Assessing the critical level of SO2 for Scots pine in situ.

A field survey was performed in eastern Finland, where measured ambient SO2 concentrations were 1.4-3.8 microg 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 microg 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 microg g(-1) may be considered a critical level for foliar St in areas with low N supply.