Field application of acid mist to a single clone of sitka spruce: effects on foliar nutrition and frost hardiness

Trees (>2.5 m tall) belonging to a single clone of physiologically mature Sitka spruce (Picea sitchensis (Bong.) Carr), growing in a base rich agricultural loam, were treated approximately twice weekly with acid mist (pH 2.5) over three consecutive growing seasons. The treatment consisted of acid mist containing H+, NH4+, NO3− and SO42− in equimolar concentrations (1.6 mol m−3) identical to that used in previous open-top chamber (OTC) studies with seedlings and grafted cuttings. The acid mist supplied approximately 48 kg N and 51 kg S ha−1 year−1 and 3.3 kg H ha−1 year−1. Treated trees were surrounded by a framework which supported temporary blinds, positioned only during the spray treatment. Control trees received no treatment. Groups of four trees were assigned to one of five height classes, to take account of a gradient in height across the site, and these groups were treated as units of replication. Shoot frost hardiness and foliar nutrient concentrations were measured for comparison with previous OTC studies. Tree size, which initially was positively correlated with differences in nutrient status (N, P, S and K), exerted a significant effect on the frost hardiness response to acid mist. Acid mist had a negative effect on frost hardiness only in the smallest trees, and only when the trees were already hardy to −20°C and treatment applications were regular and continuous up to sampling. There was no evidence of foliar mediated effects such as enhanced ion leakage rates and substantially elevated foliar S concentrations which typified OTC exposures. The usefulness of these variables with respect to monitoring field effects of acid mist is discussed together with the importance of different environmental factors which appear to control tree response in the field.