CHEMICAL WEATHERING IN A SUBTROPICAL MOUNTAINOUS WATERSHED, TAIWAN

Watershed-scale long-term weathering studies in North America and Europe are plentiful, but few data exist from other parts of the world. Comparison of weathering rates at these sites should yield important insights into controls on weathering and landscape development. We have collected stream chemistry data from a first-order stream in the Taiwan Ecological Research Network (TERN) site in the Fu-shan experimental forest, located in a semi-tropical montane watershed in northeastern Taiwan. The Fu-shan site is hosted in metamorphosed Eocene to Oligocene sandstones, siltstones and shales, and argillites interbedded with sandstones. Mean annual precipitation is 4100 mm. We compare the Fu-shan data with published information from several U.S. LTER forested sites: Hubbard Brook (West Thornton, New Hampshire), Coweeta Hydrologic Lab (Otto, North Carolina), H. J. Andrews Experimental Forest (Blue River, Oregon) and Loquillo Experimental Forest (near San Juan, Puerto Rico). Environmental factors considered were location, mean annual temperature, precipitation, elevation and aspect. Strongest correlation of chemical weathering rates for weathering of H₄SiO₄, HCO₃^- and major cations was seen with mean annual precipitation and the age of bedrock. The strongest correlation observed was for H₄SiO₄ with precipitation (r²=99%). The long term data (1–11 years) were compared to short term (instantaneous) data from the Fu-shan experimental watershed. The Fu-shan site had the highest H₄SiO₄ yield (7700 mol ha^-1 yr^-1) observed. Of all the sites, Loquillo had the highest total cation yield (8500 eq ha^-1 yr^-1) and bicarbonate yield (7700 eq ha^-1 yr^-1) observed, and the second highest H₄SiO₄ weathering yield (5300 mol ha^-1 yr^-1). Lowest yield of all solutes was seen at Coweeta and Hubbard Brook, watersheds underlain by Paleozoic rocks. The Coweeta site lies in Paleozoic crystalline metamorphics of the Coweeta Group and the overlying Tallulah Falls Formation (a coarse-grained quartz diorite gneiss). Hubbard Brook bedrock is quartz mica schists and quartzite of the Silurian Rangeley Formation. Silica weathering yield had the weakest correlation with temperature (r²=58%) observed. There were weak to no relationships observed between site elevation and solute weathering yields.