Insect defoliators as major disturbance factors in the high-altitude balsam fir forest of Mount Megantic, southern Quebec

Tree-ring data fi-om a mature balsam fir forest, located at the top of Moun t Megantic (elevation 1100 m), southern Quebec, suggest that insect defolia tors were major disturbance factors in the development of high-altitude bal sam fir forests. A comparison between the radial growth trend of balsam fir (Abies balsamea (L.) Mill.), a host species of the spruce budworm (Chorist oneura fumiferana Clem.), and paper birch (Betula papyrifera Marsh), a nonh ost species, showed that several growth depressions in the balsam fir chron ology corresponded to documented spruce budworm outbreaks in southern Quebe c in the 1910s, 1950s, 1970s, and possibly in the 1870s. Tree mortality was extensive during the last infestation because of the relatively old age (> 60 years) of many balsam fir and, possibly, to the cumulative impact of def oliation. The tree-ring series from paper birch showed several drops in rad ial growth after the 1930s, possibly related to the large-scale birch dieba ck that occurred in eastern North America. Macrofossil data (insect remains ) from one sample of the uppermost organic soil layers (F horizon) confirm the presence of the spruce budworm at the study site. The ecological role o f insect defoliators is discussed in the context of the high-altitude balsa m fir forests in northeastern North America where abiotic disturbances are considered the primary controlling factors in stand dynamics.