Response of needle sulphur and nitrogen concentrations of Scots pine versus Norway spruce to SO2 and NO2

The results of two field studies and an open-top chamber fumigation experiment showed that the response of mature Scots pine to SO2 and NO2 differed from that of mature Norway spruce. Moreover, the response of pine seedlings to SO2 and NO2 differed from that of mature trees. The greater increase in the needle total S concentrations of pine suggested more abundant stomatal uptake of SO2 compared to spruce. Both pine seedlings and mature trees also seemed to absorb more N from atmospheric deposition. Mature pine was able to assimilate SO42- derived from SO2 into organic S more effectively than mature spruce at the high S and N deposition sites, whereas both pine and spruce seedlings accumulated SO4-S under NO2+SO2 exposure. Spruce. in turn, accumulated SO4-S even when well supplied with N. Net assimilation of SO42- in conifer seedlings was enhanced markedly by elevated temperature. To protect the northern coniferous forests against the harmful effects of S and N deposition, it is recommended that the critical level for SO2 as a growing season mean be set at 5-10 mu g m(-3) and NO2 at 10-15 mu g m(-3), depending on the 'effective temperature sum' and/or whether SO2 and NO2 occur alone or in combination. (C) 2000 Elsevier Science Ltd. All rights reserved.