Future increases in temperature and altered precipitation coupled with elevated atmospheric CO2 concentrations will likely affect major ecosystem functions of old-growth temperate forests in the Pacific Northwest. Our previous work at Watershed 2 of the H. J. Andrews Experimental Forest (HJA), a 500 years old-growth forest dominated by Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla), projected future changes in the long-term and seasonal dynamics of carbon and nitrogen under the Representative Concentration Pathway (RCP) scenarios. In this study, we projected future hydrochemical responses at Watershed 2 of the HJA under the RCP4.5 and 8.5 scenarios utilizing a calibrated ecosystem model (PnET-BGC) with improved algorithms and climate change scenarios statistically downscaled from four General Circulation Models (GCMs).
Results/Conclusions
Our projections suggest future long-term changes in pH and acid-neutralizing capacity (ANC) of stream water in Watershed 2 is largely affected more by plant uptake than mineralization of calcium (Ca) and magnesium (Mg). Under the warmest future projected by HadGEM2-CC (one of the four GCMs) under the RCP8.5 scenario, large decreases in plant uptake of Ca and Mg result in large increases in their concentrations, increasing ANC and the fixation of inorganic carbon in stream water despite decreases in mineralization in response to low substrate quantity. Under the RCP4.5 scenario, however, stream water concentration of base cations were projected to show much less change compared with the high radiative forcing scenario. As a result, stream water pH and ANC were not projected to have significant long-term increase under the RCP4.5 scenario. Our projections also suggest future seasonal patterns of stream water chemistry is to a large extent affected by the ecohydrological response of the old-growth forest. Large decreases in summer transpiration and rainfall interception due to reduced foliar biomass were projected to increase summer streamflow and decrease ionic strength of stream water at Watershed 2 in HJA.