Simulations of historical impacts of climate change and atmospheric chemistry on a northeastern U.S. forest ecosystem.

Climate change, land disturbance, and atmospheric chemistry have substantially impacted northeastern hardwood forests. However, it is challenging to quantify the exacerbating or mitigating interactions among these disturbances on carbon (C), nitrogen (N), and water cycling in forest ecosystems. To evaluate these effects, we applied the PnET-CN-daily model to simulate the historical patterns of C, N, and water cycling at Harvard Forest in central Massachusetts, United States. The model was run with a reconstructed historical climate and air chemistry scenario, and results were compared with field measurements at Harvard Forest for calibration. The calibrated model was then run with a series of hypothetical scenarios to decompose the impacts of individual environmental drivers on C, N, and water cycling of the forest ecosystem. Model simulations suggest that increases in atmospheric carbon dioxide (CO2) concentrations, changes in climate, and decreases in atmospheric N deposition have contributed to historical changes in the plant C cycle. Elevated CO2 concentrations have been the dominant factor, though these effects have diminished with increasing concentrations. The combination of elevated CO2 and a warmer climate has led to increased plant growth, resulting in higher plant N storage but a decline in the soil N pool. However, elevated atmospheric N deposition has mitigated this decline in soil N and also suppressed soil decomposition. Climate has been a key driver of recent changes in the water cycle, with increased air temperatures leading to higher transpiration rates. Despite this change, soil water content at Harvard Forest remained relatively constant over the simulation period because of increasing water-use efficiency associated with increasing CO2 concentration, indicating that plant growth is not limited by water at Harvard Forest. Future investigations should use modeling approaches to project the functional responses of the forest ecosystem to the interacting effects of future climate scenarios and contrasting air quality regulations.