Response of needle sulphur and nitrogen concentrations of Scots pine versus Norway spruce to SO_2 and NO_2

摘要

The results of two field studies and an open-top chamber fumigation experiment showed that the response of mature Scots pine to SO_2 and NO_2 differed from that of mature Norway spruce. Moreover, the response of pine seedlings to SO_2 and NO_2 differed from that of mature trees. The greater increase in the needle total S concentrations of pine suggested more abundant stomatal uptake of SO_2 compared to spruce. Both pine seedlings and mature trees also seemed to absorb more N from atmospheric deposi- tion. Mature pine was able to assimilate SO_4~(2-) derived from SO_2 into organic S more effectively than mature spruce at the high S and N deposition sites, whereas both pine and spruce seedlings accumulated SO_4-S under NO_2+ SO_2 exposure. Spruce, in turn, accumulated SO_4-S even when well supplied with N. Net assimilation of SO_4~(2-) in conifer seedlings was enhanced markedly by ele- vated temperature. To protect the northern coniferous forests against the harmful effects of S and N deposition, it is recommended that the critical level for SO_2 as a growing season mean be set at 5-10 μg m~(-3) and NO_2 at 10-15 μg m~(-3), depending on the 'effective temperature sum’ and/or whether SO_2 and NO_2 occur alone or in combination.