Assessing the critical level of SO2 for Scots pine in situ Original Research Article

A field survey was performed in eastern Finland, where measured ambient SO2 concentrations were 1.4–3.8 μg m−3 a−1 and bulk S deposition 0.17–0.32 g m−2 a−1 in 1991–1993. The accumulation of sulphur (S) in needles of Scots pine (Pinus sylvestris L.) was studied with XRF, IC and FESEM analyses and the needle damage examined under a light microscope and by SEM. Foliar N concentrations were also measured. Foliar total S concentrations were observed to be above the normal S level (500–700 μg g−1) over almost the whole area. Slight chlorosis and/or necrosis of the needle tips and stomatal areas, changes in the needle surface waxes and localization of S into needle tips and mesophyll cells around the stomata suggested the impact of S deposition, as did the calculations of St/Nt, and ‘predicted’ and ‘excess’ S. A concentration of about 900 μg g−1 may be considered a critical level for foliar St in areas with low N supply.