25th Conference on Agricultural and Forest Meteorology


Eddy covariance and chamber measurements of carbon dioxide fluxes from the forest floor of a closed-canopy Douglas-fir forest

Gordon Drewitt, University of British Columbia, Vancouver, BC, Canada; and E. R. Humphreys, T. A. Black, G. Ethier, Z. Nesic, K. Morgenstern, and M. D. Novak

Carbon dioxide fluxes were measured from the forest floor of a West Coast temperate forest using an eddy covariance system and automated soil chambers operating concurrently for two months in early autumn of 2000. The forest was a 55-year-old Douglas fir stand with a leaf area index of 6.7 m2 m-2, while the forest floor had a thin layer of moss and the understory was composed of a small number of vascular plants. Conditions were not always ideal for eddy covariance flux measurements in this relatively closed-canopy stand situated on a 5-10 degree slope. In the trunk space, down slope flow persisted for much of the day, strong mixing occurred at night, and an inversion usually developed during the day. Windspeeds were low and non-stationary, while turbulence intensities were generally high, thereby often violating assumptions about surface layer turbulence required for eddy covariance flux measurements. Available energy was often very low as were latent and sensible heat fluxes measured above the forest floor resulting in poor energy balance closure. Shapes of turbulence spectra depended on wind direction and revealed effects of flow obstruction caused by the sonic anemometer mounts or tower. Data remaining after quality control checks revealed low and variable carbon dioxide fluxes from the forest floor with a very weak diurnal trend and little relationship to temperature and photosynthetically active radiation. Drainage flows are suspected of causing significant advective fluxes at this site but preliminary advection measurements indicated that very little advection occurs near the forest floor. The soil chamber system, which had six 0.2 m2 collars, measured fluxes every half hour. Chamber fluxes showed a strong dependence on soil temperature and demonstrated clear diurnal and seasonal patterns. Chamber-based estimates of forest floor carbon dioxide flux were much higher than those obtained from the understory eddy covariance system. Chamber estimates of annual soil respiration were high compared to above-canopy nighttime eddy covariance estimates of ecosystem respiration indicating that the chamber locations were likely not representative of the forest floor in the tower flux foot print.

extended abstract  Extended Abstract (48K)

Session 6, Carbon dioxide exchange Part 1: forests
Wednesday, 22 May 2002, 8:45 AM-1:15 PM

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