Up to this point, we have examined three fire behavior processes and effects which operate at an intimate level. We have considered forest fires in heat transfer terms and the effects on plants in terms of heat absorbed and combustion. However, it is also useful to step back and consider the fire's frequency and its implications for the tree population's hazard of death by fire. This landscape view has been of primary interest to ecologists (e.g. Knight 1987).

The boreal forest is a mosaic of stands each having different times since they last burned. Heinselman (1973) has called this landscape pattern the stand origin map (Figure 6.1). Most fire history studies are implicitly based on this map. The mosaic in the stand origin map is the result of a complex pattern of overlapping past fires. Only the most recent fires will appear on the stand origin map in their complete form, with all other fires being in some part obliterated by subsequent fires. The origin and meaning of this age mosaic is of central interest to any understanding of vegetation and fire dynamics in the boreal forest.

Two approaches can be used to understand the stand origin map. The fire frequency approach calculates the survivorship of stands in a landscape. The fire pattern approach is concerned with the age pattern of adjacent stands and the spatial autocorrelation in the stand origin map.

Fire frequency from the stand origin map

Fire frequency can be understood by the following examples used by Van Wagner (1978). Imagine a checkerboard which consists of a collection of 1000 squares (stands).