The occurrence of SO₂‐related foliar symptoms on Scots pine (<i>Pinus sylvestris</i> L.) in an open‐air forest fumigation experiment

Abstract

SUMMARY As part of a long‐team open‐air fumigation experiment, Scots pine was exposed continuously to SO 2 pollution following a predetermined pattern of hourly mean values based upon monitoring data from a site in central England. Although annual mean concentrations did not exceed 22 ppb, up to 20% of trees developed foliar necrosis in threw consecutive growing seasons similar to published descriptions of acute K(X damage, The number of trees affected increased with SO 2 concentration, mid there‐ wan H significant tendency for the same trees to be affected in consecutive years, implying that sensitivity lo these levels was genetically bused. As a group, trees showing necrotic symptoms were not significantly retarded in their growth, although their 1990 budburst was significantly delayed and the few trees which lost over 50% of their foliage were visibly stunted. Fumigation of potted trees for short periods in chambers with up to 1000 ppb SO 2 failed to produce any comparable effects. Regression analyses between the number of plants affected each year and a range of gas concentration parameters showed mean concentrations during a critical period in May to be most closely related to the development of symptoms, although no effects became visible until approximately 5 wk later. The start date for the critical period differed between years, and for the two years of which budburst data were available it coincided with the start of needle elongation. The duration of the critical period lay in the range 6–14 d. Linear extrapolation from the data suggested the threshold mean SO 2 concentration for visible injury to be in the range 6–8 ppb (measured during the critical period during needle expansion) although this could underestimate the true value.