STOMATAL SO2 UPTAKE AND SULFATE ACCUMULATION IN NEEDLES OF NORWAY SPRUCE STANDS (PICEA-ABIES) IN CENTRAL-EUROPE

Monthly up take rates and the annual deposition of gaseous SO2 via the stomata of six Norway spruce canopies (Picea abies (L.) Karst.) in Germany (Konigstein im Taunus, Witzenhausen, Grebenau, Frankenberg, Spessart, Furth 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 H2O 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 +/- 0.07) mu mol SO2 m(-2) (total needle surface) (nPa Pa-1)(-1) (annual mean of SO2 immission; 1 nPa (SO2) Pa-1 (air) = 1 ppb) day(-1) (vegetation period per year). Comparison of calculated SO2 uptake rates into needles with measured SO42- accumulation rates in needles from the mentioned sites and additionally from Wurzburg, Schneeberg (Fichtelgebirge) and from three sites in the eastern Erzgebirge (Hockendorf, Kahlebeg, Oberbarenburg) revealed that oxidative SO2 detoxification (SO42- 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 SO42- accumulation in needles. Cations are needed for neutralization of accumulating SO42- 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 SO-dependent SO42- 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-1 day(-1)) reductive SO2 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 approximate to 0.07 nPa Pa-1 day(-1) recommended SO2 immission limits can be deduced on a mere ecophysiological basis. These deduced values are close to the proposed SO2 immission limits of the IUFRO, WHO and the UNECE.