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 experim ent showed that the response of mature Scots pine to SO2 and NO2 differed f rom 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 m ature spruce at the high S and N deposition sites, whereas both pine and sp ruce seedlings accumulated SO4-S under NO2+SO2 exposure. Spruce. in turn, a ccumulated SO4-S even when well supplied with N. Net assimilation of SO42- in conifer seedlings was enhanced markedly by elevated temperature. To prot ect 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), dependin g on the 'effective temperature sum' and/or whether SO2 and NO2 occur alone or in combination. (C) 2000 Elsevier Science Ltd. All rights reserved.