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Boundary-Layer Meteorology

, Volume 96, Issue 1–2, pp 257–291 | Cite as

On Measuring Net Ecosystem Carbon Exchange Over Tall Vegetation on Complex Terrain

  • Dennis Baldocchi
  • John Finnigan
  • Kell Wilson
  • K. T. Paw U
  • Eva Falge
Article

Abstract

To assess annual budgets of CO2 exchange betweenthe biosphere and atmosphere over representativeecosystems, long-term measurements must be made overecosystems that do not exist on ideal terrain. How tointerpret eddy covariance measurements correctlyremains a major task. At present, net ecosystemCO2 exchange is assessed, by members of themicrometeorological community, as the sum of eddycovariance measurements and the storage of CO2 inthe underlying air. This approach, however, seemsunsatisfactory as numerous investigators are reportingthat it may be causing nocturnal respiration fluxdensities to be underestimated.

A new theory was recently published by Lee (1998, Agricultural and Forest Meteorology91: 39–50) for assessing net ecosystem-atmosphere CO2 exchange(Ne) over non-ideal terrain. Itincludes a vertical advection term. We apply thisequation over a temperate broadleaved forest growingin undulating terrain. Inclusion of the verticaladvection term yields hourly, daily and annual sums ofnet ecosystem CO2 exchange that are moreecologically correct during the growing season.During the winter dormant period, on the other hand,corrected CO2 flux density measurements of anactively respiring forest were near zero. Thisobservation is unrealistic compared to chambermeasurements and model calculations. Only duringmidday, when the atmosphere is well-mixed, domeasurements of Ne match estimatesbased on model calculations and chamber measurements. On an annual basis, sums of Newithout the advection correction were 40% too large,as compared with computations derived from a validatedand process-based model. With the inclusion of theadvection correction term, we observe convergencebetween measured and calculated values ofNe on hourly, daily and yearly time scales. We cannot, however, conclude that inclusion of aone-dimensional, vertical advection term into thecontinuity equation is sufficient for evaluatingCO2 exchange over tall forests in complexterrain. There is an indication that the neglected term,ū(∂ c¯/∂ x), isnon-zero and that CO2 may be leakingfrom the sides of the control volume during the winter. In this circumstance, forest floor CO2 effluxdensities exceed effluxes measured above the canopy.

AmeriFlux FLUXNET Carbon balance Micrometeorology Deciduous forest Eddy covariance Ecophysiology 

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Copyright information

© Kluwer Academic Publishers 2000

Authors and Affiliations

  • Dennis Baldocchi
    • 1
  • John Finnigan
    • 2
  • Kell Wilson
    • 3
  • K. T. Paw U
    • 4
  • Eva Falge
    • 5
  1. 1.Ecosystem Science Division, Department of Environmental Science, Policy and ManagementUniversity of CaliforniaBerkeley
  2. 2.CSIRO Land and WaterCanberraAustralia
  3. 3.Atmospheric Turbulence and Diffusion DivisionNOAAOak RidgeU.S.A.
  4. 4.Department of Land, Air and Water ResourcesUniversity of CaliforniaDavisU.S.A.
  5. 5.Ecosystem Science Division, Department of Environmental Science, Policy and ManagementUniversity of CaliforniaBerkeley

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