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

Abstract

To assess annual budgets of CO 2 exchange between the biosphere and atmosphere over representative ecosystems, long-term measurements must be made over ecosystems that do not exist on ideal terrain. How to interpret eddy covariance measurements correctly remains a major task. At present, net ecosystem CO 2 exchange is assessed, by members of the micrometeorological community, as the sum of eddy covariance measurements and the storage of CO 2 in the underlying air. This approach, however, seems unsatisfactory as numerous investigators are reporting that it may be causing nocturnal respiration flux densities to be underestimated. A new theory was recently published by Lee (1998, Agricultural and Forest Meteorology 91: 39-50) for assessing net ecosystem-atmosphere CO 2 exchange (N e) over non-ideal terrain. It includes a vertical advection term. We apply this equation over a temperate broadleaved forest growing in undulating terrain. Inclusion of the vertical advection term yields hourly, daily and annual sums of net ecosystem CO 2 exchange that are more ecologically correct during the growing season. During the winter dormant period, on the other hand, corrected CO 2 flux density measurements of an actively respiring forest were near zero. This observation is unrealistic compared to chamber measurements and model calculations. Only during midday, when the atmosphere is well-mixed, do measurements of N e match estimates based on model calculations and chamber measurements. On an annual basis, sums of N e without the advection correction were 40% too large, as compared with computations derived from a validated and process-based model. With the inclusion of the advection correction term, we observe convergence between measured and calculated values of N e on hourly, daily and yearly time scales. We cannot, however, conclude that inclusion of a one-dimensional, vertical advection term into the continuity equation is sufficient for evaluating CO 2 exchange over tall forests in complex terrain. There is an indication that the neglected term, u(∂c/∂x), is non-zero and that CO 2 may be leaking from the sides of the control volume during the winter. In this circumstance, forest floor CO 2 efflux densities exceed effluxes measured above the canopy.

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