The effect of fire is a complex combination of the effect of the season in which a fire occurs, the frequency of fires and the intensity with which fires burn. Each of these factors is modified, reduced or enhanced by the others, while all three are dependent on fuel load, and all three affect the fuel load and its development.
Apart from the direct effect of fire on the vegetation, fire also changes
the soil chemistry, particularly in the upper soil layers (Mitlöhner
|The effect of fire on the soil chemistry|
However, as fire frequency increases, there is increasingly less time for a fuel load to accumulate. Should insufficient fuel accumulate, fires will, of course, not burn. For instance, in an attempt to set a prescribed burn in Namibia, a fire would not start due to insufficient fuel.
In his study in the Etoscha National Park, Siegfried
(1981) found, for instance, that the incidence of fire is very much higher
after a good rain than after poor rain. Siegfried attributed this
to the amount of herbaceous fuel that was able to build up.
|The effect of fire frequency on the development of the fuel load|
As the dry season progresses, the moisture contents of the fuel decreases, and the fuel becomes more flammable. Apart from the increasing ease with which a fire will start (i.e. increased fire danger) the fuel also burns up faster, leading to a higher fire intensity.
The physiological condition of the individual plants or plant species is also of considerable importance, as it affects the short term and long term damage to the vegetation as a whole.
Both, fuel moisture and physiology are influenced by the timing of the
last rains. Should the rains extend further into the dry season,
fuel moisture will be higher, and the growing period of some plants is
|The importance of fire intensity fire intensity|
Much of the fuel may still not be completely dry, particularly woody material. Damage to woody plants is then less than later in the season since fires are not as intense (Rutherford 1981).
In the long run early burning would favor the woody component of the woodlands. Perennial grasses that are induced to flush out of season will be weakened and will not sprout as vigorously during the following growing season (Tainton 1999), thus retarding the development of the herbaceous fuels.
Trapnell (1959) reported, for instance, that continued early burning permitted the formation of clumps of regeneration of woody plants in the Ndola plots, Zambia. These clumps shade out much of the herbaceous growth with the subsequent effect on the fuel load.
In order to promote the formation of closed woodlands, Trapnell (1959), Geldenhuys (1977) and Chidomayo (1988) recommend that management burns are carried out early in the dry season.
Very late fires may harm those plants that break dormancy before the first rains. For example, Perocarpus angolensis, Burkea africana and Schinziophyton rautanenii flower and flush before the onset of the rains. A fire during this time may destroy flowers (Büschel 1999) and the subsequent fruit crop, and / or damage early leaves, reducing the plants vigor. In addition, coppice shoots may be burnt back to ground level (Lawton 1978).Fire Intensity The effect of fire is greatly dependent on fire intensity, i.e. the rate at which heat energy is released during a fire. The rate at which energy is released from fuel is much dependent on the moisture content of the fuel. As the dry season progresses, fuels become increasingly dry, so that late season fires are of higher intensity than fires burning early in the dry season.. More plants are killed by more intense burns (Rutherford 1981).
Intensity is significantly correlated with the rate of spread of a fire, the faster the rate of spread, the higher the intensity (Trollope 1981). Therefore, as wind speed increases, so does fire intensity.
The size of the fuel that is burnt also plays an important role. Smaller fuels will produce a higher fire intensity than will larger fuels (Heikkila et al. 1993) due to the larger surface to volume of the smaller fuels, and their concomitant rate of heat release..
Apart from the type of fuel that is built up, the quantity of fuel also plays an important role. As the fuel load increases, fire intensity is often also increased (Graz 1996); stands become less dense, and more herbaceous material is able to develop due to an increase in light penetration (Skarpe 1991, Lawton 1978). If the areas are grazed sufficiently, the fuel load may, however, be reduced.
It must be remembered that the sandy soils that support the dry savannah
woodlands do not always support a dense grass cover, primarily because
of a shortage of water that must be attributed to a limited water holding
capacity of the soil. In this case, the growth form of a plant must
be considered. Bushes and coppice from dead stumps accumulate leaf
litter between multiple stems. This does not only lead to a higher
fuel load, but the fuel load accumulates up against the stems of plants.
This is probably the reason why Pterocarpus angolensis coppice is
unable to re-establish, as reported by Vermeulen