LAUSR

Acid deposition is a major biogeochemical driver in forest ecosystems, but the impacts of long‐term changes in deposition on forest productivity remain unclear. Using a combination of tree ring and forest inventory data, we examined tree growth and climate sensitivity in response to 26 years of whole‐watershed ammonium sulfate ((NH4)2SO4) additions at the Fernow Experimental Forest (West Virginia, USA). Linear mixed effects models revealed species‐specific responses to both treatment and hydroclimate variables. When controlling for environmental covariates, growth of northern red oak (Quercus rubra), red maple (Acer rubrum), and tulip poplar (Liriodendron tulipifera) was greater (40%, 52%, and 42%, respectively) in the control watershed compared to the treated watershed, but there was no difference in black cherry (Prunus serotina). Stem growth was generally positively associated with growing season water availability and spring temperature and negatively associated with vapor pressure deficit. Sensitivity of northern red oak, red maple, and tulip poplar growth to water availability was greater in the control watershed, suggesting that acidification treatment has altered tree response to climate. Results indicate that chronic acid deposition may reduce both forest growth and climate sensitivity, with potentially significant implications for forest carbon and water cycling in deposition‐affected regions. Plain Language Summary While acidifying nitrogen and sulfur pollution has substantially declined in the eastern United States due to the Clean Air Act and its amendments, the legacy of acidification on forest ecosystems is projected to be long‐lasting. However, it is often difficult to discern the effects of air pollution on forests without controlled experiments, since changes in pollution have occurred alongside other long‐term environmental changes (e.g., climate change and rising atmospheric CO2 concentrations). At the Fernow Experimental Forest in West Virginia, we examined the growth and climate sensitivity of trees in a watershed that has received 26 years of experimental acidification treatments to trees in an adjacent control watershed. Trees responded to treatment in species‐specific ways, but growth of three of four examined hardwood species was greater in the control watershed. Also, trees in the acidified watershed were less sensitive to interannual variation in water availability, suggesting that forests that have experienced high levels of acid deposition respond differently to precipitation than forests that have been less impacted. However, given that trees respond to acid deposition in species‐specific ways, the impacts on changes in air pollution on forests will largely depend on the species composition in a given region.