In order for a fire to occur in the woodlands fuel must be available.
The fuel load not only determines whether or not a fire will occur, but
together with the type of fuel affect the fire intensity of a fire.
Types of fuel
Just as the the vegetation can be divided into a woody and herbaceous component,
so may the fuel be divided. Leaf litter plays an important role as
well. However, it is more appropriately considered as part of the
herbaceous fuel. The woody fuel may then again be divided into different
size classes (though these may be arbitrary).
The size of individual pieces of fuel has important implications for fire,
due to the rate at which heat may be absorbed by the individual fuel particles
/ pieces. Heat uptake is faster in smaller fuels, due to their large
surface to volume ratio. This in turn will alow faster drying of
fuels that will make fuels more flammable. Small twigs are able to
reach combustion temperature much faster than larger ones, and will burn
up much faster.
The amount of heat required to ignite fuel increases with the moisture
contents of the fuel. Dry fuel is therefore much more combustible.
Smaller twigs or branches will be able to dry out much faster than will
larger branches. This also allows the smaller pieces of fuel to burn
much earlier in the dry season than larger pieces. Additionally,
the smaller fuels will increase their moisture contents much faster than
the larger pieces.
The effect of fuel size is modified by the wood properties of individual
The importance of small fuel must be highlighted. It is generally
the grass fuel, leaf litter and small twigs that will permit a fire to
take hold in the first place since they are able to reach combustion temperature
comparatively fast. This has important implications for fuel management.
For instance, grass fuel may be reduced through grazing, reducing the fire
Fuel load development / accumulation
Fuel build up depends very much on the time available between
successive burns, and the rainfall over this period. If the time
between burns is short (i.e. fire frequency is high), less fuel is able
to accumulate, particularly woody fuel, and the accumulated material will
be comprised primarily of herbaceous plant material and leaf litter.
As the time between burns is extended, the component of larger twigs and
branches will increase.
The effect is modified by the growth rate of different species occurring
in the area as well as their growth form; The presence of shrubs will add
smaller woody fuels to the load. In other words, fuel development
is determined by the prevailing vegetation in terms of species composition,
the population structure of individual species, and the effective rainfall.
(1981) indicated that fuel accumulated between the individual stems of
Grewia flava, causing the plant to be burnt almost to ground level.
Accumulation of herbaceous fuel is also affected by grazing. With
limited or no grazing, grass fuel is able to accumulate relatively fast
(all other factors being equal).
The rate a which combustible material is able to accumulate significantly
affects the period between burns (frequency
Fuel arrangement should be considered in the horizontal and vertical planes.
While the horizontal arrangement of fuel affects the ability of a fire
to spread, the vertical arrangement affects the height to which flames
may cause damage. Discontinuation in the fuel load may hamper a fire.
This is true for the vertical and horizontal progression of a fire.
A break in the horizontal plane, such as caused by roads or fire lines,
can prevent the fire from advancing. Similarly, a vertical arrangement
of combustible fuel may cause a fire to crown.
A third variable that should be considered is the compactness of the
fuel, a combination of the vertical and horizontal distribution, and the
fuel load. The arrangement of different pieces of fuel in relation
to each other affects the air movement between them. This in turn
influences the drying of fuel and the oxygen supply for burning (Heikkila