Cross Site Analysis of Projections of the Biogeochemical Responses of Forested Watersheds in the Northeastern U.S. to Future Climate Change and Increasing CO2 over the 21st Century Using a Dynamic Model (PnET-BGC)

English Abstract:

A cross-site analysis of seven diverse, intensive study watersheds was conducted to evaluate how northeastern U.S. forests might respond to future changing climate. We used outputs of four AOGCMs; CCSM4, HadGEM2, MIROC5, and MRI-CGCM3 coupled with RCP8.5 and RCP4.5 emissions scenarios to represent possible higher- and lower-emission futures, respectively. These outputs were statistically downscaled using ARRM trained on meteorological observations from watershed stations. Meteorological projections were applied as inputs to the hydrochemical model PnET-BGC to evaluate the response of watershed attributes of management-relevant ecosystem services (NPP, soil C, stream water quantity and quality). Evapotranspiration (ET) is projected to increase across all sites under potential future conditions of warmer temperature and longer growing season. Spruce-fir forests are more susceptible to temperature stress due to their lower optimum temperature for photosynthesis and may be replaced in the future with more tolerant hardwoods. Some hardwood forests are projected to experience water stress due to early loss of snowpack, longer growing season and associated water deficit. This response is counter-intuitive to the anticipated increase in precipitation. The streamflow projections are highly variable with some sites showing significant increases in annual water yield, while others decrease. This variability in hydrologic response could challenge the future management of the Northern Forest. Following increases in temperature, ET and water stress associated with future climate change scenarios, a shifting pattern in carbon allocation in plants was evident causing significant changes in NPP. The soil humus C pool is projected to decrease with increases in temperature. This analysis suggests that dominant vegetation type and historical land disturbances coupled with climate variability will influence responses of forest ecosystem services to future changes in climate.