Forest floor CO₂ fluxes estimated by eddy covariance and chamber-based model

An intercomparison study of two methods for estimating forest soil CO₂ efflux was conducted during a 3-week period in summer. An empirical model established from nearly 2 years of chamber measurements predicted that the soil CO₂ efflux ranged from 1.2 to 1.4 μmol m^-2 s^-1. This small range was due to the lack of variability in the model parameters (soil temperature and moisture) during the study period. Eddy covariance measurements showed more variability and diurnal dependence. Turbulent fluxes of CO₂ during the day apparently were influenced by the presence of a moss layer on the forest floor; the combined effects of moss photosynthesis and respiration reduced the turbulent fluxes by an average of 0.6 μmol m^-2 s^-1 relative to the modeled soil efflux. At night, the eddy fluxes of CO₂ agreed well with the modeled soil efflux; however, the turbulent fluxes were highly variable (standard deviation exceeding the mean), due to the imperfect sampling conditions associated with the nocturnal boundary layer. These results illustrate the different processes measured by the two methods, and highlight some of the limitations of the eddy covariance technique for estimating soil CO₂ efflux. Finally, they demonstrate the need for more long-term intercomparison studies, covering a broader range in soil temperature and moisture.