KNOWLEDGE BUILDING & SHARING
The Relationship between Fynbos and Fire
Speaking of fire, Gilian van Wyk, one of our active members, contributed the following piece on the strong relationship between fynbos and fire. She is of the opinion that this relationship is centered on 4 factors – frequency of the fires, the area in which the fynbos grows, the time of year of the fires and the intensity of the fires.
However, before looking at these 4 factors, one needs to understand why fire is at all necessary for fynbos and why fynbos is referred to as a pyrophylic or fire-loving vegetation, dominated by plants that have adapted to the presence of fire. Fire has a double impact on fynbos – it plays a role in germination and it also acts as a mineralizing agent. Some fynbos species die during fire and regenerate from seed stored in the canopy e.g. Serotinous proteaceae. Other species build up seed stores in the soil, such as Minetes spelendidus. The seed can be stored in shallow soil or it can be stored deep in the soil. Germination of the seed is stimulated directly through heat or smoke, or indirectly through changed environmental conditions. Other species can re-establish by sprouting from a woody root-stock after fire, stimulating new growth to occur.
Fire acts as mineralizing agent in fynbos in that the ash left after the fire returns mineral elements that were held above ground by the plants back to the soil. The disturbance that fire causes also makes water, nutrients and light more available for a certain period after the fire. This is very important as the soil in which fynbos grows is considered low in soil fertility. Thus fire-stimulated germination could very well be an evolutionary response to the increased availability of nutrients and other resources and the reduced competition after fires. Certain bulbous species, as well as smaller perennials and shrubs struggle to compete with larger shrubs, and have the opportunity to germinate and flower after fire removes some of the competition. Thus fire can enhance the diversity of fynbos.
Frequency of fires
It is generally known that the frequency of fires has a big impact on fynbos. Fires that occur too often can in fact destroy seed banks as some species take 5 to 6 years to mature and shed seeds. Fires can thus reduce biodiversity, cause erosion and the death or migration of important pollinators and predators. Generally speaking, fires should occur between 10-25 years to ensure species richness. The best way to describe an optimum time between fires is a time when 50% of the population of the slowest maturing species in a given area has flowered for at least 3 successive seasons.
Area of growth
Another factor that plays a role in the relationship between fynbos and fire is where fynbos grows and the climatic and rainfall cycles of the areas of growth. Generally speaking fynbos in moist mountain and lowland areas should burn every 12-20 years. However, fynbos in arid mountain areas should only burn every 25 years.
Time of year
Fires at different times of the year have different impacts on fynbos species. Historically most fires occurred during summer and many species killed by fire show maximum seedling recruitment after late summer and early autumn fires. An example is Watsonia borbonica. However, intense summer fires can destroy seed banks stored in shallow soil, whereas cooler winter fires stimulate germination – such as with Stoebe plumosa. Thus fires at different times of the year impacts differently on different fynbos species.
Intensity of fires
The intensity of fires is associated with the season of the fire. Summer fires tend to be higher in intensity and winter fires have a lower intensity. The presence of alien vegetation can also affect the intensity of a fire, as the flammable oils in alien vegetation and the large biomass can increase the intensity of a fire. Many fynbos species regenerate well after a high intensity fire, such as Mimetes fimbriifolius.
Thus the relationship between fynbos and fire is a strong but complex relationship, depending on many factors, including the frequency of fires, where the fynbos is situated, the time of year of the fire and the intensity of the fire. If one looks at the regular interval of fires in the Western Cape fynbos biome, this could no doubt become a huge problem for the survival of some fynbos species.
- Cape Nature, Fire Management Fact Sheet.
- The Ecology of Fynbos, 1992, edited by R.M. Cowling. Oxford University Press: Cape Town.