Determining the fire-ecology type
There are important aspects of fire and ecology in South Africa that determine the ecologically-based risk assessment we report here.
- (a) the degree of fire dependence in a given vegetation type,
- (b) the relationship between fire dependence and both bush encroachment and the invasion of natural vegetation by invasive alien plants, and
- (c) the inherent relationship between climate, soil, vegetation and fire regime determine the nature of the natural-resource assets that can be cultivated in any given region, and hence the exposure of the asset to related fire risks.
The vegetation of South Africa can be divided broadly into two classes: fire-dependent types (FD) and climate-dependent types (CD).
The structure of the vegetation in FD vegetation is not limited by climatic conditions; without fire, the structure changes. FD vegetation requires fires to maintain its biodiversity and would become dominated by woody plant species if fires were excluded. FD types include Fynbos, Grassy Eastern Nama Karoo and areas of the Grassland and Savanna Biomes with more than about 650 mm of rain per year ([our region has between 670 and 745 mm per year]
The FD types in the Grassland and Savanna Biomes are mainly dominated by sour grasses and the CD types by sweet grasses. The FD types require high intensity fires to prevent them from becoming closed stands of woody species.
From this we see that there is a close correlation between the categories fire-dependent, and climate-dependent on the one hand, and sour and sweet grass vegetation on the other.
The distinction between “sour” and “sweet” grasses is important in the fire ecology types where grasses are the main fuel.
“Sour” grass cover typically occurs in high-rainfall (> 600-700 mm per year), in high-lying, and in cool areas. It is characterised by grass species, which grow very rapidly, produce coarse grazing and lose their nutritional value when they become dormant.
By contrast, sweet grass species predominate in climate-dependent types, in areas with lower and less reliable rainfall (< 600 mm) and produce a pasture that retains much of its nutritional value when the grasses become dormant. Sour grasses generally require frequent and regular fire that removes the unpalatable grass left behind by the grazing animals, which would otherwise inhibit grass regeneration and growth, and admit denser shrub and tree populations.
Assessment of the risk of inappropriate fire regimes
The main determinants of the nature of the veldfire management problem at any location in South Africa are vegetation, weather and terrain.
In each biome in which fire is a factor there is a fire regime that is appropriate to the maintenance of biodiversity in that biome (See addendum). A risk arises to biodiversity when there is a shift to an inappropriate fire regime.
Risk Assessment for Sour Grassland:
High – woody plant recruitment is not limited by rainfall and reductions in fire frequency and intensity will lead to bush encroachment
High – fires facilitate invasions by several species which can recover by sprouting and have fire stimulated seed germination; invasions can increase fuel loads and fire intensities, potentially leading to water repellence and soil loss.
Extract from: Classification of Veldfire Risk in South Africa for the Administration of the Legislation regarding Fire Management, F J Kruger, G G Forsyth, , L M Kruger, K Slater,D C Le Maitre and J Matshate