PARENT SESSION
Poster Session 7: Grasslands
Tuesday, August 9, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Regulation of carbon dioxide and water vapor exchange by soil moisture in a C4 grass dominated pasture in central Florida, USA.

Bracho, Rosvel^*,1, Sumner, David², Powell, Thomas³, Hinkle, Charles⁴, Drake, Bert³, ¹ National Research Council, Washington, DC, USA² U.S. Geological Survey, Altamonte Springs, FL, US³ Smithsonian Environmental Research Center, Edgewater, MD, USA⁴ Dynamac Corporation, Kennedy Space Center, FL, USA

ABSTRACT- Seasonality in precipitation and the low soil water storage capacity of sandy soils prevalent in Florida commonly produce springtime moisture stress in grasslands. To determine the effects of soil moisture () availability on evapotranspiration (ET) and on the net ecosystem carbon exchange (NEE), we conducted eddy covariance measurements of water and energy exchange at a pasture site in central Florida between June 2000 and February 2004. Carbon exchange measurements were begun at the site in February 2003. Evapotranspiration dissipated over 80% of net radiation (Rn) at > 0.15 m³m^-3 and decreased to 30% of Rn when was below 0.08m³m^-3. The decoupling coefficient () indicates that ET was strongly controlled by surface conductance (g_s) at low soil moisture. Maximum daily NEE was -6.2 g Cm^-2 during the summertime wet season when leaf area index (LAI) was at maximum. Canopy carbon assimilation (Ac) increased with and leaf area development from values near -4 g Cm^-2d^-1 in February to values around -17 Cm^-2d^-1 in August. Ecosystem respiration increased with air temperature and reached a maximum of about 11 g Cm^-2d^-1 between May and September. Canopy assimilation was modeled using a big leaf model as a function of g_s, atmospheric [CO₂] and the ratio internal to external [CO₂]. Results show that Ac calculated from eddy covariance measurements and from the model were not significantly different and were equally affected by variations in soil moisture and LAI. On an annual basis the ecosystem functioned as a carbon sink, accumulating 264 g Cm^-2. Soil water controlled water and carbon exchange through variations in stomata conductance and leaf area development.

Key words: Grasses, Eddy covariance, Evapotranspiration, Carbon exchange