19.1.1  Heat

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Heat production from burning material (Q) is described thus:


where h is the mean heat of combustion in kJ kg–1 and w is fuel loading (biomass burnt) in kg m–2, giving Q the units kJ m–2.

Rates of heat production determine the impact of surface fires on topsoil, seeds and microbes buried in the topsoil and living plants. Fires burning below ground (ground fires) are not considered here (see Gill 1995). The rate of heat production by surface fires, or ‘fire intensity’ (Byram 1959), is given by:

where I is fire intensity in kW m–1 and r is the rate of spread of the ‘fire edge’ in m s–1. Note that the unit of fire intensity is expressed as a rate per unit of fire edge rather than a rate per unit area, thereby avoiding the need to make difficult measurements of the depth of flame at the edge.

Fires pass a plant very quickly — usually within a few minutes: how long flames persist at the base of a plant seems to change little within a fuel type (e.g. grass or litter) but greatly between fuel types such as grassland and sclerophyll forest (McArthur and Cheney 1966). Rates of spread of fires vary widely and, together with the more limited variation in fuel loadings, fire intensities vary from a few kW m–1 up to about 100 000 kW m–1 (Gill and Moore 1990). Large variations in the rates of spread of fires at their edges can occur within the one fire. Heat of combustion varies much less and is usually regarded as a constant.

So where does the heat from a fire go? It is transferred by conduction (down into soil, for example), radiation (especially from flames) and convection (typified by hot gases in the ‘convection column’ or ‘smoke plume’). In practical terms, the physical effects of these heat transfers on plants and soil are assessed by duration of exposure to high temperatures.