19.1.3  Temperatures

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Physical difficulties of measuring temperatures of fires have been partly overcome by the use of thermocouples: temperatures up to about 900°C have been measured in Australian fires (Moore et al. 1995).

How long temperatures at the plant surface remain at 100°C is critical in predicting the extent of tissue death from fires (Gill 1981b). While tissues are dehydrating, the temperature stays at boiling point but once most of the water has been lost, temperatures will rise. Depths of bark affected by the elevated temperatures of fires can be quite shallow. For gumbarked species, this depth can be estimated some time after the fire has passed because bark killed by the fire is shed from the tree while live bark persists (Gill 1981b).

Soils, like bark, are good insulators. A low-intensity fire — say, about 90 kW m–1 — produces substantial temperatures in the flaming zone while temperatures in the surface soil only rise to about 100°C ( Shea et al. 1979). However, more intense fires — up to 2500 kW m–1 — heat soil to a depth of 6 cm and increase surface soil temperatures more than 250°C above ambient (Shea et al. 1979). Heating of surface soil can kill microorganisms, seeds and other plant tissue. Changes in soil chemistry may also occur which, together with sterilisation, can temporarily increa