Acidic cloud water and cation loss from red spruce foliage

Abstract

Declines of red and Norway spruce in North America and Europe are occurring at high elevations where cloud interception is a major source of water and chemical deposition and the mean pH of cloud water is markedly lower than that of rainfall. At the summit of Whitetop Mountain (1700 m) in the southern Appalachians the chemistry of ambient cloud water was compared on a cloud event basis to that of cloud water-generated throughfall from red spruce saplings. Large shifts occurred in the relative importance of all the cations measured as cloud water became throughfall, whereas the relative importance of the major anions (SO42−, NO3−, Cl−) remained relatively constant. Hydrogen ion and NH4+ percentage contributions were reduced to two-thirds and one-third of their original percentages respectively, whereas the other four major cations (K+, Na+, Mg2+, Ca2+) all at least doubled in importance. Losses of Ca and Mg from native red spruce foliage were observed to intensify markedly with increases in the acidity of cloud water. On sites where the available pools of certain cations are already marginal, losses of cations caused by acidic cloud water may contribute to nutrient deficiencies.