A stand and landscape comparison of the effects of a spruce budworm (Choristoneura fumiferana (Clem.)) outbreak to the combined effects of harvesting and thinning on forest structure

Spruce budworm outbreaks are a major disturbance determining boreal forest dynamics in forests dominated by balsam fir. However, these forests have been anthropogenically disturbed in recent decades by clearcutting with protection of advance regeneration and soils which it has been suggested, emulates spruce budworm outbreaks by leaving a seedling bank while removing the overstory. The main objective of this study is to characterize and compare the effects of a spruce budworm outbreak to the effects of this harvesting treatment in balsam fir stands in the Gaspe Peninsula. The study was conducted at two different scales; the stand scale, by field sampling vegetation, and at the landscape scale, through the interpretation of aerial photographs. At the stand scale, results showed a more important structural variability in stands affected by the spruce budworm outbreak. There was a greater diversity of saplings and trees, especially paper birch and white spruce, in post-outbreak stands by comparison to post-logged stands. At the landscape scale, canopy openings created by clearcutting with protection of advance regeneration presented less regular shapes, but still had smaller perimeter/area ratio because of their larger size. Spruce budworm openings are thus more likely to be influenced by the surrounding forest and recolonisation by tree species should be faster than for clearcut openings. Our results suggest that efforts should be made to maintain the associated species (paper birch (Betula papyrifera Marsh.) and white spruce (Picea glauca (Moench) Voss)) by preserving more overstory trees of these species while managing the balsam fir dominated forest. Aggregation of cutting units should be limited and there is a need to create more small canopy openings in the landscape if we are to emulate naturally created patterns.