Impacts of Air Pollutants on Cold Hardiness

Air pollutants can have detrimental effects on the physiology of north temperate conifer species, with immediate consequences for vitality (Schulze et al. 1989; Eagar and Adams 1992). Both sulfur- (S) and nitrogen- (N) based pollutants, and ozone (O3) can predispose conifers to freezing stress (Barnes et al. 1996). Experimental support for effects of wet deposited N and S on cold hardiness has, however, generally been derived from exposures restricted to the growing season. Effects of multi-freezing events, as experienced in the field, remain poorly understood. Over winter, subzero temperatures and low photon flux densities mean metabolically regulated detoxification processes are slow or absent. We have limited information on whether negative effects accumulate over time and/or the extent to which they may be counterbalanced each year if conditions favor carbon (C) assimilation. In trying to establish if, and how, perturbations to cold hardiness are driven by air pollutants we should remind ourselves that we have only a poor understanding of the day-to-day responses of unpolluted trees to sub-freezing temperatures.