NEEDLE S FRACTIONS AND S TO N RATIOS AS INDICES OF SO2 DEPOSITION

Needles of Scots pine (Pinus sylvestris L.) from 25 and 40 sampling plots in southern and northern Finland, respectively, that had earlier been analysed for total sulphur concentration (S_t) were reanalysed for foliar sulphate sulphur (SO₄–S) and total nitrogen (N_t). Organic sulphur content (S_o) was calculated as the difference between S_t and SO₄–S. Current (c) and previous-year (c+1) needles were collected from southern Finland in December 1989 and c – c+2 needles from northern Finland in September-October 1990/September 1992. The results show that the S_t concentration and S_t/N_t ratio in Scots pine needles are good indices of dry deposition of SO₂ in general, while SO₄–S concentrations and SO₄–S/S_o ratios can be used in areas with low N supply from the soil and/or low wet deposition of N. The normal S_t concentration in needles of Scots pines growing on a podzol with low N supply is considered to be 500–700 μg g^-1 and that of SO₄–S 100–200 μg g^-1. An increase of 100 μg g^-1 in needle S_t may be attributed to a rise of 1.4 μg m^-3 in ambient SO₂ concentration in areas with relatively low SO₂ concentrations (>15 μg m^-3). A critical level of 5 μg m^-3 as an annual and growing season mean is proposed for forestry in northern Europe (north of 60°N).