Seedling responses to forest canopy disturbance following a spruce budworm outbreak in Maine

Abstract

Patterns of tree mortality and seedling responses to canopy disturbance were investigated in northern Maine, where an outbreak of the spruce budworm (Choristoneurafumiferana (Clem.)) affected the forests of balsam fir (Abiesbalsamea (L.) Mill.) and spruce (Picea spp.) continuously between 1972 and 1984. The outbreak created a gradient of canopy tree mortality that ranged between 8.5 and 100% of the cumulative basal area in 1984. This was a result of the difference in vulnerability among the host species (balsam fir > spruce) and of their spatial distribution patterns along the site drainage gradient. Two groups of plant species responded differently to the gradient of canopy disturbance: balsam fir, spruce, and white birch (Betulapapyrifera Marsh.) regenerated mostly at the intermediate levels of mortality (≈20%by basal area) of the canopy balsam fir; raspberry (Rubusidaeus L.) and pin cherry (Prunuspensylvanica L.) were most abundant at ≈100% fir mortality. Overall, the observed responses in space and time of the seedlings to budworm-caused canopy disturbance could be mostly explained by the concept of patch dynamics. Long-term changes in species composition of the spruce–fir forests cannot be predicted with precision with the present knowledge. However, I hypothesize, based on the species-specific vulnerability to budworm damage and patterns of regeneration, that the proportion of spruce to fir trees would not differ very much in the long run regardless of extensive tree mortality by the spruce budworm.