EFFECTS OF ACID MIST ON MATURE GRAFTS OF SITKA SPRUCE .1. FROST HARDINESS AND FOLIAR NUTRIENT CONCENTRATIONS

Mature grafts of five clones of Sitka spruce (Picea sitchensis Bong. S arg.) were exposed to simulated acid mist composed of an equimolar mix ture of sulphuric acid and ammonium nitrate at pH 2.5 and pH 5.0 in op en-top chambers from May to November 1991. Treatments were applied on consecutive days, four times a week. The pH 2.5 treatment provided an overall dose three times higher than that received by forests in uplan d areas of Britain. Frost hardiness was assessed in November by freezi ng detached current year shoots at a range of temperatures and assessi ng the rate of electrolyte leakage Foliar nutrient concentrations were determined on the same shoots. Acid mist at pH 2.5 significantly redu ced frost hardiness in four of the five clones; the temperature causin g 50% shoot death (LT50) was increased by 0 to 7-degrees-C The clones varied in their level of hardiness, one clone being exceptionally fros t sensitive. The frost hardiness of the frost sensitive clone was foun d to be less perturbed by acid mist than the hardiness of the more fro st resistant clones. Mature grafts showed a smaller reduction in hardi ness at an equivalent dose than that found previously with Sitka spruc e seedlings. Compared with seedlings, grafts had lower absolute concen trations of foliar sulphur. Exposure to acid mist at pH 2.5 increased %S in current year foliage by <0.05% compared with absolute increases of more than 0.10% in current year foliage of seedlings. We conclude t hat the effect of acid mist on frost hardiness is likely to be less on mature trees than on seedlings and that the increased frost risk to m ature trees of Sitka spruce from occult deposition alone is small.