evolved with fire; fires are either absent or very infrequent due to a lack of fuel in the
Nama Karoo, Succulent Karoo, Desert Biomes, and arid parts of the Savanna Biome;
and in the Forest and Albany Thicket Biomes fires are largely excluded due to the
non-flammable vegetation (van Wilgen et al. 1990).
Fires can either promote or retard invasions, depending on the ability of individual species to respond to fires. There are four broad types of responses of plants
to fire.
1. Serotiny
Serotiny has evolved specifically as a mechanism for plant populations to persist in
regions characterised by frequent fires (Lamont et al. 1991). Several major invasive
plant species in South Africa (e.g. Hakea and Pinus species) accumulate seed banks
in serotinous cones or follicles over several flowering seasons. These plants are
typically killed by fires and spread over considerable distances by means of winged
seeds that germinate in the post-fire environment. Spread and densification is
therefore facilitated by fires which occur at intervals that allow the plants to mature
and accumulate large seed banks during inter-fire periods (Richardson et al. 1987).
Without such fires, invasions are either very slow or do not happen (e.g., Geerts et al.
2013b).
2. Soil-Stored Seed Banks
Trees and shrubs in the genera Acacia and Paraserianthes have soil-stored seed
banks whose germination is stimulated by fire (Richardson and Kluge 2008). The
hard-coated seeds are shed each year and accumulate in the soil. The heat from fires
stimulates mass germination, so that stands of these invasive plants become denser
after each fire.
3. Resprouting
Species pre-adapted to survive fires by means of re-sprouting do so either from
underground rootstocks or from epicormic buds at the base of the stem or below the
bark in the canopy (e.g. Eucalyptus and Populus species, and some alien perennial
grass species). In cases where species resprout vigorously after a fire, the lack of
competition in the post fire environment can mean that regular fires enhance invasion
[e.g. Kudzu Vine (Pueraria montana) (Geerts et al. 2016)].
4. Fire Sensitive Species
Alien plant species native to areas where fires do not occur are unlikely to possess
mechanisms to persist in fire-prone areas. Examples include plant species that invade
forests [Sweet Pittosporum (Pittosporum undulatum), Bugweed (Solanum
mauritianum)] or very arid areas that seldom experience fire (Cactaceae). An
intolerance to natural fire regimes has been cited as a main reason why some alien
plants are limited to disturbed road-sides and do not invade natural ecosystems
(Geerts et al. 2013a; Holmes et al. 2018). Native forest trees that are embedded in
fire-prone fynbos vegetation are fire-sensitive, but native forest patches are able to
persist because of differences in their fuel properties that exclude fires (van Wilgen
et al. 1990). The seeds of native forest trees can germinate on recently-burnt fynbos
13 The Role of Environmental Factors in Promoting and Limiting Invasions
373
Nama Karoo, Succulent Karoo, Desert Biomes, and arid parts of the Savanna Biome;
and in the Forest and Albany Thicket Biomes fires are largely excluded due to the
non-flammable vegetation (van Wilgen et al. 1990).
Fires can either promote or retard invasions, depending on the ability of individual species to respond to fires. There are four broad types of responses of plants
to fire.
1. Serotiny
Serotiny has evolved specifically as a mechanism for plant populations to persist in
regions characterised by frequent fires (Lamont et al. 1991). Several major invasive
plant species in South Africa (e.g. Hakea and Pinus species) accumulate seed banks
in serotinous cones or follicles over several flowering seasons. These plants are
typically killed by fires and spread over considerable distances by means of winged
seeds that germinate in the post-fire environment. Spread and densification is
therefore facilitated by fires which occur at intervals that allow the plants to mature
and accumulate large seed banks during inter-fire periods (Richardson et al. 1987).
Without such fires, invasions are either very slow or do not happen (e.g., Geerts et al.
2013b).
2. Soil-Stored Seed Banks
Trees and shrubs in the genera Acacia and Paraserianthes have soil-stored seed
banks whose germination is stimulated by fire (Richardson and Kluge 2008). The
hard-coated seeds are shed each year and accumulate in the soil. The heat from fires
stimulates mass germination, so that stands of these invasive plants become denser
after each fire.
3. Resprouting
Species pre-adapted to survive fires by means of re-sprouting do so either from
underground rootstocks or from epicormic buds at the base of the stem or below the
bark in the canopy (e.g. Eucalyptus and Populus species, and some alien perennial
grass species). In cases where species resprout vigorously after a fire, the lack of
competition in the post fire environment can mean that regular fires enhance invasion
[e.g. Kudzu Vine (Pueraria montana) (Geerts et al. 2016)].
4. Fire Sensitive Species
Alien plant species native to areas where fires do not occur are unlikely to possess
mechanisms to persist in fire-prone areas. Examples include plant species that invade
forests [Sweet Pittosporum (Pittosporum undulatum), Bugweed (Solanum
mauritianum)] or very arid areas that seldom experience fire (Cactaceae). An
intolerance to natural fire regimes has been cited as a main reason why some alien
plants are limited to disturbed road-sides and do not invade natural ecosystems
(Geerts et al. 2013a; Holmes et al. 2018). Native forest trees that are embedded in
fire-prone fynbos vegetation are fire-sensitive, but native forest patches are able to
persist because of differences in their fuel properties that exclude fires (van Wilgen
et al. 1990). The seeds of native forest trees can germinate on recently-burnt fynbos
13 The Role of Environmental Factors in Promoting and Limiting Invasions
373