The role of climate variability and global warming in the dieback of Northern Hardwoods

The severity of dieback in Northern Hardwood Forests of Canada and the United States this century (1910-1990) was reconstructed from pathology records and compared to indices of extreme weather stresses in the region, and to changes in global temperatures and climatic variability over the same period. Thaw-freeze and root-freeze events in winter and early spring were key factors in triggering (and synchronizing) severe episodes of dieback. Once trees were injured by freezing, forest dieback correlated significantly with heat and drought stress. Freezing (but not drought) stresses in Northern Hardwoods correlated significantly (r = 0.70, p < 0.001) with increasing global mean annual temperatures and low values of the Pacific tropical Southern Oscillation Index. Major diebacks did not occur early in the century at a time of notable freezing stress. Prior to 1940, the Northern Hardwoods consisted of relatively young populations of trees regrowing following extensive cutting and forest burning in the late 19th century (1860-1890). It appears that forest maturation is the key factor preconditioning trees to climatic injury, and dieback. A simple projection of climate and forest maturation ages suggested the recurrence of major dieback episodes on white/yellow birch, sugar maple and red spruce in the latter half of the 21st century (2045-2085).