Nocturnal Carbon Efflux can estimates of eddy covariance and chamber measurements be reconciled? Eva van Gorsel1, Ray Leuning1, Helen A. Cleugh1, Heather Keith2 and Tanja Suni1 1Marine and Atmospheric Research 2Forestry and Forest Products September 06

BACKGROUND // MOTIVATION Comparisons between micrometeorological and chamber measurements almost consistently show: Respiration is underestimated NEE overestimated The problem occurs due to advection (not usually measured) What is // can be done? Nocturnal Carbon Efflux - September 06

MEASUREMENTS (CHAMBER) Leaf Respiration open-flow-through (IRGA, Licor6400) Wood Respiration (Stems and Root) transient (IRGA, Licor6200) Soil Respiration non-flow-through steady state (Soda lime chambers) Soil Respiration: 30 plots Wood Respiration: transient measurement with IRGA of incrasing CO2 conc over time Leaf respiration: leaf cuvettes (with temperature control) 8 trees Nocturnal Carbon Efflux - September 06 Keith H. et al. (2006): Measurement of soil CO2 efflux using soda lime absorption […] Soil Biol. and Biochem. Keith H. et al. (in preparation): Net carbon exchange in a eucalypt forest […]

MEASUREMENTS (EDDY COVARIANCE) EC-System (70 m) Ultrasonic Anemometer (Gill HS) IRGA (open path Licor7500) Profile Measurement (9 levels) IRGA (closed path Licor6262) Advection Experiment (50 x 50 x 6 m control volume) 40 m tall Eucalypt forest (evergreen, wet sclerophyll) Lai~2.4 Forest cover and species homogenous within radius > 5 km Nocturnal Carbon Efflux - September 06 Leuning et al. (2005) Agric. For. Meteorol. 129, 151-173

MEASUREMENTS (EDDY COVARIANCE) 40 m tall Eucalypt forest (evergreen, wet sclerophyll) Lai~2.4 Forest cover and species homogenous within radius > 5 km 40 m tall Eucalypt forest (evergreen, wet sclerophyll) Lai~2.4 Forest cover and species homogenous within radius > 5 km Nocturnal Carbon Efflux - September 06

MEASUREMENTS ( SITE // TOPOGRAPHY) ↑ N Main mast CV Streamlines tilted within a range of [-6,2] degrees, depending an wind direction. 1km X 1km topographic map tumba Shallow basin sloping towards N Nocturnal Carbon Efflux - September 06

MEASUREMENTS ( SITE // TOPOGRAPHY) Shallow basin sloping towards N 40 m tall Eucalypt forest (evergreen, wet sclerophyll) Lai~2.4 Forest cover and species homogenous within radius > 5 km Streamlines tilted within a range of [-6,2] degrees, depending an wind direction. Nocturnal Carbon Efflux - September 06 Photographs : Dale Hughes

METHOD (EDDY COVARIANCE) Nocturnal Carbon Efflux - September 06 Graphics (Schematic tower): Andreas Christen

METHOD (EDDY COVARIANCE) Advection terms not measured? measurements in homogeneous flat terrain good vertical mixing strong turbulence Nocturnal Carbon Efflux - September 06 Graphics (Schematic tower): Andreas Christen

METHOD (EDDY COVARIANCE) Advection terms not measured? measurements in homogeneous flat terrain good vertical mixing strong turbulence Nocturnal Carbon Efflux - September 06 Graphics (Schematic tower): Andreas Christen

METHOD (EDDY COVARIANCE) Nocturnal Carbon Efflux - September 06 Graphics (Schematic tower): Andreas Christen

RESULTS Nocturnal Carbon Efflux // Participation Eddy Flux and Storage Term not measured fraction of NCE cumulative frequency (u*) Nocturnal Carbon Efflux - September 06

METHOD (EDDY COVARIANCE) CALM INTERMITTENT WINDY 1) even rather shallow topography can lead to cold air drainage, as the slope length rather than the slope angle is a key controlling parameter. Nocturnal Carbon Efflux - September 06

RESULTS Nocturnal Carbon Efflux // Storage term Nocturnal Carbon Efflux - September 06

RESULTS Nocturnal Carbon Efflux // Storage term Nocturnal Carbon Efflux - September 06 graph: Aubinet et al.,(2005) Boundary Layer Meteorol., 116: 63-94

RESULTS Nocturnal Carbon Efflux // Storage term // Advection Total NCE Chamber (Soil + Leaf + Wood) 5.18 ± 0.18 mmol m-2 s-1 4.97 mmol m-2 s-1 5.23 mmol m-2 s-1 1.70 mmol m-2 s-1 0.46 mmol m-2 s-1 Nocturnal Carbon Efflux - September 06

RESULTS Nocturnal Carbon Efflux // Storage term // Advection Nocturnal Carbon Efflux - September 06

SUMMARY // CONCLUSIONS Maximum in storage term in early evening regularly observed observed at several stations Nighttime decrease in storage term is not counter-balanced by eddy flux Advection leads to (usually) unmeasured loss of CO2 from control volume. Hypothesis: the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. It has been shown that… Nocturnal Carbon Efflux - September 06

CONCLUSIONS We propose taking advantage of this time delay to develop relationships between Rmax and Tsoil. Comparison of chamber measurements and results from the developed temperature response function lead to excellent agreement. The new parameterization leads to realistic values of nighttime respiration, and therefore improves estimates of net ecosystem exchange. It has been shown that… Nocturnal Carbon Efflux - September 06

Thank You CSIRO Marine and Atmospheric Research OzFlux Dr. Eva van Gorsel Research Scientist – Land and Atmosphere +61 (0)2 6246 5611 eva.vangorsel@csiro.au www.cmar.csiro.au/ OzFlux www.dar.csiro.au/lai/ozflux/ Thank You Contact CSIRO Phone 1300 363 400 +61 3 9545 2176 Email enquiries@csiro.au Web www.csiro.au