Two-year-old seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were grown in open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Between May and November 1988, plants were exposed to mists containing NH4+, SO42− and NO3− ions at concentrations of 1.6 mol m−3 and H+ at 3.2 mol m−3, in pairwise combination and all together. The mists were applied twice weekly at a rate of 2 mm precipitation equivalent per application. Treatment with H2SO4 severely damaged the plants after 3 wk; the concentration for this treatment was therefore reduced from 1.6 mol m−3 to 0.5 mol m3, and new plants were substituted, in June 1988. Frost hardiness was assessed by freeze-testing detached shoots, then measuring rates of electrolyte leakage. No effects of the mist treatments were detected during the early stages of frost hardening, but on 31 October, shoots that had received sulphate and ammonium ions were less hardy than ‘control’ shoots treated with deionized water. The temperature which killed 20% of shoots was 14 °C higher, and the temperature which killed 56% of shoots was 7 °C higher than for ‘control’ shoots. Acidity per se had no effect on frost hardiness; nitrate ions had no effect and may have mitigated the effects of sulphate when applied together.
The results confirm earlier data, and indicate that uptake of NH4+ and SO42− ions can disturb the frost-hardening process. There are important implications for pollution control strategies in the northeastern United States, if damaging interactions between frost hardiness of red spruce and pollutant deposition are linked to ammonium and sulphate rather than nitrate ions.