EARLY NEEDLE SENESCENCE AND THINNING OF THE CROWN STRUCTURE OF PICEA-ABIES AS INDUCED BY CHRONIC SO2 POLLUTION .2. FIELD DATA BASIS, MODEL RESULTS AND TOLERANCE LIMITS

Field data on the sulphur and cation budget of growing Norway spruce c anopies (Picea abies [L.] Karst.) are summarized. They are used to tes t a spruce decline model capable of quantifying effects of chronic SO2 pollution on spruce forests. At ambient SO2 concentrations, acute SO2 damage is rare, but exposure to polluted air produces reversible thin ning of the canopy structure with a half-time of a few years. Canopy t hinning in the spruce decline model is highest (i) at elevated SO2 pol lution, (ii) in the mountains, (iii) at unfertilized sites with poor K +, Mg2+ or Zn2+ supply, (iv) at low spruce litter decomposition rates, and (v) acidic, shallow soils at high annual precipitation rates in t he field and vice versa. Model application using field data from Wurzb urg (moderate SO2 pollution, alkaline soils, no spruce decline) and fr om the Erzgebirge (extreme SO2 pollution, acidic soils in the mountain s, massive spruce decline) predicts canopy thinning by 2-11% in Wurzbu rg and by 45-70% in the Erzgebirge. The model also predicts different SO2-tolerance limits for Norway spruce depending on the site elevation and on the nutritional status of the needles. If needle loss of more than 25% (damage class 2) is taken to indicate 'real damage' exceeding natural variances, then for optimum soil conditions SO2 tolerance lim its range from (27.3 +/- 7.4) mu g m(-3) to (62.6 +/- 16.5) mu g m(-3) . For shallow and acidic soils, SO2 tolerance limits range from (22.0 +/- 5.5) mu g m(-3) to (37.4 +/- 7.5) mu g m(-3). These tolerance limi ts, which are calculated on an ecophysiological data basis for Norway spruce are close to epidemiological SO2-tolerance limits as recommende d by the IUFRO, UN-ECE and WHO. The observed statistical regression sl ope of the plot (damaged spruce trees vs. SO2-pollution) in west Germa ny is confirmed by modelling (6% error). Model application to other fo rest trees allows deduction of the observed sequence of SO2-sensitivit y: Abies > Picea > Pinus > Fagus > Quercus. Thus, acute phytotoxicity of SO2 seems not to be involved in 'forest decline'. Chronic SO2-pollu tion induces massive canopy thinning of Abies alba and Picea abies onl y at unfavourable sites, where natural stress factors and secondary ef fects of SO2 pollution act together to produce tree decline.