Effect of a native forest canopy on rainfall chemistry in China’s Qinling Mountains

Abstract

This study investigated the effect of a pine/oak forest canopy on rainfall chemistry in the Qinling Mountains. The area is an important water source for China’s North-to-South Water Transfer Project. Rainfall and throughfall samples were collected at the Huoditang Natural Forest in 1999, 2004, and 2009. Analyses of the samples indicated that the forest canopy had several important effects on rainfall chemistry. Rainfall pH generally increased as water passed through the canopy. On average, the rainfall pH increased by 0.54 pH units. The canopy’s effect declined after deciduous trees lost their leaves late in the sampling season. Rainfall NO3 − concentrations generally declined as water passed through the forest canopy, but PO4 3− concentrations generally increased. On average, rainfall NO3 − concentration declined by 0.135 mg/L as it passed though the forest canopy and PO4 3− increased by 0.85 mg/L. The forest canopy had a mitigating effect on the base cation content of throughfall. Specifically, K+, Na+, Ca2+ and Mg2+ were leached from the canopy when the concentration of these cations in rainfall was low. In contrast, K+, Na+, Ca2+ and Mg2+ were absorbed by the canopy when the concentration of these cations in rainfall was high. The pH of rainfall, as well as its K+, Ca2+ and Mg+ concentration, influenced the effect of the forest canopy on the base cation content of throughfall. The concentration of Cd, Pb, and Zn in rainfall generally decreased as water passed through the forest canopy, but the concentration of Fe in rainfall generally increased. The Cd concentration decreased by an average of 3.938 μg/L, the Pb concentration decreased by an average of 8.457 μg/L, and the Zn concentration decreased by an average of 0.986 mg/L. The Fe concentration increased by an average of 0.009 mg/L. The canopy’s ability to absorb Cd declined after several rainfall events in which rainfall Cd concentrations were relatively high.