Mercury Drives Metal Stress Response in Red Spruce Foliage in High-Elevation Appalachian Forests of New England, USA

The dieback of Picea rubens Sarg. (red spruce) in the Appalachian Mountains of New England has been correlated with emissions transported from the Great Lakes region, including acids, metals, and oxidants. In 1994–1995, metal stress in red spruce foliage evidenced by phytochelatin concentrations increased with red spruce damage index in spruce-fir dominated stands in high elevation forests. In this study, we revisited those same forests after two decades to examine metal stress impacts on high-elevation forests following reductions in atmospheric pollutant loading. We measured metal concentrations in soils, lichens, and foliage, and concentrations of phytochelatin and its precursors in foliage of red spruce trees at 1000 m along a west–east transect from New York to New Hampshire, and along an 800–1000 m elevational transect on Whiteface Mountain, NY. Path analysis showed that foliar Hg had a direct positive effect on foliar phytochelatins, metal-binding peptides produced by the metal stress response in plants. Essential metals Cu and Zn decreased the concentration of Hg in foliage. However, we could not determine the relative importance of atmospheric vs soil pathways for metal exposure. While metal stress was still occurring on Whiteface Mountain in 2013, the overall visual health of red spruce trees across the region was significantly improved compared to 1993–1995. Thus, although metal stress is still measurable in red spruce, the physiological impact may be lessened by decreases in the deposition of metals and acids, thus providing evidence of positive forest health outcomes from improvements in regional air quality in the Northeastern US.