INFLUENCE OF ACIDIC MIST ON FROST HARDINESS AND NUTRIENT CONCENTRATIONS IN RED SPRUCE SEEDLINGS .2. EFFECTS OF MISTING FREQUENCY AND RAINFALL EXCLUSION

Two-year-old red spruce (Picea rubens Sarg.) of Pittston provenance an d 3-yr-old plants of Chatham provenance were exposed to acid mist in r eplicated open-top chambers, supplied with charcoal-filtered air near Edinburgh, Scotland. Plants of Chatham provenance had already been exp osed to acid mist throughout the previous growing season. The plants w ere exposed to mist, equivalent to 4 mm rainfall per week, containing an equimolar mixture of sulphuric acid and ammonium nitrate at pH 2.5 or pH 5.0 (1.6 or 0.01 mol m3) from May to November. This weekly dose was delivered at a low frequency (2 mm twice a week), or high frequenc y (1 mm on 4 consecutive days each week) to chambers fitted with ceili ngs to exclude rain. The low frequency dose was also a plied to chambe rs without ceilings, to examine the effect of natural washing by rain. Frost hardiness, estimated by exposing detached shoots to controlled freezing and then measuring rates of electrolyte leakage, was determin ed during the misting period at the end of October and in December. Fo liar nutrient concentrations were measured during the dormant period a fter treatment had ceased. At the end of October, plants which had rec eived acid mist were less frost hardy than plants receiving mist at pH 5. The temperature causing 50 % shoot death (LT50) increased by 6-deg rees-C for low frequency application, and by 10-degrees-C at high freq uency, relative to the plants receiving mist at pH 5. Exclusion of amb ient rainfall had no detectable effect on the frost hardiness response to acid mist. In December, 3 wk after the cessation of misting, all p lants were more frost hardy than in October. Significant effects of th e acid mist treatment could no longer be detected. Differences in nutr ient concentrations were small among treatments, although K+ concentra tions in the low frequency treatment with acid mist with rain exclusio n were 50 % below those in other treatments. Ca concentrations were 50 % larger in the acid mist treatment with rain exclusion than without. The data suggested enhanced sulphate uptake resulting from increasing the frequency of exposure, but the increase was not significant. Ther e was no clear relationship between the pattern of frost hardiness and nutrient concentrations except for S, which was 30 % smaller in the c ontrol plants (pH 5) than in the high frequency pH 2.5 treatments. It is concluded that excluding rainfall, an experimental artifact introdu ced in evaluating effects of acid mist, does not influence the frost h ardiness response of red spruce seedlings. The much greater effect of exposure to the same dose at double the frequency suggests that such e xperiments may underestimate effects in the field, if those trees are exposed to more frequent episodes of polluted cloud water than experim ental plants.