settlements are more numerous and are more likely to act as launching sites for plant
invasions into natural areas as they share proportionally greater boundaries with their
surroundings (Fig. 11.1d; Foxcroft et al. 2008; McLean et al. 2017). For example, a
survey inside a military base near Kimberley in the Northern Cape showed that alien
fleshy-fruited trees cultivated mostly as ornamentals, for shade, or to provide fruit,
spread beyond the confines of the base into the surrounding savanna (Milton et al.
2007). Dean and Milton (2019) describe the invasion of Myrtillocactus geometrizans
(Bilberry Cactus) into natural vegetation near the town of Prince Albert, Western
Cape, from a 40-year-old cactus garden in the town (Fig. 11.2f). Cilliers et al. (2008)
found that the cover of alien species increases with increasing proximity to the edge
of native grassland patches surrounded by urban and rural landscapes in
South Africa and Australia. Such examples provide evidence of a key process
driving many urban plant invasions.
During the Dutch and British colonial periods (1652–1871), most alien plant
species were introduced for timber production (forestry), fuelwood, shade and dune
reclamation (Richardson et al. 2003). Such introductions have left a major imprint
on plant invasions in natural and semi-natural ecosystems (Richardson et al. 2020,
Chap. 3). However, by the twentieth century there was an increasing emphasis on
amenity plantings in gardens and urban open spaces (Richardson et al. 2003). This
trend increased exponentially, and there has been an explosion of large and small
organisations (e.g. online traders) specialising in the dissemination of plants (usually
in the form of seeds or bulbs) worldwide in the last decade (e.g. Humair et al. 2015).
Despite a recent upsurge in the popularity of wild, drought-tolerant gardens
comprising mainly native plants, alien plants remain conspicuous features in all
South African cities. Early European settlers wanted to reconstruct the gardens of
Europe and these introductions were, in many cases, assimilated into local culture,
which perpetuated their further use (e.g. Davoren et al. 2016). Examples include oaks
(Quercus species) in Stellenbosch (nicknamed Eikestad in Afrikaans, i.e. “Oak
City”), and Jacaranda mimosifolia in Pretoria (“Jacaranda City”). At least 25 alien
tree species are protected as “Champion Trees of South Africa” under the National
Forests Act of 1998. Many of these occur in urban settings—68% of species listed as
“champion trees” are alien and 25% are currently listed as invasive. Updated information from the South African Plant Invaders Atlas (SAPIA, accessed 12 December
2018) shows that 18% of invasive alien plant taxa recorded for South Africa occur in
urban open spaces or around human habitation (Henderson and Wilson 2017). The
proportion of “urban invaders” in South Africa’s invasive flora is, however, much
greater than this, as 76% of taxa listed in Henderson’s (2001) book on “Alien weeds
and invasive plants” are ornamental plants that are grown in urban areas, and because
Henderson’s book focusses largely on natural-area invaders.
Life-history traits such as flower and fruit size and shape, growth rates, and the
capacity to flourish under harsh environmental conditions have driven the importation of many alien plants into urban areas of South Africa. As a result, large showy
flowers, colourful fruits, the capacity for rapid growth and the ability to survive
without irrigation are features of many widely planted alien plants in South Africa.
Waves of interest in new types of alien trees with particular features have occurred in
282
L. J. Potgieter et al.
invasions into natural areas as they share proportionally greater boundaries with their
surroundings (Fig. 11.1d; Foxcroft et al. 2008; McLean et al. 2017). For example, a
survey inside a military base near Kimberley in the Northern Cape showed that alien
fleshy-fruited trees cultivated mostly as ornamentals, for shade, or to provide fruit,
spread beyond the confines of the base into the surrounding savanna (Milton et al.
2007). Dean and Milton (2019) describe the invasion of Myrtillocactus geometrizans
(Bilberry Cactus) into natural vegetation near the town of Prince Albert, Western
Cape, from a 40-year-old cactus garden in the town (Fig. 11.2f). Cilliers et al. (2008)
found that the cover of alien species increases with increasing proximity to the edge
of native grassland patches surrounded by urban and rural landscapes in
South Africa and Australia. Such examples provide evidence of a key process
driving many urban plant invasions.
During the Dutch and British colonial periods (1652–1871), most alien plant
species were introduced for timber production (forestry), fuelwood, shade and dune
reclamation (Richardson et al. 2003). Such introductions have left a major imprint
on plant invasions in natural and semi-natural ecosystems (Richardson et al. 2020,
Chap. 3). However, by the twentieth century there was an increasing emphasis on
amenity plantings in gardens and urban open spaces (Richardson et al. 2003). This
trend increased exponentially, and there has been an explosion of large and small
organisations (e.g. online traders) specialising in the dissemination of plants (usually
in the form of seeds or bulbs) worldwide in the last decade (e.g. Humair et al. 2015).
Despite a recent upsurge in the popularity of wild, drought-tolerant gardens
comprising mainly native plants, alien plants remain conspicuous features in all
South African cities. Early European settlers wanted to reconstruct the gardens of
Europe and these introductions were, in many cases, assimilated into local culture,
which perpetuated their further use (e.g. Davoren et al. 2016). Examples include oaks
(Quercus species) in Stellenbosch (nicknamed Eikestad in Afrikaans, i.e. “Oak
City”), and Jacaranda mimosifolia in Pretoria (“Jacaranda City”). At least 25 alien
tree species are protected as “Champion Trees of South Africa” under the National
Forests Act of 1998. Many of these occur in urban settings—68% of species listed as
“champion trees” are alien and 25% are currently listed as invasive. Updated information from the South African Plant Invaders Atlas (SAPIA, accessed 12 December
2018) shows that 18% of invasive alien plant taxa recorded for South Africa occur in
urban open spaces or around human habitation (Henderson and Wilson 2017). The
proportion of “urban invaders” in South Africa’s invasive flora is, however, much
greater than this, as 76% of taxa listed in Henderson’s (2001) book on “Alien weeds
and invasive plants” are ornamental plants that are grown in urban areas, and because
Henderson’s book focusses largely on natural-area invaders.
Life-history traits such as flower and fruit size and shape, growth rates, and the
capacity to flourish under harsh environmental conditions have driven the importation of many alien plants into urban areas of South Africa. As a result, large showy
flowers, colourful fruits, the capacity for rapid growth and the ability to survive
without irrigation are features of many widely planted alien plants in South Africa.
Waves of interest in new types of alien trees with particular features have occurred in
282
L. J. Potgieter et al.