Following the coastline in the southwestern corner of South Africa is the Cape Floristic Kingdom, the smallest and richest of the world’s six floral kingdoms. This Mediterranean climate region is home to more than 9000 plant species, of which 69% are endemic (B.W. van Wilgen, 2009). Most of these plants grow as fynbos, a shrubland well adapted to the nutrient-poor soil and frequent wildfires that are characteristic of this area.
Fynbos vegetation consists mainly of shrubs, bulbs and short-lived herbaceous plants and annuals. The four distinct planting groups in this region are the Restios, Protea, Ericas and Geophytes (bulbous plants) (J. Kettle, 2016). Fynbos plants are recognised by their microphyllous and sclerophyllous leaves, the first being small, simple leaves and the latter being hard, tough and leathery leaves, allowing these plants to resist dry conditions by preventing wilting. Their leaves also contain lignin, which allows them to grow in nutrient-deficient soil.
Fire in fynbos ecology
Fire is necessary to rejuvenate the vegetation, by removing dying plants, and stimulate seed release and germination in fynbos, making it an important factor in its ecology (B.W. van Wilgen, 2009).
Fire cycles and post-fire vegetation age affect the plant species and soil quality in the area, thus affecting the ecology. Variation in fire-intervals and fire intensity induces a variation in density of overstorey shrubs, which in turn affects the diversity in understorey species. Pre-fire vegetation density also influences the density of post-fire recruitment, resulting in alternating densities and species diversity on the same site between different fires. Therefore, frequent fires can act as a buffer, saving plant populations from extinction by ensuring stable co-existence over time, despite local eradication by individual fires (B.W. van Wilgen, G.G. Forsyth, P. Prins, 2012).
Fire rejuvenates fynbos by burning it down before alien species have a chance to prevail; without fire, deciduous forest trees would invade and become dominant, eliminating the fynbos vegetation. The cycle of rejuvenation begins post-fire; fast-growing bulbous, short-lived plants and annuals grow into maturity first; while these species flourish, the slow-growing shrubs start to appear. Once the fast-growing species start dying and slowly disappear from the vegetation, the shrubs will prosper and dominate the area. When this mark has been reached, wildfires will occur once again and the cycle will start over (J. Kettle, 2016).
Long fire-free intervals lead to senescence and poor regeneration, causing local extinction of fire-dependant populations, thus giving alien species a chance to invade. On the other hand, increasingly shorter fire cycles cause fire to terminate the growth-cycle of some species prematurely; being unable to surpass their juvenile stage, they will be unable to sprout, causing these species to be eliminated from the vegetation (B.W. van Wilgen).
Adaptations to fire
The plants found in fynbos have a wide range of regeneration strategies and fire-survival mechanisms. The majority of these species are able to resprout shortly after fire. Most fynbos plants flower within one year of a fire and only 2% of species takes longer than 3 years to flower. Short-lived species (<10yr) are thought to have reasonably long-lived seed banks, enabling them to reappear after fire. In both reseeders and resprouters post-fire recruitment is more important than inter-fire recruitment, demonstrating the value of regular fires in this ecosystem (N. Allsopp, J.F. Colville, G.A. Verboom, 2014).
Some of the major fire survival and persistence strategies are listed below:
- Obligate reseeders – serotiny: These plants retain seeds in persistent fruits, protecting them for overlapping seasons. This type releases its entire seed bank post-fire after the parent plant has died. These seeds are short-lived once they have been released and germinate as soon as conditions are favourable; they do not require conditions associated with wildfires to germinate and do so best under cold conditions (<15°C).
- Obligate reseeders – ant- or rodent-dispersed: These plants use ants and rodents to disperse their seeds; the animals bury large protein-rich seeds, which are triggered to germinate by fire. Stimulants of germination are scarification, charate (plant-derived smoke) or appropriate soil and temperature regimes provided by fire.
- Facultative resprouter- reseeders: These species have adapted a dual strategy to survive fire; reproduction occurs through post-fire resprouting as well as through seedling recruitment from a fire-stimulated dormant seed bank.
- Obligate vegetative resprouters: Post-fire these plants exclusively display vegetative resprouts; seedling recruitment does not occur at this time.
- Heat-stimulate germination from soil-stored seed: Germination in these hard-coated, soil-stored seeds is triggered by sufficiently high fire intensities.
- Smoke-stimulated germination: Germination occurs when seeds are exposed to butenolide, a chemical present in plant-derived smoke. This species is fire-dependent for its reproduction.
- Fire-stimulated flowering: Profuse flowering occurs for one or more years after fire; this is largely due to indirect effects of fire, rather than direct effects. Altered soil temperatures and changed nutrient availability are examples of indirect effects.
- Fire ephemerals: these include any ephemerals, annuals or short-lived perennials, which exhibit a degree of stimulated germination.
- Fire avoiders: These species grow where fire does not occur, thus avoiding fire altogether. They are not fire-dependant for seedling reproduction (N. Allsopp, J.F. Colville, G.A. Verboom, 2014).
