4 - Fire intensity

Summary

Fires in the boreal forest are characterized by high intensities which result in extensive mortality in canopy and understory plants. This chapter will examine the heat transfer from a flaming fire front (fire intensity), how the high intensities in the boreal forest come about and how these intensities can be coupled to the effects on plants.

The flame is the essence of a fire to most people. To characterize this flame, the rate at which heat is given off by the flame (fire intensity) is more useful than the flame's temperature. Temperature is a quantification of the degree of hotness of a body, while heat is the quantification of the work transferred from a body at higher temperature to one at lower temperature. The temperature of a single burning twig can be the same as a large crown fire yet clearly the crown fire is transferring more heat from the flaming front to the immediate environment than is the burning twig. It is this heat transfer (intensity) which causes the adjacent fuels to be heated and burn, thereby releasing more heat and propogating the fire. Also, plant death and injury will be dependent on the heat transferred to them and on how much of this heat is absorbed so as to raise the plant's temperature to the lethal level.

The concept

Combustion of forest fuels involves pyrolysis, the chemical decomposition of the fuel by the action of heat, followed by ignition. Two types of combustion are recognized, flaming and glowing (Shafizadeh and De Groot 1976). Flaming combustion involves the fuel's extractive and holocellulose chemical components.