Fire intensity is the term used to describe how hot the fire has burned. In fire management circles, we use the terms “hot fires”, and “cold fires”.
A hot fire is one that burns at a very high temperature, moves very slowly, and as a result tends to burn all the material present. In contrast, a cold fire would be one that moves very quickly, is not as hot, and leaves some of the heavier fuels (plant material such as tree trunks) behind. There is, of course, a large variation between these two extremes, and the general factor that determines a hot or cold fire is the amount and type of fuel present.
Fuel refers to the amount of combustible vegetation, usually in the form of dead or dry shrubs and trees, but can include plants that have volatile compounds such as oils (many Fynbos species) and tannins (Gum trees). The intensity of a fire has a dramatic impact on the environment. High intensity fires (hot fires) can be extremely destructive, causing damage to seedbeds located underground, the killing of plants that would normally re-sprout after the fire, the death of burrowing animals, and the changing of the upper soil structure (baking the soil) that affects the germination potential of some seeds.
Cold fires too can be counter-productive to the environment, with not enough heat being generated to cause some seeds to germinate, not being able to burn enough older plants to allow space for young plants to grow, or not being able to remove sufficient fuel loads which could result in a destructive hot fire next time. Due to much of our environment now needing to be actively managed by Man (as a result of development, introduction of alien plants, fragmentation of land, and land use), the management of fuel loads has now become necessary.