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

Authors

Dennis Bladocchi, Ecosystem Science Division, Department of Environmental Science, Policy and Management,151 Hilgard Hall, University of California, Berkeley, CA
John Finnigan, CSIRO Land and Water, GPO Box 821, Canberra, ACT 2601, Australia
Kell Wilson, Atmospheric Turbulence and Diffusion Division, NOAA, PO Box 2456, Oak Ridge, TN
K. T. Paw U, 4Department of Land, Air and Water Resources, University of California, Davis, CA
Eva Falge, Ecosystem Science Division, Department of Environmental Science, Policy and Management,151 Hilgard Hall, University of California, Berkeley, CA

Date of this Version

2000

Comments

Published in Boundary-Layer Meteorology 96: 257–291, 2000.

Abstract

To assess annual budgets of CO₂ 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 CO₂ exchange is assessed, by members of the micrometeorological community, as the sum of eddy covariance measurements and the storage of CO₂ 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 CO₂ 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 CO₂ exchange that are more ecologically correct during the growing season. During the winter dormant period, on the other hand, corrected CO₂ 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 CO₂ exchange over tall forests in complex terrain. There is an indication that the neglected term, u(c/x), is non-zero and that CO₂ may be leaking from the sides of the control volume during the winter. In this circumstance, forest floor CO₂ efflux densities exceed effluxes measured above the canopy.