Canopy modification of precipitation chemistry in deciduous and coniferous forests affected by acidic deposition

Abstract

Ionic concentrations and fluxes of throughfall and precipitation were measured during a period of 3 y in a deciduous stand (Df-site) and a coniferous stand (Cf-site) affected by acidic precipitation in the north-western part of Toma-komai in Hokkaido. Throughfall pH at the Df-site was higher than precipitation pH in many cases, indicating that the deciduous canopy neutralized a proton in acidic precipitation. In contrast, pH in throughfall at the Cf-site fluctuated independently of the acidity of precipitation without any significant difference between pH in throughfall and that in precipitation. Concentrations of SO42-, K+, Na+, Cl- in throughfall were higher than in precipitation at both sites, especially during the growing period (from June to October), and the magnitude of increment at the Cf-site was larger than that at the Df-site. Na+ and Cl- in net throughfall flux (throughfall flux minus precipitation flux) for both sites were markedly affected by sea salt, while other ions in net throughfall flux were supplied by non sea salt, namely, external terrestrial, biological and anthropogenic materials and internal sources of living leaves. Net throughfall flux of protons showed that proton consumption on the leaf surface was a major process of canopy modification. At the Df-site, the absolute value of net H+ flux during the growing period (from June to October) was almost equivalent to that of K+ and HCO3-, suggesting that the mechanism of H+ consumption during canopy modification was closely dependent upon K+ release from internal sources of foliage and surface reactions which were controlled by bicarbonate and/or weak organic acid equilibrium. Annual proton consumption in deciduous and coniferous canopies were 0.53 and 0.28 kmolc ha-1 y-1 respectively, and these values corresponded to about 80 and 44% of the proton load as wet deposition, respectively. The smaller value of proton consumption at the Cf-site compared with the Df-site may reflect the high capturing efficiency of the coniferous canopy which acts as a collector of dry acidic deposition, especially non sea salt SO42-. © 1996, Taylor and Francis Group, LLC.