trees. In the context of the Catabolic Theory, poverty of boron will reduce anabolism, reducing the demand for catabolic nutrients, preventing plants from building up
a surplus of photosynthates, and thereby favouring short herbaceous plants over tall
woody plants. By contrast, phosphorus is predominantly catabolic as it is needed for
producing adenosine triphosphate (the molecule that stores the energy released from
the breakdown of carbohydrates and so the ultimate endpoint of catabolism in
plants). A deficiency of phosphorus, like deficiencies of copper and zinc, results in
accumulation of carbohydrates in plant tissues (Broadley et al. 2012; Graham 1980).
Further research is needed to establish whether soil amendments that bind boron or
increase phosphorus availability can constrain the establishment of alien tree
seedlings.
Tree invasions can also be explained by increases in carbon dioxide levels. As
carbon is a limiting anabolic element (as per The Catabolic Theory), increases in
carbon in the soil boosts anabolism relative to catabolism, resulting in a surplus of
carbohydrates. Thus, greater carbon dioxide levels shift the competitive balance
towards carbohydrate-rich plants such as trees.
The impact of edaphic factors on invasions is still, however, an area ripe for more
research. There have been a few fairly limited autecological studies [e.g. granite
specialists such as Sweet Hakea (Hakea drupacea) occur on granite; and the invasive
New Zealand Christmas Tree (Metrosideros excelsa) in the Western Cape requires
moist organic-rich substrates for germination and establishment, allowing fine-scale
habitat suitability to be accurately predicted based on native species with similar
edaphic preferences (Rejmánek et al. 2005)]. There is, however, also on-going
broader-scale research in the Fynbos Biome. Although fynbos soils are very poor
in key nutrients, similarly nutrient-poor soils elsewhere in the world, e.g. Western
Australia, support forest vegetation. The paucity of trees in fynbos has been attributed to nutrient and/or water limitations. However, the success of alien trees such as
acacias and pines dispels the myth of such resources as a major barrier to tree growth
in the fynbos (Richardson and Cowling 1992). Moreover, the interaction between
nitrogen-fixing alien Acacia species and the nitrogen-poor soils has resulted in
dramatic ecosystem-level impacts and regime shifts (Gaertner et al. 2014; Holmes
et al. 2020, Chap. 23). Again, this emphasises the importance of considering bioticabiotic interactions in mediating invasions and their impacts, e.g. the fynbos is
highly susceptible to soil-altering invaders. Another result of the fact that many of
the landscapes, particularly in the greater Cape Floristic Region, can be characterised
as OCBILs i.e. “old, climatically buffered, infertile landscapes” (Hopper 2009), is
that the edaphic fauna is composed of many ancient lineages. Such lineages show
little biotic resistance to invasion, although this has not been well studied (JanionScheepers et al. 2016).
13 The Role of Environmental Factors in Promoting and Limiting Invasions
363
a surplus of photosynthates, and thereby favouring short herbaceous plants over tall
woody plants. By contrast, phosphorus is predominantly catabolic as it is needed for
producing adenosine triphosphate (the molecule that stores the energy released from
the breakdown of carbohydrates and so the ultimate endpoint of catabolism in
plants). A deficiency of phosphorus, like deficiencies of copper and zinc, results in
accumulation of carbohydrates in plant tissues (Broadley et al. 2012; Graham 1980).
Further research is needed to establish whether soil amendments that bind boron or
increase phosphorus availability can constrain the establishment of alien tree
seedlings.
Tree invasions can also be explained by increases in carbon dioxide levels. As
carbon is a limiting anabolic element (as per The Catabolic Theory), increases in
carbon in the soil boosts anabolism relative to catabolism, resulting in a surplus of
carbohydrates. Thus, greater carbon dioxide levels shift the competitive balance
towards carbohydrate-rich plants such as trees.
The impact of edaphic factors on invasions is still, however, an area ripe for more
research. There have been a few fairly limited autecological studies [e.g. granite
specialists such as Sweet Hakea (Hakea drupacea) occur on granite; and the invasive
New Zealand Christmas Tree (Metrosideros excelsa) in the Western Cape requires
moist organic-rich substrates for germination and establishment, allowing fine-scale
habitat suitability to be accurately predicted based on native species with similar
edaphic preferences (Rejmánek et al. 2005)]. There is, however, also on-going
broader-scale research in the Fynbos Biome. Although fynbos soils are very poor
in key nutrients, similarly nutrient-poor soils elsewhere in the world, e.g. Western
Australia, support forest vegetation. The paucity of trees in fynbos has been attributed to nutrient and/or water limitations. However, the success of alien trees such as
acacias and pines dispels the myth of such resources as a major barrier to tree growth
in the fynbos (Richardson and Cowling 1992). Moreover, the interaction between
nitrogen-fixing alien Acacia species and the nitrogen-poor soils has resulted in
dramatic ecosystem-level impacts and regime shifts (Gaertner et al. 2014; Holmes
et al. 2020, Chap. 23). Again, this emphasises the importance of considering bioticabiotic interactions in mediating invasions and their impacts, e.g. the fynbos is
highly susceptible to soil-altering invaders. Another result of the fact that many of
the landscapes, particularly in the greater Cape Floristic Region, can be characterised
as OCBILs i.e. “old, climatically buffered, infertile landscapes” (Hopper 2009), is
that the edaphic fauna is composed of many ancient lineages. Such lineages show
little biotic resistance to invasion, although this has not been well studied (JanionScheepers et al. 2016).
13 The Role of Environmental Factors in Promoting and Limiting Invasions
363