thus
altering ecosystem functions such as nutrient-cycling. Their rapid growth to canopy
closure halts native seed set, resulting in depletion of native seed banks (Holmes
2002; Mostert et al. 2017). By contrast, the legacy effects of pines are less pronounced, relating more to altered local hydrology through increased water use and
increased biomass. However, long-duration, dense pine invasions cause the depletion of fynbos seed banks and reduced recovery potential (Galloway et al. 2017), and
in grasslands the loss of the forb resprouter guild (Zaloumis and Bond 2011).
Where an alien species is adapted to fire in the invaded ecosystem, management
interventions need to be carefully integrated to promote ecosystem recovery and
avoid exacerbating the invasion through fire-stimulated germination and spread of
the alien species. In the case of serotinous invaders such as Hakea and Pinus species,
fire may be used after felling adult populations to kill their released seed, seedlings
and saplings (van Wilgen et al. 1994; Table 23.1, Langeberg case study). The case
study of Cape Flats Sand Fynbos restoration at Blaauwberg Nature Reserve highlights the legacy effects to be overcome following Acacia invasion for a critically
endangered ecosystem in which restoring and conserving biodiversity is the ultimate
goal (Table 23.1; Figs. 23.2 and 23.3).
In riparian ecosystems, invasions of alien Acacia species and River Red Gum,
Eucalyptus camaldulensis, also induce legacy effects. Whereas nutrient enrichment
was documented after Acacia invasions, concentrations of soil available nutrients
did not change following Eucalyptus invasion along the Berg River, although soil
pH and moisture decreased significantly, and soil water repellency increased (Tererai
et al. 2015a; Ruwanza et al. 2013a, b). A decrease in soil nutrients was documented
following clearance of Acacia species along the Palmiet River, Eastern Cape (Ndou
and Ruwanza 2016). In the lowland Berg River case study (Table 23.1; Fig. 23.4),
E. camaldulensis produced allelopathic chemicals that suppressed germination and
growth of four native species (Ruwanza et al. 2015). The soil seed banks retained
some potential to initiate recovery after alien control but contained a high proportion
of alien herbaceous weedy species that caused secondary invasions post-control
(Tererai et al. 2015b). By contrast, in mountain stream and foothill reaches, fynbos
riparian ecosystems are relatively resilient to invasion and good spontaneous regeneration followed the removal of large alien trees (Blanchard and Holmes 2008).
Although not all riparian species have soil-stored seeds, seed banks had potential to
initiate recovery after alien control (Fourie 2008; Vosse et al. 2008), as was also
found along the Sabie River in the Kruger National Park (Morris et al. 2008;
Table 23.1; Fig. 23.5).
Most lowland rivers traverse highly modified landscapes, and suffer profound
alterations to hydrological regimes and geomorphology, due to upstream impoundments and conversion of riparian zones to agriculture or dense stands of alien trees
(Holmes et al. 2005). Such large-scale alterations to ecological processes limit the
23 Biological Invasions and Ecological Restoration in South Africa
679
species quicker than do pines. Their legacy effects include large soil-stored seed
banks, increased leaf litter and biomass accumulation, increased pressure from
folivorous insects and phylopathogenic fungi and changes to soil chemistry (Maoela
et al. 2016; Nsikani et al. 2017; Strydom et al. 2012; Yelenik et al. 2004), thus
altering ecosystem functions such as nutrient-cycling. Their rapid growth to canopy
closure halts native seed set, resulting in depletion of native seed banks (Holmes
2002; Mostert et al. 2017). By contrast, the legacy effects of pines are less pronounced, relating more to altered local hydrology through increased water use and
increased biomass. However, long-duration, dense pine invasions cause the depletion of fynbos seed banks and reduced recovery potential (Galloway et al. 2017), and
in grasslands the loss of the forb resprouter guild (Zaloumis and Bond 2011).
Where an alien species is adapted to fire in the invaded ecosystem, management
interventions need to be carefully integrated to promote ecosystem recovery and
avoid exacerbating the invasion through fire-stimulated germination and spread of
the alien species. In the case of serotinous invaders such as Hakea and Pinus species,
fire may be used after felling adult populations to kill their released seed, seedlings
and saplings (van Wilgen et al. 1994; Table 23.1, Langeberg case study). The case
study of Cape Flats Sand Fynbos restoration at Blaauwberg Nature Reserve highlights the legacy effects to be overcome following Acacia invasion for a critically
endangered ecosystem in which restoring and conserving biodiversity is the ultimate
goal (Table 23.1; Figs. 23.2 and 23.3).
In riparian ecosystems, invasions of alien Acacia species and River Red Gum,
Eucalyptus camaldulensis, also induce legacy effects. Whereas nutrient enrichment
was documented after Acacia invasions, concentrations of soil available nutrients
did not change following Eucalyptus invasion along the Berg River, although soil
pH and moisture decreased significantly, and soil water repellency increased (Tererai
et al. 2015a; Ruwanza et al. 2013a, b). A decrease in soil nutrients was documented
following clearance of Acacia species along the Palmiet River, Eastern Cape (Ndou
and Ruwanza 2016). In the lowland Berg River case study (Table 23.1; Fig. 23.4),
E. camaldulensis produced allelopathic chemicals that suppressed germination and
growth of four native species (Ruwanza et al. 2015). The soil seed banks retained
some potential to initiate recovery after alien control but contained a high proportion
of alien herbaceous weedy species that caused secondary invasions post-control
(Tererai et al. 2015b). By contrast, in mountain stream and foothill reaches, fynbos
riparian ecosystems are relatively resilient to invasion and good spontaneous regeneration followed the removal of large alien trees (Blanchard and Holmes 2008).
Although not all riparian species have soil-stored seeds, seed banks had potential to
initiate recovery after alien control (Fourie 2008; Vosse et al. 2008), as was also
found along the Sabie River in the Kruger National Park (Morris et al. 2008;
Table 23.1; Fig. 23.5).
Most lowland rivers traverse highly modified landscapes, and suffer profound
alterations to hydrological regimes and geomorphology, due to upstream impoundments and conversion of riparian zones to agriculture or dense stands of alien trees
(Holmes et al. 2005). Such large-scale alterations to ecological processes limit the
23 Biological Invasions and Ecological Restoration in South Africa
679