Canopy throughfall of Picea abies (L.) Karst. as depending on trace gas concentrations

At six sites in central Germany consequences of SO₂, NO_X and O₃ 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^-1 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^-1)^-1 is 2.3 times higher than the specific stomatal SO₂ uptake. The NO₂-dependent nitrogen deposition of (2.464±0.707) mmol N kg^-1 needle dw a^-1 (nPa NO₂ Pa^-1)^-1 is 2.2 times higher than the specific stomatal NO_X (NO₂+NO) uptake. These ratios (2.32.2) are explained by the percentage of annual hours with open needle stomata. The shape of observed epicuticular SO₂ and NO_X deposition curves and of stomatal SO₂ and NO_X uptake curves are congruent. As for stomatal NO_X 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>Al>Mg>Fe in this order of relative importance. NO_X deposition in spruce canopies reduces K⁺ throughfall and it weakly promotes throughfall of Mn²⁺ and Zn²⁺. There was no significant codeposition of sulphate and ammonium and no ion exchange of intercepted H₃O⁺ with nutrient cations at the measured ambient pH values of the precipitation water. In the presence of O₃, throughfall of Mn²⁺ is reduced and throughfall of K⁺, Ca²⁺ and Al³⁺ is enhanced. In the cooperative presence of SO₂, NO₂ and O₃ pollution in the field there is a 1.3-fold increase of the annual K⁺ demand and a 1.5-fold Mg²⁺ 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 Mg²⁺ throughfall could not be recycled in spruce ecosystems. Observed canopy thinning ranges from (13 to 26)% at the investigated six spruce stands.Abbreviations A_spec Specific needle surface area per kg needle dry matter (m²kg^-1 needle dw) - A_tot Total needle surface of spruce stands (ha ha^-1) - [gas]_a Ambient trace gas concentration (gas=SO₂; NO₂ or O₃) in air (nPa Pa^-1=ppb) - GP Number of days per annual growth period d a^-1) - IC_H30⁺ Acid interception rate (Eq H₃O⁺ kg^-1 needle dw a^-1) - k_o Trace gas-independent ion throughfall rate constant (mmol kg^-1 needle dw a^-1) - k_gas 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) - k_H30 Specific H₃O⁺/Me⁺ exchange ratio (mol mol^-1) - L_o Background throughfall rate at [gas]_a=0 (mmol kg^-1 needle dw a^-1) - L_ion Canopy throughfall rate of ions (mmol kg^-1 needle dw a^-1) - L'_ion Trace gas dependent ion throughfall (mEq kg^-1 needle dw a^-1 (nPa Pa^-1)^-1) - LAI Leaf area index of the canopy (m² projected needle surface m^-2 ground) - Me⁺ Equivalents of metal cations (Eq) - N Stock of needles of spuce stands in the field (kg needle dw ha^-1) - P_% Percentage of needle loss relative to a healthy reference (%) - r Pearson correlation coefficient (no dimension) - R COO^--Sum of all organic anion equivalents Cat⁺ - An^- (Eq kg^-1 needle dw a^-1) - An^- Sum of all measured inorganic anion equivalents (Eq kg^-1 needle dw a^-1) - Cat⁺ Sum of all measured inorganic cation equivalents (Eq kg^-1 needle dw a^-1)