Environmental control of forest ecosystem δ13CO2 discrimination

We used Picarro isotopic CO₂ analyzers to make profile measurements of ¹²CO₂, ¹³CO₂, and water vapor at 5 forested AmeriFlux sites (Wind River, GLEES, Howland, Silas Little, and Coweeta) across the continental U.S., starting in late 2010. We describe our experimental system and validation efforts. Concentration data and convective boundary layer budgeting methods are used to infer regional surface-atmosphere CO₂ exchange. Whole ecosystem discrimination varied markedly at diurnal and seasonal timescales, reflecting the influence of vapor pressure deficit, temperature, and light. Comparison of Howland results to nearby tall tower data suggests that these data reflect regional scale patterns of CO₂ discrimination.