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Increasing atmospheric CO2 can enhance photosynthesis and intrinsic water use efficiency (iWUE) in trees, resulting in increased woody biomass production and reduced tree sensitivity to water stress. However, these benefits may only be realized when other environmental factors, such as climate or nutrients, are not limiting. Tree response to increasing CO2 is modulated by a variety of environmental variables, including water availability, vapor pressure deficit, atmospheric N and S deposition, and soil fertility. Further, many studies of tree iWUE assess only a single species, making it difficult to assess how CO2 and other change drivers will impact forest carbon and water balance at ecosystem scales. We examined trends in, and drivers of, intrinsic water use efficiency in seven broadleaf deciduous and three evergreen conifer species from seven watersheds spanning a~ 6 latitudinal gradient in the Appalachian …