Fire regime

Fire affects different species and size classes in different ways, and plants may die as their fire tolerance is overcome.  The effect of fire on the plants themselves depends on the ability of the plants to recover in the time available between burns.  As the frequency of fires increases, this time interval is reduced.

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 1997).
See also:
The effect of fire on the soil chemistry

Fire Frequency

Fire frequency refers to the number of fires over a given period of time.  If plants are not killed outright by fire, the effect of frequency of burning is very  much dependent on the degree to which plants are able to recover in  the intervals between fires.  As the frequency increases, the vegetation has progressively less time to recover.

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.
See also:
The effect of fire frequency on the development of the fuel load

Fire Season

Fires generally occur in the dry season or at the beginning of the wet season, primarily due to the moisture content of the fuel load.  However, the timing within the dry season, that is at the beginning or the end of the season, has important consequences for the overall effect of fire.

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 effectively lengthened.
See also:
The importance of fire intensity fire intensity

Early in the dry season
At the beginning of the dry season most of the woody plants have reached dormancy (Lawton 1978), although species such as Baikiaea plurijuga and Burkea africana may continue growing further into the dry season.

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.

Late in the dry season
Later in the dry season the fuel will have dried thus increasing its rate of combustion and concomitant intensity.  Damage to woody plants is therefore higher than for early season fires (Rutherford 1981).  Lawton (1978) also indicates that the crowns of canopy trees may be damaged if there is sufficient fuel.

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 (1990).
See also:
Fire season