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Deposition of nitrogen (N), sulfur (S), and H+ pollutants has been a major biogeochemical driver in eastern North American forests in the past century, substantially altering the availability of essential plant nutrients. While acid deposition has been shown to affect both tree stomatal function and carbon allocation, relatively few studies have examined the influence of deposition on forest productivity and water use efficiency (WUE). Using evidence from tree ring stable C isotope ratios (δ 13 C), and forest growth and catchment water balance data, we examined tree growth and WUE in response to both experimental whole-watershed acidification treatments (Fernow Experimental Forest, West Virginia and Bear Brook Watersheds, Maine), and base cation amendment (Hubbard Brook Experimental Forest, New Hampshire). We observed species and site-specific differences in response to nutrient manipulations. At Bear …