CANOPY MODIFICATION OF PRECIPITATION CHEMISTRY IN DECIDUOUS AND CONIFEROUS FORESTS AFFECTED BY ACIDIC DEPOSITION

Ionic concentrations and fluxes of throughfall and precipitation were measured during a period of 3 y in a deciduous stand (Df-site) and a c oniferous stand (Cf-site) affected by acidic precipitation in the nort h-western part of Tomakomai in Hokkaido. Throughfall pH at the Df-site was higher than precipitation pH in many cases, indicating that the d eciduous canopy neutralized a proton in acidic precipitation. In contr ast, 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 si tes, 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 p recipitation 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 proton s showed that proton consumption on the leaf surface was a major proce ss of canopy modification. At the Df-site, the absolute value of net H + flux during the growing period (from June to October) was almost equ ivalent to that of K+ and HCO3-, suggesting that the mechanism of H+ c onsumption during canopy modification was closely dependent upon K+ re lease from internal sources of foliage and surface reactions which wer e controlled by bicarbonate and/or weak organic acid equilibrium. Annu al proton consumption in deciduous and coniferous canopies were 0.53 a nd 0.28 kmol(c) ha(-1) y(-1) respectively, and these values correspond ed to about SO and 44% of the proton load as wet deposition, respectiv ely. The smaller value of proton consumption at the Cf-site compared w ith the Df-site may reflect the high capturing efficiency of the conif erous canopy which acts as a collector of dry acidic deposition, espec ially non sea salt SO42-.