EFFECTS OF ACID MIST ON NEEDLES FROM MATURE SITKA SPRUCE GRAFTS .2. INFLUENCE OF DEVELOPMENTAL STAGE, AGE AND NEEDLE MORPHOLOGY ON VISIBLE DAMAGE

Mature grafts of five Sitka spruce (Picea sitchensis (Bong.) Carr.) cl ones were exposed to simulated acid mist comprising an equimolar mixtu re of H2SO4 and NH4NO3 (1.6 and 0.01 mol m(-3)) at pH 2.5 and 5.0. Mis t was applied to potted plants growing in open-top chambers on consecu tive days, four times a week, at a precipitation equivalent of 1 mm pe r day. The total exposure to polluted mist was equivalent to three tim es that measured at an upland forest in SE Scotland. The aim of the ex periment was to characterize the response of juvenile foliage produced by physiologically mature grafts (on seedling root stock) and compare it with the behaviour of juvenile foliage on seedlings. Development o f visible foliar damage was followed through the growing season. Measu rements of needle length, diameter, weight, surface area, surface wax weight and wettability were made on current year needles to determine whether particular foliar characteristics increased susceptibility to injury. Significant amounts ( > 10%) of visible needle damage was obse rved on only one of the five clones. Damage was most severe on the clo ne with the most horizontal branch and needle habit, but over the five clones there was no relationship between angle of branch display and damage. Likewise no combination of needle characteristics (length, wid th, area, amount of wax) was indicative of potential susceptibility. A comparison with previous acid misting experiments using seedlings sug gests that juvenile foliage on physiologically mature trees is equally susceptible to visible injury as juvenile seedling foliage. Date of b udburst differed among clones, and in this experiment exerted the over -riding influence on development of injury symptoms. Foliage exposed t o a combination of strong acidity and high sulphate concentrations ove r the few weeks immediately following budburst suffered most visible i njury. The absence of significant amounts of visible damage in UK fore sts probably reflects the general low susceptibility to visible injury of Sitka spruce exposed to acid mist.