The recent sugar maple decline in southern Quebec: probable causes deduced from tree rings
We used tree-ring chronologies from sugar maple (Acersaccharum Marsh.) stands showing various degrees of dieback (i.e., 16 sugar maple chronologies from healthy trees and 11 from damaged trees), distributed throughout the species range in southern Quebec, to analyze the spatial extent and timing of the recent sugar maple decline. Furthermore, six tree-ring chronologies of American beech (Fagusgrandifolia Ehrh.) from six damaged sugar maple stands were used to compare for differential responses associated with factors such as insect defoliation (from the forest tent caterpillar, Malacosomadisstria Hbn., for which American beech is a nonpreferred species), drought, and other climatic events. It was found that several small-scale drought-induced disturbances occurred repetitively over the last 100 years in the western part of the species range in southern Quebec. Most sugar maple chronologies from stands located west, north, and south of Québec City also show extreme narrow tree rings, indicating the incidence of three large and deep growth depressions from the early to mid-1910s, mid-1950s, and early 1980s. The factors explaining the large growth depression of dominant sugar maple of the early 1980s, in the region where the 1980s maple decline was the most severe, are likely associated with the synergistic influence of drought and insect defoliators. The recovery of sugar maple stands from the 1980s growth decline emphasizes the positive responsiveness of the robust native trees to frequent natural disturbances. The 1980s maple decline corresponds to the category of natural disturbances affecting stand dynamics by the combination of events such as drought and insect infestations, and possibly (but to a minor extent) winter thaw-frost, which has yet to be demonstrated, rather than by anthropogenic pollution.