Modeling nighttime ecosystem respiration from measured CO 2 concentration and air temperature profiles using inverse methods

A major challenge for quantifying ecosystem carbon budgets from micrometeorological methods remains nighttime ecosystem respiration. An earlier study utilized a constrained source optimization (CSO) method using inverse Lagrangian dispersion theory to infer the two components of ecosystem respiration (aboveground and forest floor) from measured mean CO 2 concentration profiles within the canopy. This method required measurements of within‐canopy mean velocity statistics and did not consider local thermal stratification. We propose a Eulerian version of the CSO method (CSO E ) to account for local thermal stratification within the canopy for momentum and scalars using higher‐order closure principles. This method uses simultaneous mean CO 2 concentration and air temperature profiles within the canopy and velocity statistics above the canopy as inputs. The CSO E was tested at a maturing loblolly pine plantation over a 3‐year period with a mild drought (2001), a severe drought (2002), and a wet year (2003). Annual forest floor efflux modeled with CSO E averaged 111 g C m −2 less than that estimated using chambers during these years (2001: 1224 versus 1328 gCm −2 ; 2002: 1127 versus 1230 gCm −2 ; 2003: 1473 versus 1599 gCm −2 ). The modeled ecosystem respiration exceeded estimates from eddy covariance measurements (uncorrected for storage fluxes) by at least 25%, even at high friction velocities. Finally, we showed that the CSO E annual nighttime respiration values agree well with independent estimates derived from the intercept of the ecosystem light‐response curve from daytime eddy covariance CO 2 flux measurements.