ESTIMATING OF 2 DIFFERENT KINDS OF FOREST DAMAGE MECHANISMS IN SOUTHEASTERN FINLAND .1. THE NITROGEN-SULFUR HYPOTHESIS

Under northern conditions, coniferous trees provide versatile material for studies on variable effects caused by air pollutants. Pinus silve stris is a particularly suitable bioindicator, since it is widely dist ributed and sensitive to many air pollutants. In projects carried out by the Finnish Meteorological Institute, we have recently obtained res ults which support the so-called nitrogen-sulphur hypothesis. We sugge st that the excess of nitrogen compounds like ammonia/ammonium, nitrog en oxides and nitrate deposited on forests, together with gaseous sulp hur compounds and anthropogenic acids, trigger severe forest damages w ithin large areas and distances around their emission sources. Around several urban areas, atmospheric concentrations of sulphur compounds h ave decreased due to the emission decrease of these compounds. In thes e cases, the total S(sulphur)-concentrations of the pine needles have decreased over large areas around the emission sources, while the tota l N(nitrogen)concentrations of the needles have increased. On the othe r hand , there are many urban areas where the ambient air concentratio ns of S-compounds have stayed at rather high levels. In these situatio ns, high total S-concentrations and low total N-concentrations have be en noticed in the pine needles. The basis of two different kinds of fo rest damage mechanisms seems to depend on the relationship of N- and S -compounds in ambient air as indicated by the different concentrations of total sulphur and nitrogen in the needles and the relationship the reof. If the total sulphur concentrations of the needles are high, it is conceivable that the sulphur compounds, especially sulphur dioxide in the air, are so high that, when concentrated in the needles, they s top the function of nitrite reductase enzyme and the needles cannot re duce nitrogen compounds (oxides). As a result, pines cannot use concen trated nitrogen as nutrient and nitrite-nitrogen causes toxic effects. It is also possible that sulphur compounds alone cause toxic effects. Damages caused in this way hinder the nitrogen compound uptake by pla nts from the air. Around those urban areas where the emissions of sulp hur compounds have decreased and the total sulphur contents of the nee dles are not high, the needles can take in nitrogen compounds from the air and use them as nutrients. The effects on and damages of needles in these situations are caused by excess nitrogen. It is conceivable t hat the sulphur dioxide concentrations 'in the air are so low that the y do not stop the function of nitrite reductase enzyme. In this situat ion, both compounds are used as nutrients and the damages are so-calle d wintering failures (i.e., winter or frost damages). The disturbances in nitrogen metabolism and the deficiencies of several mineral nutrie nts may be the main causes of those damages seen during cold periods.