biological control agents, as this will reduce future costs of restoration. For example
in the case of the gall-forming rust fungus Uromycladium tepperianum on Port
Jackson Willow, Acacia saligna, biological control substantially reduces the growth
rate and canopy density of infected trees, allowing some fynbos species to persist in
dense alien stands (Wood 2017). This slows the invasion process and maintains
spontaneous regeneration potential in the invaded stands.
23.5.3 Implementation: Active Restoration
Where abiotic thresholds have been breached, interventions may be required to
manipulate the abiotic environment before native species can be re-established. An
example for nutrient-poor ecosystems includes reducing excess nutrients through
removing or burning the litter layer and applying C-rich organic matter to
immobilise nutrients (Zink and Allen 1998; Török et al. 2000; Gaertner et al.
2011). In riparian zones, steep banks and sediment accumulation resulting from
stands of alien trees may first need to be re-contoured and flushed out, respectively,
to create a more natural geomorphology before native wet and dry bank communities
can be successfully re-established (Richardson et al. 2007). Exposure of bare soil
and reduction in herbaceous cover following felling operations in riparian zones can
result in secondary invasions (Beater et al. 2008; Table 23.1) which may be
countered by planting native riparian species. If mutualistic microbial communities
(e.g. specialist mycorrhizae) required for plant growth have been lost following
long-term invasion or severe fires, soils or plants may first need to be inoculated to
ensure successful re-introductions (Nsikani et al. 2018).
Native species may be restored from seed and propagated material. Seed is the
preferred method for a number of reasons: it allows for more species and genetic
diversity to be returned to the site, avoids the potential introduction of foreign soil,
pests and parasites from nursery-grown stock, and potentially is more cost-effective,
allowing larger areas to be restored. In grasslands, where the goal is to promote
diversity, it is recommended to sow a propagule mix of grass species with low
invasion potential (species with short stature, slow growth, low leaf mass and few
tillers, Fynn et al. 2009). However, not all species are easily restored by seed. For
example, fynbos obligate resprouter species produce few viable seeds (Marais et al.
2014) and in other species seeds are recalcitrant (Walters and Berjak 2013) thus
cannot be collected and stored ahead of sowing. Some large-seeded species are
nutritious and are highly parasitised or produce few viable seeds owing to granivory
(PMH pers. obs.), while others are dependent on ants for dispersal and germination
(Bond and Slingsby 1983). For threatened ecosystems there may be few intact
remnants remaining as a source of suitable seeds for restoration, further limiting
the available options. For some of the above examples, propagating new plants from
cuttings and splits, or germinating from seed and first growing on before planting,
may be more successful. In some cases, a combination of sowing and planting might
be recommended to optimise the restoration outcomes.
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P. M. Holmes et al.
in the case of the gall-forming rust fungus Uromycladium tepperianum on Port
Jackson Willow, Acacia saligna, biological control substantially reduces the growth
rate and canopy density of infected trees, allowing some fynbos species to persist in
dense alien stands (Wood 2017). This slows the invasion process and maintains
spontaneous regeneration potential in the invaded stands.
23.5.3 Implementation: Active Restoration
Where abiotic thresholds have been breached, interventions may be required to
manipulate the abiotic environment before native species can be re-established. An
example for nutrient-poor ecosystems includes reducing excess nutrients through
removing or burning the litter layer and applying C-rich organic matter to
immobilise nutrients (Zink and Allen 1998; Török et al. 2000; Gaertner et al.
2011). In riparian zones, steep banks and sediment accumulation resulting from
stands of alien trees may first need to be re-contoured and flushed out, respectively,
to create a more natural geomorphology before native wet and dry bank communities
can be successfully re-established (Richardson et al. 2007). Exposure of bare soil
and reduction in herbaceous cover following felling operations in riparian zones can
result in secondary invasions (Beater et al. 2008; Table 23.1) which may be
countered by planting native riparian species. If mutualistic microbial communities
(e.g. specialist mycorrhizae) required for plant growth have been lost following
long-term invasion or severe fires, soils or plants may first need to be inoculated to
ensure successful re-introductions (Nsikani et al. 2018).
Native species may be restored from seed and propagated material. Seed is the
preferred method for a number of reasons: it allows for more species and genetic
diversity to be returned to the site, avoids the potential introduction of foreign soil,
pests and parasites from nursery-grown stock, and potentially is more cost-effective,
allowing larger areas to be restored. In grasslands, where the goal is to promote
diversity, it is recommended to sow a propagule mix of grass species with low
invasion potential (species with short stature, slow growth, low leaf mass and few
tillers, Fynn et al. 2009). However, not all species are easily restored by seed. For
example, fynbos obligate resprouter species produce few viable seeds (Marais et al.
2014) and in other species seeds are recalcitrant (Walters and Berjak 2013) thus
cannot be collected and stored ahead of sowing. Some large-seeded species are
nutritious and are highly parasitised or produce few viable seeds owing to granivory
(PMH pers. obs.), while others are dependent on ants for dispersal and germination
(Bond and Slingsby 1983). For threatened ecosystems there may be few intact
remnants remaining as a source of suitable seeds for restoration, further limiting
the available options. For some of the above examples, propagating new plants from
cuttings and splits, or germinating from seed and first growing on before planting,
may be more successful. In some cases, a combination of sowing and planting might
be recommended to optimise the restoration outcomes.
688
P. M. Holmes et al.