Fuel load

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).
Fuel Size
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.
Fuel moisture
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 species.

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 danger..

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. Rutherford (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 of fire).

Fuel arrangement
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 et al. 1993).
See also:
Fuel load and the species composition of woody plants
The effect of woody population structure on fuel load
Fuel load accumulation and frequency of fire
The importance of effective rainfall for fuel load development
Fuel load development and the effect of grazing