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 ma ture Sitka spruce (Picea sitchensis (Bong.) Carr), growing in a base r ich agricultural loam, were treated approximately (pH2.5) over three c onsecutive growing seasons. treatment consisted of acid mist containin g H+, NH4+, NO3- and SO42- in equimolar concentrations (1.6 mol m(-3)) identical to that used in previous open-top chamber (OTC) studies wit h 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). Tre ated trees were surrounded by a framework which supported temporary bl inds, positioned only during the spray treatment. Control trees receiv ed no treatment. Groups of four trees were assigned to one of five hei ght classes, to take account of a gradient in height across the site, and these groups were tl eaten as units of replication. Shoot frost ha rdiness and foliar nutrient concentrations were measured for compariso n with previous OTC studies. Tree size, which initially was positively correlated with differences in nutrient status (N, P, S and K), exert ed a significant effect on the frost hardiness response to acid mist. Acid mist had a negative effect on frost hardiness only in the smalles t trees, and only when the trees were already hardy to -20 degrees C a nd 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 respec t to monitoring field effects of acid mist is discussed together with the importance of different environmental factors which appear to cont rol tree response in the field. (C) 1998 Elsevier Science Ltd. All rig hts reserved.