PARENT SESSION
Organized Oral Session 52: Casting light on nocturnal stomatal and canopy conductance
Organizer(s): NG Phillips and M Barbour
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 510a, Level 5, Palais des congrès de Montréal
Challenges to estimating nocturnal terrestrial ecosystem carbon dioxide and water vapor fluxes using micrometeorological techniques.
Gu, Lianhong1, Hanson, Paul1, Wullschleger, Stan1, Yang, Bai1, Pallardy, Steve2, Meyers, Tilden3, 1 Oak Ridge National Lab, Oak Ridge, TN, USA2 University of Missouri, Columbia, MO, USA3 NOAA / ATDD, Oak Ridge, TN, USA
ABSTRACT- The eddy covariance technique has been proven to be a useful tool in investigating ecosystem-level functions and their responses to environmental changes. However, this technique is fundamentally one-dimensional and requires that net flux exchanges occur only vertically. Such a requirement is usually met in relatively flat terrains during daytime when turbulent mixing is strong. Fluxes observed during daytime from sites around the world generally show consistency with our understanding of fundamental ecophysiological processes and with mechanistic ecosystem models. During nighttime, however, the atmospheric boundary layer is frequently stably stratified; as a result, vertical mixing is suppressed and net fluxes may be 2D or 3D and thus evade the detection by the eddy covariance system. Because of this, nighttime ecosystem respiration is often underestimated by the eddy covariance system. Although there has been no direct evaluation on water vapor fluxes observed during nighttime, it is likely that water vapor fluxes can be also underestimated if there is significant dew formation or if stomata remain partially open and air is dry. In general, the energy balance closure problem is worse during nighttime than during daytime. This may be an indication of water vapor flux underestimation. Although in theory the problem of flux underestimation can be solved by monitoring three dimensional wind and scalar concentration fields with high precision, such an approach would be too costly. Therefore more practical solutions are needed. Several methods have been used to derive nighttime fluxes, including scaling up chamber measurements, extrapolation of daytime response curves, and filtering nighttime eddy flux observations based on atmospheric conditions. In this presentation, we will discuss advantages and weaknesses of each of these three methods and make some recommendations based on recently obtained research results. We will also discuss the implications of incomplete stomatal closure at night in the context of limitations with the eddy covariance technique, and address possible rates of nighttime water loss using available micrometeorological data and FAPIS, a model for carbon and water exchange in terrestrial ecosystems.
Key words: Eddy covariance, Nocturnal fluxes, Stomatal conductance, Turbulence