CANOPY THROUGHFALL OF PICEA-ABIES (L) KARST AS DEPENDING ON TRACE GASCONCENTRATIONS

At six sites in central Germany consequences of SO2, NOx and O-3 depos ition and of acid precipitation on canopy throughfall of sulphate, nit rate, ammonium, organic acids and of metal cations from Norway spruce crowns were investigated in the field. Measured canopy throughfall rat es (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 inte rception)-dependent throughfall rates at ambient trace gas concentrati ons. Based on synchronously measured pollution, precipitation and cano py throughfall data, statistical response functions are given, which a llow the separate estimation of annual rates of sulphur and nitrogen d eposition into spruce canopies if only annual means of SO2 or NO2 conc entrations in air are known. The specific SO2 deposition rate of (0.84 1+/-0.214) mmol S kg(-1) needle dw a(-1) (nPa SO2 Pa-1)(-1) is 2.3 tim es higher than the specific stomatal SO2 uptake. The NO2-dependent nit rogen 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 NOx (NO2 + NO) uptake. These ratios (2.3 approximate to 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 stomata l 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. The re is significant SO2-dependent canopy throughfall of Ca>K>Al>Mg>Fe in this order of relative importance. NOx 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 a nd no ion exchange of intercepted H3O+ with nutrient cations at the me asured ambient pH values of the precipitation water. In the presence o f O-3, throughfall of Mn2+ is reduced and throughfall of K+, Ca2+ and Al3+ is enhanced. In the cooperative presence of SO2, NO2 and O-3 poll ution in the field there is a 1.3-fold increase of the annual K+ deman d and a 1.5-fold Mg2+ demand of spruce canopies relative to the situat ion 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 s pruce ecosystems. Observed canopy thinning ranges from (13 to 26)% at the investigated six spruce stands.