Effects of acid mist on the frost hardiness of red spruce seedlings

Seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were exposed to mists containing equimolar (NH4)2 SO4 and HNO3 at pHs of 2.5, 2.7, 3.0, 3.5, 4.0 or 5.0. The mists were applied twice each week, amounting to 2 mm precipitation equivalent on each occasion, between July and December, to open-top chambers supplied with charcoal-filtered air. Frost hardiness of shoots excised from seedlings was determined on 6 occasions starting on 21 September, and was found to be strongly influenced by acid mist treatments, seedlings subject to the most acidic mists being the least frost hardy. On 21 September when the first sample was taken the lethal temperature for killing 50% of shoots (LT50) was -11⚬C for the least acidic (pH 5.0) mist and -7⚬C for the most acidic (pH 2.5). By 19 October, the LT50s of pH 5.0 and pH 2.5 mists were -27 and -15⚬C respectively. All intermediate treatments ranked according to treatment concentration, with the smaller concentrations causing lower LT50 values. The treatment at pH 3.0 provided S and N inputs to the seedlings similar to those experienced by red spruce at elevations of about 1000 m in the southern Appalachians. At pH 3.0, the frost hardiness LT10 during October was typically 8⚬C higher than the pH 5.0 treatment, leading to a significant increase in the probability of frost damage at the LT10 level in an average October. The proximity of minimum night temperatures during September to December to the LT10 temperatures of red spruce shoots receiving large inputs of SO4 2-, NO3 -, NH4 + and H+ suggests that decreases in frost hardiness caused by intercepted cloud water containing large concentrations of these ions may play a significant part in the observed decline at mountain-top locations