Measuring and modeling ecosystem photosynthesis and the carbon isotope composition of ecosystem-respired CO2 in three boreal coniferous forests

Abstract

Photosynthetic capacity in boreal coniferous forests varies on a seasonal basis in response to the strong fluctuations in environmental conditions, and this contributes significantly to temporal changes in the concentration and stable isotope composition of atmospheric carbon dioxide. Our objectives in this study were to compare measurements of seasonal variation in ecosystem-scale photosynthesis and the carbon isotope composition of ecosystem-respired CO2 (δR) with calculations done using a model that included seasonal changes in the temperature acclimation of photosynthesis. Our measurements and model calculations were conducted in three boreal coniferous forests during the main growing season months (May–September) in three different years (2004–2006) as part of the Fluxnet-Canada Research Network. We observed good agreement between measured and modeled ecosystem photosynthesis, with measured ecosystem photosynthesis based on eddy covariance measurements of net ecosystem CO2 exchange. In addition, good agreement was observed between measured and modeled δR values, which helped to provide validation of our calculations of ecosystem-scale carbon isotope discrimination. There were important seasonal changes in both ecosystem photosynthesis rate and carbon isotope discrimination that affect the concentration and stable isotope composition of atmospheric CO2. The seasonal patterns of change in ecosystem photosynthesis and carbon isotope discrimination determined in this study closely matched the timing of measured changes in the concentration and isotope composition of atmospheric CO2 recorded at Cold Bay, Alaska.