Seasonal Characteristics of Acidic Deposition onto the Mountain Forests Adjacent to Cities

Abstract

The seasonal amounts of dry and fog water depositions onto a mountain forest canopy were estimated together with those of stem flow and leaching (or absorption) by a method based on both water and material balances. The maximum water retained on the forest canopy that was estimated from the intercept of the regression line of the bulk precipitation versus throughfall was used for calculating the water balance. The coefficient of interception loss (k) was used in the equation for calculating the interception loss. Two conditions were set for calculation of the k values. One is the high correlation (r2> 0.95) between the volume of the fog sample and that of the fog water deposition in their seasonal data. Another is the high correlation (r2> 0.95 ) between the seasonal average of the atmospheric concentration of ions (Ca²⁺, K⁺ and Na⁺) and the corresponding seasonal amount of dry deposition. The variation of the k value in its suitable range gave - 28.3±0.6 % and 11.0±0.7 % changes in the dry deposition amounts of ions and those amounts depositing with fog water, respectively. The similarity of the dry deposition ratios among several ions to those of the atmospheric concentration and the result of the leaching test confirmed the reliability of the calculation using this method. Throughfall was collected every 2 weeks in the pinus densiflora forests (altitude 850 m) adjacent to cities. The seasonal change in the throughfall pH, high in the summer and low in the winter, are repeated every year. The calculation results explain the mechanism of seasonal change as follows. A significant amount of sulfate and nitrate ions deposit on the leaves in both the spring and summer so that the corresponding H⁺ tends to be generated. However, the H⁺ generation is simultaneously depressed in the summer by both the exchange reaction with K⁺ and the absorption of nitrate ion by the leaves. As a result, the throughfall pH increases in the summer. The pH decreases in the winter because only a small amount of K⁺ leaches from the leaves in that season in spite of the supply of sulfate and nitrate ions from the fog and dry depositions.