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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.

These are:

(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

A fire break a year later

The photograph below is taken of a 2010's firebreak, which was not repeated in 2011. The new growth on the break is distinctly different from the surrounding (unburnt) grassland as may be expected. However, closer examination shows that the growth on the firebreak is almost entirely Hyparrhenia hirta (thatch grass) with some Cymbopogon excavatus (turpentine grass). What is significant, is that when Egoli Granite Grassland is considered degraded, Hyparrhenia hirta dominates. GJ Bredenkamp explains that "anthropogenic Hyparrhenia hirta dominated grassland" is what REPLACES the threatened Egoli Granite Grassland. In other words, the activity of man - in the burning of a fire break - is transforming the grassland.

Changes in Egoli Grassland

Addendum

The study quoted above references "Vegetation in Southern Africa" by RM Cowling, DM Richardson and SM Pierce (2004); V International Conference on Forest Fire Research, 2006

These authors state:

Fire is widely used for management purposes in more humid "sourveld" grasslands to maintain the vigour and palatability of the grass sward. Fires are burnt at annual or biennial frequencies (1-2 year intervals), since fuel accumulates rapidly in these systems. Season of burn is determined by efforts on sward productivity and composition, the condition of plant cover in relation to wind and water erosion and the hazards of runaway fires. In general fires in the dormant season or early in the growing season are most favoured for maintaining grass productivity and composition. Fires in the summer growing season have detrimental effects on grass cover and composition. Many grasslands are also being colonised by invasive alien species (including Australian Acacia species and pines (especially pinus patula). Frequent fires can be used to suppress the invasion process.

SAVANNAS

The key principle driving the use of fire for controlling bush encroachment involves burning at frequencies and intensities that will prevent juvenile trees from escaping the height where the canopy is no longer killer by fire. Fires burnt at the end of the dry season, after trees have leafed out but before rains have initiative grass growth, are most effective at reducing tree biomass. Fires burnt during or toward the end of the rainy season, favour woody plants at the expense of grass.

Fire and Conservation

Fire prescriptions developed for livestock farming have often been applied to nature reserves to promote the large mammal component. Recently this farming approach to fire has come under criticism, since it might promote some components of the biota at the expense of others.

...in Grass and Savanna biomes, calls have been made for more variable fire-regimes (varied frequencies, varied seasons, varied burn intensities) in conservation areas in line with the objectives of conserving biotic diversity.

Where feasible "natural" fire regimes have been proposed, meaning in practice, the lightning fires be allowed to burn and other fires suppressed. The rationale is that the fire regime under which the biota evolved is most likely to ensure survival.

One problem with this approach is that it has no predictive value. A "Natural" fire -regime may produce undesirable changes in terms of other management objectives, but the cause of these changes requires understanding of fire/plant interactions. ...Human have ignites fire in the region for hundreds of thousands of years at unknown season, frequencies and intensities. The spread of the fires from an ignition site has been curtailed by extensive fragmentation of nearly all fire-prone vegetation by roads, buildings, croplands, etc. Fires ignited by lightening in most contemporary landscapes will not be able to travel as far as they did in the past, this confounding the attempts to recreate "natural" fire-regimes. Nevertheless, for economic reasons along, fire management is likely to become more expensive and fire-regimes less rigid in conservation areas in the future.