Early effects of acid mist on Sitka spruce planted on acid peat

Ten year old Sitka spruce [Picea sitchensis (BONG.) CARR.], growing in a co mmercial plantation, on a drained, acid peat were sprayed with simulated mi st providing a 'no spray', N alone, S alone and a combination of N, S and a cidity at pH 5 and 2.5, (single and double dose) treatments. Approximately twice weekly from May (budburst) to December, six treatments (equivalent to 2 mm precipitation) Na2SO4, NH4NO3, H2SO4 + NH4NO3 at pH 2.5 (single and d ouble dose), and pH 5 and an unsprayed control were applied, to four replic ate chambers of 10 trees. The treatment 'chambers' comprised a framework of poles, supporting 2 full cone sprayers per tree mounted within 0.5 m of th e tops of the trees, without wall structures. This paper reports some sensi tive indicators of treatment effects, stemwood growth, foliar chemistry and frost hardiness, together with measurements of soil chemistry after one an d two treatment seasons. In the first year of treatment no significant effects on growth were found although stem basal area increment increased by approximately 50%. After on e year of treatment there was a tendency for shoots receiving N/acidity to be least frost hardy when tested in December (by approximately 4 degrees C) . Foliar S, N and Mg concentrations were significantly increased in respons e to S and N additions. In the second year, no biologically significant eff ects were found. Two years of treatment with N and S, both with and without acidity, still failed to affect growth measured as relative stem basal are a increment. However, relative increments in stem volume indicated a signif icant positive response to the acidified S + N treatments after two treatme nt seasons. Initial findings are discussed in relation to results from simi lar studies in controlled environments and the field. Early indications are that the sail nutrient supply is dominating the tree response to N, S and acid treatment. and that acidified S and N inputs are not damaging the tree s but have stimulated growth over the first 2 years of treatment.