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

At the intersection of micrometeorology and ecology: Under what conditions can we measure nocturnal transpiration and CO₂ using bulk methods?

Fitzjarrald, David^*,1, Sakai, Ricardo ¹, Czikowsky, Matthew¹, ¹ Jungle Research Group, Albany, New York, USA

ABSTRACT- Micrometeorological approaches for flux measurements are at their worst in nocturnal conditions. Hourly estimates of net ecosystem exchange (NEE) and evaporation for relatively large sections of an ecosystem are now commonly reported at a number of sites, but quality flux data cannot be maintained on calm nights (limited atmospheric mixing). One standard technique used in micrometeorology, the eddy covariance approach, did not result in ecologically plausible estimates of CO2 efflux on calm nights--insufficient flux is typically reported. The approaches used to produce seamless data sets for annual carbon balance--so-called gap-filling strategies--are not useful to the ecologists trying to figure out how plants are functioning in specific cases. Note that the transpiration and NEE measurements are diurnally antisymmetric: CO2 flux is the net of upward (respiratory) and downward (photosynthetic uptake) fluxes in the day; it is unidirectional upward at night. Evaporation is the opposite: unidirectional (upward) evapotranspiration in the day, but at night transpiration, evaporation (upward fluxes) coexist with dew deposition. Evaluating to what extent the nocturnal energy budget can be closed observationally is one way to assess how imp points The desire by ecologists to obtain believable nocturnal CO2 flux estimates forced the micrometeorologists to return to the problem of measuring nocturnal fluxes, addressing intermittent turbulent effects, subcanopy horizontal advection and other issues they had preferred to ignore for some decades. Three case studies of meteorologically measured nocturnal evapotranspiration and (for some cases) CO2 flux will be discussed and assessed: a) Midlatitude temperature forest (Harvard Forest MA); b) equatorial pasture (LBA Santarem); c) rain forest (LBA Santarem); and dry savannah (Namibia). In a), particular attention will be made to effects of horizontal subcanopy motions and to the limitations of the eddy covariance and boundary layer accumulation approaches at night.

Key words: nocturnal CO2 flux and evapotranspiration measurement