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Canopy and Soil Modification of Precipitation Chemistry in a Temperate Rain Forest

College of Forest Resources, Univ. of Washington, Seattle, WA 98195

Ted B. Thomas

U.S. Forest Service, Forestry Sciences Lab., Olympia, WA 98502

Jon J. Rhodes

Columbia River Intertribal Fishery Commission, 975 SE Sandy Blvd., Suite 202, Portland, OR 97214

*Corresponding author.

ABSTRACT

Little is known about the chemistry of solutions moving through old-growth coastal temperate coniferous rain forests. The major objectives of this study were to examine precipitation, throughfall, stemflow, and soil solution chemistry in an old-growth temperate rain forest in the Hoh River valley on the northwest Washington coast 32 km from the Pacific Ocean, and to determine mechanisms involved with changes in solution chemistry. Dominant species were Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), western hemlock (Tsuga heterophylla [Raf.] Sarg.), western redcedar (Thuja plicata Donn), and Pacific silver fir (Abies amabilis [Dougl.] Forbes). Stemflow was more acidic (avg. pH 4.3) than throughfall (avg. pH 5.0) and precipitation (avg. pH 5.3). This precipitation pH is typical for a remote site. Organic acids were important contributors to acidity in throughfall and stemflow. Soil solution pHs were much higher as a result of acid neutralization processes, averaging 5.7 in the forest floor and 6.2 at 40-cm depth. Sodium and Cl dominated precipitation, followed by Ca and SO₄, indicating a strong oceanic influence. Throughfall and stemflow were generally enriched in cations, especially K, but concentrations in soil solutions were less than those in stemflow. Organic anions contributed greatly to cation leaching in the canopy, with a much smaller contribution from SO₄. Like precipitation, Na and Cl dominated throughfall, stemflow, and soil solutions. The highest concentrations of NH₄ were found in stemflow, suggesting N fixation in the canopy. Throughfall and stem-flow under Pacific silver fir had the highest concentrations of both cations and anions. Phosphate, NH₄, and NO₃ concentrations were low in the soil solution, indicating strong retention of N and P in this ecosystem.

Received for publication February 16, 1990.