Dynamics Of Isotopic Exchange Of Carbon Dioxide In A Tennessee Deciduous Forest

The combination of isotopic measurements and micrometeorological flux measurements is a powerful new approach that will likely lead to new insight into the dynamics of CO₂ exchange between terrestrial ecosystems and the atmosphere. Since the biological processes of photosynthesis and respiration modify the stable isotopic signature of atmospheric CO₂ in different ways, measurements of the net fluxes of CO₂, ¹³CO₂, and C¹⁸OO can be used to investigate the relative contribution of each process to net ecosystem CO₂ exchange. We used two independent approaches to measure isotopic fluxes of CO₂ over a Tennessee oak-maple-hickory forest in summer 1998. These approaches involved (1) a combination of standard eddy covariance with intensive flask sampling, and (2) a modification to the relaxed eddy accumulation technique. Strong isotopic signals associated with photosynthesis and respiration were observed and persisted in forest air despite the potential for mixing due to atmospheric turbulence. Calm nights allowed a buildup of respiratory CO₂ below the canopy and were associated with isotopically depleted forest air in the morning. Windy nights were followed by a relatively more enriched early-morning isotopic signal. Entrainment of air from above the decaying nocturnal boundary layer during daytime mixed layer growth exerted strong control on isotopic composition of forest air, resulting in similar isotope ratios in the late afternoon despite different isotopic starting points following calm or windy nights. The influences of the convective boundary layer and turbulent mixing within the forest cannot be ignored when using isotopes of CO₂ investigate biological processes.