High pressure ridges tend to develop over Alaska, the Yukon and the Northwest Territories of Canada. As summer progresses, the center of origination shifts to the east. Once formed, the ridges generally move in a southeasterly direction (Johnson, 1992, p. 19).

After a period of high temperatures, any breakdown in an atmospheric ridge can lead to forest fire. Thunderstorms and lightning are generally associated with the warmer, unstable air of low pressure areas. In addition, high and variable winds in front of or behind the atmospheric trough provide a mechanism for fire spread. Brotak and Refsnyder (1976) found that of major wildland fires in North America, most follow the passage of a cold front (Johnson, 1992, p. 19).

Once a forest fire has begun, a number of complex processes begin to occur. Initially, the fire spreads by heating and then igniting unburned woody and herbaceous fuels (Johnson, 1992, p. 22). This involves the evaporation of any remaining moisture within the fuel and the thermal breakdown of cellulose, as well as the ignition of volatiles to produce a visible flame. Heat is transferred primarily by convection and radiation as the front c

It is during this phase of the forest fire that flames may extend upwards and ignite tree crowns (Johnson, 1992, p. 47). The crown layer includes live branches and foliage within the canopy. Between the crown and the surface exist the ladder fuels. These may consist of intermediate sized trees, saplings, arboreal lichens, and live or dead lower branches.

As a result of this variability, forests actually tend to be mosaics of tree stands, each having different times since they last burned. Tree populations are therefore never examined on an individual basis. Instead, trees are usually examined as groups. These populations are divided on the basis of birth and death schedules, etc.

After flaming combustion has passed, glowing combustion subsequently takes place. This second phase of forest fire oxidizes any residual carbon at the surface. In most forests, the ground is covered by layers of duff. Duff consists of soil and a litter layer. This litter layer may include leaves, needles, herbaceous vegetation, low and medium shrubs, tree seedlings, stumps and dead tree boles and branches. The ligin components of these materials are very stable when heated; they produce a carbonaceous compound called char (Johnson, 1992, p. 3940). Glowing combustion is surface oxidation of this char. The ultimate result of the process is preparation of the forest floor for the growth of new seedlings (Johnson, 1992, pp. 2247).

In purely deciduous forests, fire frequency may have some effect on stand structure. By injuring trees, fire may contribute to the number of treefall gaps. These gaps are subsequently exploited by an array of trees which are adapted to different degrees of shade, canopy closure, and