Natural Disturbances Have Broad Boundaries

The transition between a stand affected by wildfire or mountain pine beetles and an undisturbed forest is seldom abrupt. In south-central British Columbia near Arrow Lakes, where the edges of natural disturbances were examined, the proportion of dead trees declines over a distance averaging 51 m between the fully-disturbed and unaffected forests.

The boundaries around a recent rampage of fire or beetles are highly variable, ranging from 0 to 127 m wide. Characteristics of the understory vegetation do not correspond to the drop in tree mortality across a boundary. Instead, plant species richness and diversity often increase then decrease across the zone. The area with intermediate tree mortality can support either the most or the least amount of vegetation cover for a given species.

Since harvesting patterns typically leave abrupt transitions between cutblocks and undisturbed forests, they do not mimic the boundaries researchers found surrounding natural disturbances. If these ecologically complex portions of natural landscapes were extrapolated to cutblocks of 40 to 200 ha, boundary zones would extend over 14 to 30 percent of forested land.