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Monthly uptake rates and the annual deposition of gaseous SO2 via the stomata of six Norway spruce canopies (Picea abies (L.) Karst.) in Germany (K6nigstein im Taunus, Witzenhausen, Grebenau, Frankenberg, Spessart, Ftirth im Odenwald) were calculated (i) from statistical response functions of stomatal aperture depending on meteorological data, and (ii) from the synchronously measured SO2 immission at these stands. The stomatal response functions had been derived on the basis of thorough stomatal water conductance measurements in the field. Calculations of the SO2 conductance of spruce twigs and SO2 uptake rates via stomata need continuously measured complete data sets of the (i) light intensity, (ii) air temperature, (iii) air humidity and (iv) SO2 concentration in spruce forests from all the year. These data were recorded half hourly in different German spruce forests. The apparent needle water vapour pressure difference and transpiration rates were calculated from meteorological data. Additional use of canopy through flow data in dry years allowed the estimation of the mean stomatal conductance for H20 and SO2 of whole spruce canopies. The annual SO2 uptake of a mean unit needle surface in spruce forests was 32% of the SO2 uptake rate of exposed needles at the top of spruce crowns. There is significant SO2 uptake all the year. The mean SO2 dose at all sites and years received through the stomata was (0.25 4-0.07) #mol SO2 m -2 (total needle surface) (nPa Pa-l)-l (annual mean of SO2 immission; 1 nPa (SO2) Pa-1 (air) = 1 ppb) day-l (vegetation period per year). Comparison of calculated SO2 uptake rates into needles with measured SO4 z-accumulation rates in needles from the mentioned sites and additionally from Wtirzburg, Schneeberg (Fichtelgebirge) and from three sites in the eastern Erzgebirge (H6ckendorf, Kahleberg, Oberb/arenburg) revealed that oxidative SO2 detoxification (SO ]-formation) dominates only at sites with high SO2 immission and short vegetation periods. Under these conditions 70 to 90% of the annual stomatal SO2 uptake is detoxified via SO]-accumulation in needles. Cations are needed for neutralization of accumulating SO 2-which are inavailable to support growth. Thus, SO2 induces a dominant and competitive additional nutrient cation demand, cation deficiency symptoms and enhanced needle loss ("spruce decline symptoms") mainly at sites, where the ratio R = (SO2 immission) : (length of the vegetation period) is higher than R = 0.07 nPa Pa-1 day-1. Correlation analysis of the relative needle loss versus the SO2-dependent SO]-formation rate revealed a significant increase of needle loss at the 98% level (Student). At sites with small SO2 immission and long vegetation periods (R < 0.07 nPa Pa-I day-l) reductive S Q detoxification via growth (and/or phloem export of SO42-) is not kinetically overburdened. Under these conditions only 30% of the annual SO2 uptake is detoxified via SO42-formation and spruce decline is small or absent. On the basis of the critical value R ~ 0.07 nPa Pa-l day-1 recommended...

A b s t r a c tAt six sites in central Germany consequences of SO2, NOx and 03 deposition and of acid precipitation on canopy throughfall of sulphate, nitrate, ammonium, organic acids and of metal cations from Norway spruce crowns were investigated in the field. Measured canopy throughfall rates (mmol ion kg-l needle dw a-1 are separated in (i) "background" ion throughfall rates in clean air and (ii) trace gas-(or acid interception)-dependent throughfall rates at ambient trace gas concentrations. Based on synchronously measured pollution, precipitation and canopy throughfall data, statistical response functions are given, which allow the separate estimation of annual rates of sulphur and nitrogen deposition into spruce canopies if only annual means of SO2 or NO2 concentrations in air are known. The specific SO2 deposition rate of (0.841+0.214) mmol S kg -1 needle dw a -1 (nPa SO2 Pa-l) -1 is 2.3 times higher than the specific stomatal SO2 uptake. The NO2-dependent nitrogen deposition of (2.464 4-0.707) mmol N kg-1 needle dw a-l (nPa NO2 Pa-1)-1 is 2.2 times higher than the specific stomatal NOx (NO2 + NO) uptake. These ratios (2.3~2.2) are explained by the percentage of annual hours with open needle stomata. The shape of observed "epicuticular" SO2 and NOx deposition curves and of stomatal SO2 and NOx uptake curves are congruent. As for stomatal NOx uptake, there is an apparent compensation point at (5 to 8) nPa NO2 Pa-1 There is significant SO2-dependent canopy throughfall of Ca>K>AI>Mg> Fe in this order of relative importance. NOx deposition in spruce canopies reduces K + throughfall and it weakly promotes throughfall of Mn z+ and Zn 2+. There was no significant codeposition of sulphate and ammonium and no ion exchange of intercepted H30 + with nutrient cations at the measured ambient pH values of the precipitation water. In the presence of 03, throughfall of Mn 2+ is reduced and throughfall of K +, Ca 2+ and A13+ is enhanced. In the cooperative presence of SO2, NO2 and 03 pollution in the field there is a 1.3-fold increase of the annual K + demand and a 1.5-fold Mg 2+ demand of spruce canopies relative to the situation in clean air. This trace gas-dependent additional cation demand of spruce canopies corresponds to a needle loss percentage of (23 to 33)% if the additional K + and Mg 2+ throughfall could not be recycled in spruce ecosystems. Observed canopy thinning ranges from (13 to 26)% at the investigated six spruce stands.Abbreviations: Aspee -Specific needle surface area per kg needle dry matter (m2kg -1 needledw);Atot-Totalneedle surface of spruce stands (ha ha-l); [gas]a-Ambient trace gas concentration (gas = SO2; NO2 or 03) in air (nPa Pa-1 = ppb); GP -Number of days per annual growth period d a-1 );iC+30_ Acid interception rate (Eq H3 O + kg-1 * Fax no: +499310886158 296 needle dw a-t); ko-Trace gas-independent ion throughfall rate constant (mmol kg -l needle dw a -1); kgas-SO2-,NO2-or O3-dependent ion throughfall rate per unit of trace gas pollution (mmol kg-1 needle dw a-1 (nPa Pa-1 )-1); KH30 -Spec...

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