Poor nutritional values in the soil have led many plant species in fynbos to establish specialised root systems. One of these strategies for accessing nutrients, such as nitrogen (N) and phosphorous (P), is the formation of cluster roots; dense clusters of rootlets of determinate growth, growing along an elongating root axis. Cluster roots are highly variable, ranging from small collections of root hairs (<1cm ∅), to large complex structures (>5cm ∅) (N. Allsopp, J.F. Colville, G.A. Verboom, 2014). “Cluster roots proliferate in a restricted volume of soil and solubilise P from mineral complexes through the release of carboxylates and acid/alkaline phosphatases, which hydrolyse organic P complexes improving access to both mineral and organic P forms” (N. Allsopp, J.F. Colville, G.A. Verboom, 2014, p. 249).
Mycorrhizal symbiosis is another adaptation some plant species have implemented in their root system. Mycorrhizal symbiosis is a form of mutualism between roots and soil fungi, where the fungi colonise the roots of the plant. Mycorrhizal hyphae increase the volume of soil from which nutrients can be extracted, allowing a greater direct ‘interception’ of P (N. Allsopp, J.F. Colville, G.A. Verboom, 2014).
Mycorrhizal symbiosis and cluster roots are not mutually exclusive; some plant species have developed both cluster roots and mycorrhizas (N. Allsopp, J.F. Colville, G.A. Verboom, 2014).
Biological control – Alien species
There are different ways of approach when it comes to controlling invasive alien plants, including chemical, mechanical and biological control. Although the first two methods are useful for containing infestations, they do not provide a permanent or sustainable solution to the problem. Biological control options, however, provide more sustainable solutions; unfortunately, this method is only available for some of the invasive plant species in fynbos (B.W. van Wilgen, 2009).
There are several compelling arguments favouring biological control; such as the cost-effectiveness compared to the expense associated with herbicides; a higher degree of safety, especially in comparison to the risks linked to herbicides and prescribed fires; achievability of successful integration with other management practices; and, most attractive of all, this method of control is self-sustaining (B.W. van Wilgen, 2009).
Using biological control to tackle alien species in fynbos began in the 1960’s when seed-feeding insects were introduced to the area to initially combat Hypericum perforatum, and later also Hakea sericea. This project expanded over the next two decades and seed-feeding and gall-producing insects were released, targeting various alien species (B.W. van Wilgen, 2009). As a result of these releases, two terrestrial species are under complete control, and eight are under a substantial degree of control (N. Allsopp, J.F. Colville, G.A. Verboom, 2014).
Aside from the undeniable positive aspects of biological control, this method also has its risks. The outcome of the introduction of a foreign species into the fynbos biome cannot be predicted beforehand. Therefore, it is not possible to anticipate whether or not the benefits will outweigh the environmental costs. Some undesired outcomes could include unintentional impacting of non-target species and the disruption of food webs (B.W. van Wilgen, 2009). However, in contrast to other countries, in South Africa no substantial effects have yet been recorded on non-target species (N. Allsopp, J.F. Colville, G.A. Verboom, 2014).
Threats and future
The fynbos vegetation is constantly under threat, with 26 plant species already being extinct and many more facing the possibility of extinction (J. Kettle, 2016). There are several factors contributing to the increased number of species going extinct in the fynbos biome.
An increase in human population density and alongside this, the expansion of agriculture and commercial development are steadily eradicating fynbos.
Another important threat to fynbos is the change of timing and frequency of fires. Although they are essential to the fynbos ecology, when fires burn in the wrong season or when they occur either too frequently or not frequently enough, this can be detrimental to certain species. If intervals between fires are protracted, it can lead to alien plants dominating the area, but if these periods are too brief, some fynbos species will be unable to reach adulthood, leading to their elimination from this region.
The most significant threat to the conservation of fynbos ecosystems are invading alien species; over 150 species of alien plants can be found in the fynbos biome. The most important groups of invading species include the pines, hakeas, wattles and gums. Pines and hakeas are serotinous trees and shrubs, which produce copious amounts of seeds that are released upon the parent plant’s death in fires. These winged seeds are capable of spreading over great distances after fires and occur across the entire fynbos biome. These species form dense and impenetrable stands, overshadowing the fynbos vegetation. Another species disrupting the fynbos ecology is wattle; this species also produces copious seeds, but these are released on ripening. These hard-coated seeds accumulate in the soil and are spread along rivers banks, either by birds or when soil is moved around; when fire occurs the soil-stored seed banks are triggered to germinate in dense stands. One gum species, in particular, imposes a threat for the fynbos vegetation. The red river gum Eucalyptus camaldulensis), is aggressively invasive near river courses and in lower-lying areas (B.W. van Wilgen, 2009).
Although the Cape Floristic Kingdom will face new challenges in the future and will require continuous adaptation in order to survive, it needs to be conserved as best we can. This biodiversity hotspot is home to an impressive amount of endemic animal and plant species and should be preserved for this reason alone. Sadly, the world we live in is more concerned about economic advantages than ecological preservation, so hopefully the commercial importance of this area will be enough to help protect this beautiful and diverse region from extinction.