Estimating nocturnal ecosystem respiration from the vertical turbulent flux and change in storage of CO2

Micrometeorological measurements of nighttime ecosystem respiration can be systematically biasedwhen stable atmospheric conditions lead to drainage flows associated with decoupling of air flow aboveand within plant canopies. The associated horizontal and vertical advective fluxes cannot be measuredusing instrumentation on the single towers typically used at micrometeorological sites. A commonapproach to minimize bias is to use a threshold in friction velocity, u *, to exclude periods when advectionis assumed to be important, but this is problematic in situations when in-canopy flows are decoupledfrom the flow above. Using data from 25 flux stations in a wide variety of forest ecosystems globally, weexamine the generality of a novel approach to estimating nocturnal respiration developed by van Gorselet  al.  (van  Gorsel,  E.,  Leuning,  R.,  Cleugh,  H.A.,  Keith,  H.,  Suni,  T.,  2007.  Nocturnal  carbon  efflux:reconciliation of eddy covariance and chamber measurements using an alternative to the u * -thresholdfiltering technique. Tellus 59B, 397–403, Tellus, 59B, 307-403). The approach is based on the assumptionthat advection is small relative to the vertical turbulent flux (F C ) and change in storage (F S ) of CO 2 in thefew hours after sundown. The sum of F C and F S reach a maximum during this period which is used to derive a temperature response function for ecosystem respiration. Measured hourly soil temperatures arethen used with this function to estimate respiration R Rmax. The new approach yielded excellent agreement with (1) independent measurements using respiration chambers, (2) with estimates using ecosystem light-response curves of Fc + Fs extrapolated to zero light, RLRC, and (3) with a detailed process-based forest ecosystem model, Rcast. At most sites respiration rates estimated using the u*-filter, Rust, were smaller than RRmax and RLRC. Agreement of our approach with independent measurements indicates that RRmax provides an excellent estimate of nighttime ecosystem respiration.