Deposition and forest canopy interactions of airborne sulfur: Results from the integrated forest study

The Integrated Forest Study (IFS) was a long-term research project designed to determine the effects of atmospheric deposition on forest nutrient cycles. Concentrations and fluxes of airborne sulfur compounds were determined for several years at the 13 IFS research forests in North America and Europe using a standard set of protocols. Annual mean air concentrations of sulfur ranged from ～1.5 to 8 μgSm^?3 and were generally dominated by SO₂ (～60% of total sulfur on the average). Atmospheric deposition of sulfate at these forests was highest at the high elevation (～ 1000–2000 eq ha^?1yr^?1) and at the southeastern U.S. sites (～800–1000 eq ha^?1yr^?1), and lowest in the Pacific northwest (～300 eq ha^?1yr^?1). Cloud water contributed significantly to the sulfur flux at the mountain sites (45–50%), and dry deposition was comparable to wet at the drier southeastern sites (>40% of total). Deposited sulfur appeared to behave more or less conservatively in these canopies, showing little net uptake (ofSO₂) and minor foliar leaching (of soil-derived, internal SO₄^2?) relative to the total atmospheric flux. The estimated fluxes in total deposition were generally within 15% of the measured fluxes in throughfall plus stemflow, indicating that useful estimates of total atmospheric deposition of sulfur can be derived from measurements of throughfall.