19.1  Physics and chemistry of an ecosystem on fire

Printer-friendly version

Fire creates heat and gases (‘smoke’) as living and dead plant material burns. However, the immediate impact of fires varies, depending on the amount of combustible material, how fast it burns and how fast heat is transferred to the surrounding biota. Fire is analysed here as a physical phenomenon by quantifying the release of heat energy as plant materials burn. However, fires do more than release energy; they have chemical impacts on plants through smoke and ‘ash-bed’ effects that are dealt with in Section 19.2. In turn the biological responses of plants to fire intensities are highly variable, depending on species and time scales (Sections 19.2 and 19.3). The effect of fire on an ecosystem is a function of: (1) intervals between fires; (2) fire intensities, that depend on fuels, atmospheric conditions and terrain; (3) seasonalities of fires; and (4) whether or not fires burn any peat that may be present. These variables are the components of ‘fire regimes’ (Gill 1975, 1981a).

Complex impacts of fires can be seen in Figure 19.1, showing differential effects of fire on woodland and open forest growing on a dissected sandstone landscape north of Sydney. The extent of burnt and unburnt plant material left after this fire varies dramatically, giving rise to differential fuel loads that will affect subsequent fire regimes. Erosion patterns also vary locally after such large fires, affecting rates of revegetation and production of new fuel.


Figure 19.1 A dissected sandstone landscape north of Sydney (latitude 33°S) before (a) and after (c) a severe fire in January 1994. Complex effects of burning are well illustrated in this area of east-west ridges and valleys. Larger inputs of solar energy on north-facing slopes than south-facing slopes affect vegetation type, fire regimes and subsequent erosion patterns that form east—west valleys. (a) Vegetation on ridges and north-facing slopes is eucalypt woodland and shrubland with sparse foliage cover and much bare ground; on south-facing slopes and along creeks vegetation is denser open eucalypt forest with little or no bare ground. (b) Diagram to show: ridge crests (dashed lines), sometimes in the form of roads (thick solid lines); creeks (thinner solid lines); open forest (stippled) and woodland and shrubland (no symbol). (c) Immediately after fire and before scorched leaves are shed from the tree canopy. Woodland and shrubland is extremely exposed while in open forest a dense but scorched leaf canopy remains. Some creeklines still have green vegetation that could burn in a future fire. (Aerial photographs courtesy New South Wales government: (a) Homes Valley New South Wales, Run 13, Frame 86, 27 November 1992 and (c) Hunter Region Bushfires January 1994, New South Wales 4185 (M1979), Run 19, Frame 159, 4 February 1994; (b) illustration courtesy D.A. Adamson and P.M. Selkirk)