Cross site analysis of northern forested watersheds responses to future changes in climate and CO2 using a dynamic biogeochemical model (PnET-BGC)

Effects of global climate change will be manifested differently across land areas with differing biogeographic characteristics. Understanding the nuances of forest watershed response to future climate change and the characteristics that drive this varied response is critical to assessments of effects. To assess the impacts of climate change, a multi-faceted approach is required that is capable of resolving multiple climatic drivers and other anthropogenic stressors likely to simultaneously affect ecosystems over the coming decades. Dynamic hydrochemical models are useful tools to understand and predict the interactive effects of climate change, atmospheric CO₂, and atmospheric deposition on the hydrology and water quality of forested watersheds. In this study, we used the biogeochemical model, PnET-BGC, to assess, compare and contrast the effects of potential future changes in temperature, precipitation, solar radiation and atmospheric CO₂ on pools, concentrations, and fluxes of major elements at four forested watersheds in the northeastern U.S.; the Hubbard Brook Experimental Forest and Cone Pond Watershed in New Hampshire, Sleepers River Watershed in Vermont, and Huntington Wildlife Forest in New York. Future emissions scenarios were developed from monthly output from three atmosphere-ocean general circulation models (HadCM3, GFDL, PCM) in conjunction with potential lower and upper bounds of projected atmospheric CO₂ (550 and 970 ppm by 2099, respectively). These climate projections indicate that over the 21^st century, average air temperature will increase at all sites with simultaneous increases in annual average precipitation.

The modeling results suggest that climate change is projected to cause substantial temporal shifts in hydrologic patterns. Modeling results indicate that spring (April–June) snowmelt will occur earlier and will be less extreme in the future. Low flows associated with enhanced evapotranspiration during the summer months (July–September), will extend earlier into the spring and later into the fall (October–December). Future streamflow in late fall and early winter (January–March) will increase because of less snowpack accumulation due to warmer air temperatures and concurrent declines in the ratio of snow to rain. Over the summer period, higher rates of evapotranspiration are predicted to decrease streamflow. Model results show that under elevated temperature, net soil nitrogen mineralization and nitrification markedly increase, resulting in acidification of soil and streamwater, although the extent varies with site land disturbance history. The timing, patterns, and magnitude of stream water NO₃^- concentrations are highly variable depending on the climate scenarios and site characteristics. Since NO₃^- is the main driver of acid-base status of the ecosystem, Ca²⁺, pH, and ANC follow similar patterns. Warmer temperatures in the future caused a decrease in soil moisture and an increase in vapor pressure deficit, despite the increase in precipitation. These factors decrease evapotranspiration and cause midsummer drought stress, the extent of which is dependent on the climate-change scenario considered. Future projections of DOC concentrations are highly sensitive to NPP and midsummer drought. These results suggest that as climate change will likely alter the overall element concentrations and fluxes, these changes will be manifested in the seasonal patterns of elements concentrations and fluxes and the timing of these changes.