Precipitation and throughfall chemistry for a montane hardwood forest ecosystem: potential contributions from cloud water

Abstract

This study analyzed the chemical composition of cloud water, bulk precipitation, and throughfall in a hardwood forest at the top of a watershed in the Virginia Blue Ridge Mountains to further investigate canopy–atmosphere interactions. The composition of both cloud water and bulk precipitation was dominated by H+, NH4+, SO42−, and NO3−. On average, cloud water was four times more acidic than bulk precipitation (251 vs. 63 μequiv./L in 1986 and 158 vs. 40 µequiv./L in 1987, respectively) and had higher NH4+/Ca2+ ratios (22.2 vs. 0.7 in 1986 and 9.4 vs. 1.2 in 1987, respectively), indicating possibly below-cloud scavenging of primary aerosols by rain, dry deposition of Ca2+, and (or) differential dissolution of NH3 gas in cloud water and rainwater. Acidity in throughfall was less than that in bulk precipitation (H+ of 40 vs. 63 μequiv./L in 1986 and 25 vs. 40 μequiv./L in 1987, respectively). Potassium and Ca2+ were leached from the canopy, whereas NH4+ and NO3− were retained by the canopy. Cloud water was a major source for the retained NH4+ and NO3−. Deposition of cloud water during the warm season was estimated from precipitation-throughfall volume relationships to be approximately 5.21 cm in 1986, an increase of about 23% over precipitation inputs, and approximately 5.42 cm in 1987, an increase of about 8% over precipitation. The resulting inputs of major ions from cloud water may, however, equal or exceed that of precipitation at the ridgetop site and may contribute significantly to canopy fertilization and acidification. Further study is needed to determine the spatial extent down the watershed and the ultimate effects of this enhanced deposition to high elevation forests.