There are substantial opportunities to investigate the interaction between geomorphology and invasions. Some areas, e.g. mountain tops, are much less invaded,
and an evaluation of which elements of the South Africa landscape are currently
more invaded and an assessment of the potential for future invasions would be very
useful.
13.3 Soils
Soil properties such as pH, texture, redox potential, and nutrient status have myriad
effects on the flora and fauna, but our understanding of how they influence invasions
is still rudimentary. In general, however, invasibility might be expected to be
correlated with the degree of disturbance (e.g. human-mediated physical or chemical
disturbance). This is because each soil type has a unique array of chemo-physical
properties, and through evolutionary time the native flora and fauna would have
developed specific adaptations to deal with these. For example, alien grass species
are known to thrive in the Fynbos Biome on old agricultural lands that have in the
past been fertilised (Milton 2004). The native fynbos plants did not in general evolve
on nutrient-rich soils and this places them at a disadvantage in such environments
(Cramer et al. 2014). Similar issues certainly affect the distribution of some alien
animal species, such as earthworms (Janion-Scheepers and Griffiths 2020, Chap. 7;
Janion-Scheepers et al. 2016).
Given the above principles, the effects of soils on invasions should be analysed on
a case-by-case basis. However, it might be possible to generalise for alien trees
invading Fynbos and Grassland. In particular the shift from a relatively short plant
form (shrubs or grasses) to a tall plant form (trees) can be explained in part by the
Catabolic Theory (Milewski and Mills 2010; Mills et al. 2016, 2017). The theory has
three main premises: first the availability of catabolic versus anabolic nutrients has
marked effects on vegetation structure; second that short plants (e.g. shrubs and
grasses) are more competitive than tree seedlings where demand for catabolic
nutrients is met by supply; and third that demand for catabolic nutrients is dependent
on the rate of photosynthate production (Milewski and Mills 2015; Mills et al.
2013a, b, 2016).
The presence or absence of alien trees in fynbos and encroaching native trees in
grassland environments of South Africa have been linked to a wide range of
nutrients, soil properties, and soil treatments, including pH, acid saturation, Mg,
Ca, Mn, Cu, Zn, B, P, and N-fertilisation (Mills and Allen 2018; Mills et al. 2017).
Unpublished data collected from 25 diverse sites across South African Fynbos,
Grassland and Savanna sites have shown that soils in sub-sites relatively poor in
boron or relatively rich in phosphorus tend to be less wooded than adjacent sub-sites.
Boron is of particular interest with regards to tree invasions because the physiological demand for boron per unit photosynthate produced from short, monocotyledonous plants such as grasses is considerably less than for dicotyledonous trees. Indeed,
it is the only nutrient that has this distinct difference in demand between grasses and
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and an evaluation of which elements of the South Africa landscape are currently
more invaded and an assessment of the potential for future invasions would be very
useful.
13.3 Soils
Soil properties such as pH, texture, redox potential, and nutrient status have myriad
effects on the flora and fauna, but our understanding of how they influence invasions
is still rudimentary. In general, however, invasibility might be expected to be
correlated with the degree of disturbance (e.g. human-mediated physical or chemical
disturbance). This is because each soil type has a unique array of chemo-physical
properties, and through evolutionary time the native flora and fauna would have
developed specific adaptations to deal with these. For example, alien grass species
are known to thrive in the Fynbos Biome on old agricultural lands that have in the
past been fertilised (Milton 2004). The native fynbos plants did not in general evolve
on nutrient-rich soils and this places them at a disadvantage in such environments
(Cramer et al. 2014). Similar issues certainly affect the distribution of some alien
animal species, such as earthworms (Janion-Scheepers and Griffiths 2020, Chap. 7;
Janion-Scheepers et al. 2016).
Given the above principles, the effects of soils on invasions should be analysed on
a case-by-case basis. However, it might be possible to generalise for alien trees
invading Fynbos and Grassland. In particular the shift from a relatively short plant
form (shrubs or grasses) to a tall plant form (trees) can be explained in part by the
Catabolic Theory (Milewski and Mills 2010; Mills et al. 2016, 2017). The theory has
three main premises: first the availability of catabolic versus anabolic nutrients has
marked effects on vegetation structure; second that short plants (e.g. shrubs and
grasses) are more competitive than tree seedlings where demand for catabolic
nutrients is met by supply; and third that demand for catabolic nutrients is dependent
on the rate of photosynthate production (Milewski and Mills 2015; Mills et al.
2013a, b, 2016).
The presence or absence of alien trees in fynbos and encroaching native trees in
grassland environments of South Africa have been linked to a wide range of
nutrients, soil properties, and soil treatments, including pH, acid saturation, Mg,
Ca, Mn, Cu, Zn, B, P, and N-fertilisation (Mills and Allen 2018; Mills et al. 2017).
Unpublished data collected from 25 diverse sites across South African Fynbos,
Grassland and Savanna sites have shown that soils in sub-sites relatively poor in
boron or relatively rich in phosphorus tend to be less wooded than adjacent sub-sites.
Boron is of particular interest with regards to tree invasions because the physiological demand for boron per unit photosynthate produced from short, monocotyledonous plants such as grasses is considerably less than for dicotyledonous trees. Indeed,
it is the only nutrient that has this distinct difference in demand between grasses and
362
J. R. Wilson et al.