We have discussed stand-level silviculture without addressing how these stands should be distributed over the landscape. Here again knowledge of natural disturbance regimes provides some clues to this question. Based on a study of forest composition as a function of time since the last fire developed a simple model that ofthelondscape Proportion of hardwood. mixedwood and softwood stands in the landscape in relation to the fire cycle. enables prediction of the proportion of each stand type as a function of the tire cycle. If we assumeth at fires occur randomly, age class distribution follows a negative exponential where about two-thirds of stands are younger than the fire cycle By associating each age class of a given composition, it is possible to determine the proportion of each stand type for different fire cycles. The model may be adapted to landscapes with different proportions of site types presents results for the Lake Duparquet Forest. Hardwoods dominate the landscape up to a fire cycle of 100 years and the proportion of softwood- dominated stands increases as the fire cycle lengthens. Mixedwood stands, however, remain abundant even for very long fire cycles. This is due to the constant presence and periodic outbreaks of spruce budwoxm, which contribute to the maintenance of a good proportion of mixedwood stands. In assuming a natural fire cycle of approximately 100 years, the landscape should be made up of about 50% hardwood, 35% mixedwood and 15% softwood stands The model does not, however, provide information
about spatial distribution and size of stands that will be subject to management. The area covered by natural fires provides considerable flexibility in this respect For example? the 1916 fire burned only a few hectares while the 1760 tire burned almost the entire area of the Lake Duparquet Forest. Most fires, however, leave many unburned islands or
patches within the fire limits. More study is needed to determine how our understanding of natural disY. Bergeron. B. Harvey/ Forest Ecology and Management 92 (1997) 235-242 241 turbance occurrences can guide the spatial distribution
of forestry interventions. Criteria should be drawn from our understanding of natural disturbances as well as from objectives aimed at conservation and economic potential of non-wood resources Elements such as biodiversity
conservation, and the importance of maintaining sufficiently extensive residual forest parcels to maintain interior wildlife species should be taken into consideration Consequences of spatial distribution of softwood stands on spruce budworm vulnerability are also relevant to management in the southern boreal forest. For example, the observation that isolated softwood stands in hardwood-dominated forests are less vulnerable to outbreaks (Bergeron et al., 1995) could
provide some guidance to an optimal spatial arrangement of stand types.