Dendrochronological analysis of balsam fir (Abies balsamea) radial tree-ring growth across edges created by a spruce budworm outbreak

Abstract

Outbreaks of spruce budworm can form patches or large expanses of dead trees and surviving trees at the edge of these areas experience a detectable decline in growth. However, we do not know how far this pattern extends from the edge. The growth of trees at budworm-induced edges was evaluated within the boreal forests of Cape Breton Highlands National Park in northern Cape Breton, N.S. The objectives of this study were (i) to analyze radial growth before, during and after the spruce budworm outbreak to see if there was a change over time, (ii) to determine how far budworm-altered radial growth extends into the forest, and (iii) to discern if edge balsam fir experienced more of an increase in radial growth after the budworm outbreak compared with the forest interior. Balsam fir trees were cored at three sites in plots at different distances from either side of the edge. Dendrochronological analysis was performed to observe changes in annual tree growth patterns of balsam fir trees. Each site experienced a significant decrease in growth during the period of spruce budworm outbreak (1974-1984). Surprisingly, decreases in tree growth were detectable far away from the edge. After the budworm outbreak subsided, two sites showed an increase in growth near the edge but not right on the edge. The increase in growth near the edge may have been due to release from light competition modified by local spatial pattern (e.g. gaps), while the lack of a gradient in tree growth reduction into the forest could indicate that a second wave of severe budworm defoliation stopped at the current edge, perhaps due to weather effects.