STEM GROWTH REDUCTION IN MATURE SITKA SPRUCE TREES EXPOSED TO ACID MIST

An eighteen-year-old clone of Sitka spruce (Picea sitchensis (Bong.) C arr) growing in the field was used to evaluate the whole tree response of 'mature' Sitka spruce to acid mist treatment. The mist, an equimol ar mixture of H2SO4 and NH4NO3 at pH 2.5 with or without particles (so da glass ballotini <20 mu m diameter), was applied twice weekly (equiv alent to 4mm precipitation week(-1)) throughout the growing season, Ma y-November. 1990-1992. The annual dose of S, N, H applied as mist (at 51, 48 and 3.3 kg ha(-1), respectively) was 2.5 times that measured in the Scottish uplands. Throughout the experiment there was no evidence of visible injury symptoms, yet there was a highly significant reduct ion (p < 0.02) in the stem-area increment relative to the stem area at the start, measured using vernier dendrometer bands. There was no sig nificant difference between the (acid mist+particle) and the acid mist only treatments. The mean relative stem-area increment over two compl ete growing seasons (1991-1992) was 65% for control trees, but only 53 % for acid-misted trees. (C) 1997 Elsevier Science Ltd.