Stomatal uptake of SO2, NOx and O3 by spruce crowns (Picea abies) and canopy damage in Central Europe

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Abstract
The stomatal uptake of SO2, NO2 and O3 by Norway spruce canopies (Picea abies (L.) Karst.) has been integrated at six sites in central Germany (Königstein in the Taunus mountains. Witzenhausen, Grebenau, Frankenberg, Fürth in the Odenwald mountains and Biebergemünd in the Spessart mountains), Results are based on 788000 half-hourly available sets of field data on air pollution and site meteorology measured since 1984 (45 site years). Data on stomatal water conductance are available from statistical response functions obtained in the field at all times of day and seasons of the year. From this, stomatal conductance was calculated fur different atmospheric tract gases. Statistical response functions are presented which allow the separate estimation of annual stomatal net uptake of trace gases in the field if only (i) measured annual means of SO2, NO2, and O3, pollution and (ii) the length of the trunk growth period (defined by temperature) are known. The following specific annual doses of stomatal trace-gas net uptake in the field (μmol m−2total needle surface d−1annual trunk growth period/(nPa Pa−1) annual mean of trace gas concentration) are obtained; SO2: (0.157 ± 0.011) μmol m−2d−1/(nPa SO2, Pa−1), NOx: (0.4774 ± 0.034) μmol m−2d−1/(nPa NO2 Pa−1), O3: (0.474 ± 0.034) μmol m−2d−1/(nPa O3 Pa−1). There is an apparent NO2, compensation point at 7 nPa Pa−1in the field if NOx (=NO2, + NO) fluxes are integrated; the individual NO2, compensation point equals c 3.2 nPa Pa−1. Additionally, a statistical response function is presented, which estimates actual stomatal water transpiration rates in the field on the basis of measured water vapour pressure differences. VPD (mPa Pa−1). Complex canopy effects were calculated on the basis of available data using an effective leaf area which is 0×32 times the morphological leaf area index. Results are consistent (i) with available water balance data, (ii) with observed SO2-dependent sulphate accumulation rates in spruce needles and (iii) with observed epicuticular SO2, deposition rates. Pollution data are compared with observed damage of spruce canopies in the field. There was a statistically significant correlation between observed damage to spruce canopies.