Weather and climate control fire occurrence and spread in the boreal forest. As we shall see in this chapter, airstream movements and boundaries limit the length of the fire season and determine the seasonal geographic progression of fires. Further, a critical synoptic weather pattern of upper level (50 kPa) ridge build-up and breakdown is often responsible for the ignition and spread of large and widespread fires. The weather determines these patterns by generating the principle ignition source (lightning), controlling the most significant variable of fuel flammability (fuel moisture) and providing the high winds required for rapid spread.

Fire occurrence

The occurrence of forest fires in the boreal forest of Canada is given in Figure 2.1. This map was constructed from over 40 000 individual forest fires from all causes for 1961–6. Although the period is short, and not all areas are covered with the same thoroughness (some areas, e.g. northern Quebec, have no records at all), the map still presents the characteristic pattern of fires in the boreal forest (Simard 1975). Because of the scale of the map, the localized high fire occurrence in Figure 2.1 associated with population centers (e.g. La Ronge, Saskatchewan and Chicoutimi, Quebec) and transportation corridors (e.g. south from Moosonee, Ontario) have been removed. Also less obvious is the higher fire occurrence in the summer cabin region running from north of Toronto to north of Montréal and the large area of reduced fire occurrence southeast of North Bay, Ontario caused by Algonquin Provincial Park.