Box 3.1 South Africa: World Capital of Tree Invasions
The dominance of trees among invaders of natural and semi-natural vegetation
is a striking feature of South Africa’s naturalised flora. Of South Africa’s
759 naturalised plant taxa (Supplementary Appendix 3.1), roughly a third are
trees (240; 32%), following the criteria for separating trees from shrubs
proposed by Richardson and Rejmánek (2011) (“perennial woody plants
with many secondary branches supported clear of the ground on a single
main stem or trunk with clear apical dominance”). Another 36 taxa are
generally classified as shrubs, but some may assume tree-like stature.
Together, these 276 woody plant taxa make up 36% of South Africa’s
naturalised flora. Taxa classified primarily as trees belong to 56 families and
120 genera. Myrtaceae (45 species from 11 genera) and Fabaceae (38 species
from 11 genera) are the dominant families.
Genera of alien trees for which invasions have been well studied in
South Africa are Acacia, Casuarina, Eucalyptus, Pinus, Prosopis, and Schinus.
Insights on invasions of these taxa have contributed substantially to the understanding of tree invasions globally (Richardson et al. 2014; Rundel et al. 2014).
The phenomenal success of trees as invaders in South Africa is probably at
least partly due to the massive propagule pressure and long residence time
because of repeated introductions and widespread plantings over more than a
century. However, several ecosystem types in South Africa appear to be
extraordinarily susceptible to invasion and transformation by alien trees.
Species-rich fynbos shrublands are highly vulnerable to invasion by trees
from other fire-prone regions of the world. Serotinous Pinus species from
Europe and Central and North America, and Australian Acacia species with
soil-stored seeds that are stimulated to germinate by fire have invaded vast
areas of fynbos, transforming shrubland vegetation into woodlands or forests
over several decades (Richardson and Brown 1986; Richardson and Kluge
2008; Richardson and Cowling 1992).
Riparian habitats throughout South Africa have been severely invaded by alien
trees, especially species in the genera Acacia, Eucalyptus, Populus, and Salix.
These invasions are driven primarily by dispersal of propagules along rivers and
through disturbance caused by flood events. These invasions are self-reinforcing in
that stands of naturalised plants trap sediments, thereby creating abundant habitat
for further establishment of seedlings and detached plant parts (Galatowitsch and
Richardson 2005; Holmes et al. 2005). Invasions by these species are widespread
in the wetter parts of the country, and also extend along perennial rivers throughout
the arid Karoo, and in the Grassland and Savanna Biomes.
Inundation of floodplains during periods of above-average rainfall has triggered invasions of several species in South Africa, notably of Prosopis spp. in the
arid interior of the county (Harding and Bate 1991). Groundwater availability
appears to limit the extent of these invasions; water in floodplain aquifers is easily
(continued)
3 The Biogeography of South African Terrestrial Plant Invasions
77
The dominance of trees among invaders of natural and semi-natural vegetation
is a striking feature of South Africa’s naturalised flora. Of South Africa’s
759 naturalised plant taxa (Supplementary Appendix 3.1), roughly a third are
trees (240; 32%), following the criteria for separating trees from shrubs
proposed by Richardson and Rejmánek (2011) (“perennial woody plants
with many secondary branches supported clear of the ground on a single
main stem or trunk with clear apical dominance”). Another 36 taxa are
generally classified as shrubs, but some may assume tree-like stature.
Together, these 276 woody plant taxa make up 36% of South Africa’s
naturalised flora. Taxa classified primarily as trees belong to 56 families and
120 genera. Myrtaceae (45 species from 11 genera) and Fabaceae (38 species
from 11 genera) are the dominant families.
Genera of alien trees for which invasions have been well studied in
South Africa are Acacia, Casuarina, Eucalyptus, Pinus, Prosopis, and Schinus.
Insights on invasions of these taxa have contributed substantially to the understanding of tree invasions globally (Richardson et al. 2014; Rundel et al. 2014).
The phenomenal success of trees as invaders in South Africa is probably at
least partly due to the massive propagule pressure and long residence time
because of repeated introductions and widespread plantings over more than a
century. However, several ecosystem types in South Africa appear to be
extraordinarily susceptible to invasion and transformation by alien trees.
Species-rich fynbos shrublands are highly vulnerable to invasion by trees
from other fire-prone regions of the world. Serotinous Pinus species from
Europe and Central and North America, and Australian Acacia species with
soil-stored seeds that are stimulated to germinate by fire have invaded vast
areas of fynbos, transforming shrubland vegetation into woodlands or forests
over several decades (Richardson and Brown 1986; Richardson and Kluge
2008; Richardson and Cowling 1992).
Riparian habitats throughout South Africa have been severely invaded by alien
trees, especially species in the genera Acacia, Eucalyptus, Populus, and Salix.
These invasions are driven primarily by dispersal of propagules along rivers and
through disturbance caused by flood events. These invasions are self-reinforcing in
that stands of naturalised plants trap sediments, thereby creating abundant habitat
for further establishment of seedlings and detached plant parts (Galatowitsch and
Richardson 2005; Holmes et al. 2005). Invasions by these species are widespread
in the wetter parts of the country, and also extend along perennial rivers throughout
the arid Karoo, and in the Grassland and Savanna Biomes.
Inundation of floodplains during periods of above-average rainfall has triggered invasions of several species in South Africa, notably of Prosopis spp. in the
arid interior of the county (Harding and Bate 1991). Groundwater availability
appears to limit the extent of these invasions; water in floodplain aquifers is easily
(continued)
3 The Biogeography of South African Terrestrial Plant Invasions
77