THE ROLE OF CLIMATE VARIABILITY AND GLOBAL WARMING IN THE DIEBACK OF NORTHERN HARDWOODS ALLAN N. D. AUCLAIR, JOHN T. LILL and CARMEN REVENGA Science and Policy Associates, Inc. Suite 400, West Tower, 1333 H Street, NW Washington, D.C. 20005 (Received 9 June, 1995; accepted 10 October, 1995) Abstract. 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 21 st century (2045-2085). Key words: birch, cavitation, climate change, climatic variability, decline, dieback, drought, freezing, global warming, Northern Hardwoods, temperate forests, sugar maple 1o Introduction Extreme weather events are now recognized as an important mechanism inciting forest dieback in Northern Hardwoods and elsewhere (Auclair, 1992, 1993a, b; Auclair et al., 1990, 1992, 1995; Pomerleau, 1991). The onset of each of five successive, major episodes of dieback on tree species in Quebec coincided with unusually prolonged winter thaw followed by sudden, severe freezing (Auclair et al., 1992). Marked winter thaw-freezes were also apparent at the onset of dieback on western white pine in the Pacific Northwest in 1936 (Auclair et al., 1990) and on Norway spruce and silver fir in Europe in 1976 (Auclair, 1993b). At another scale, major episodes of dieback in Northern Hardwoods this century have coincided with rapid increases in the global annual mean temperature; recov- ery occurred as global temperatures dropped (Auclair et al., 1992). That changes in global climate could be a significant factor in forest dieback is supported by several simulation studies on forest response. Of particular interest is the study of Solomon (1986) showing a high sensitivity of Northern Hardwoods to dieback under continued climatic warming. His and related studies have emphasized the need to consider seriously the possibility that global warming may lead to dieback Water, Air, and Soil Pollution 91: 163-186, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.