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

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.