PARENT SESSION
Oral Session # 66: Ecological Studies at Biosphere II.
Presiding: ML Martinez Vazquez
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 104.

Influence of drought and VPD on net ecosystem carbon exchange and CO₂ uptake of cottonwoods.

Barron-Gafford, Greg^*,1, Grieve, Katie¹, Paige, Terence¹, Bobich, Edward¹, Patterson, Randy¹, Marshall, John², Murthy, Ramesh¹, ¹ Biosphere 2 Center, Oracle, AZ, USA² University of Idaho, Moscow, ID, USA

ABSTRACT- Under the controlled conditions of a sealed greenhouse system at the Intensive Forestry Mesocosm (Biosphere 2 Center), eastern cottonwoods (Populus deltoides Bartr.) established in 1998 were used to examine the concomitant effects of atmospheric vapor pressure deficit (VPD) and drought on net ecosystem carbon exchange (NECE), sap flux, carbon dioxide (CO₂) uptake, conductance, and transpiration under ambient and elevated levels of CO₂. Under low VPD (1.0 kPa) and high soil water content (0.3 v/v), CO₂ uptake for plants growing under 400, 800, and 1200 ppm CO₂ averaged 18.8, 34.4, and 40.2 mol m^-2 s^-1, respectively. Under high VPD (3.3 kPa) and high soil water content (0.3 v/v), CO₂ uptake for plants under 400, 800, and 1200 ppm CO₂ averaged 17.3, 30.9, and 39.0 mol m^-2 s^-1, respectively. Four weeks into the drought volumetric soil water content declined to 0.1 v/v, resulting in a 40% decrease in CO₂ uptake under low VPD for all of the CO₂ treatments. Under high VPD, CO₂ uptake decreased by an average of 80% for all CO₂ treatments. In addition, transpiration, conductance, respiration, and sap flux all decreased significantly as soil water content declined for plants in all of the CO₂ treatments. Leaf-level CO₂ uptake data were used to estimate stand-level CO₂ uptake values using leaf area measurements. NECE for each treatment was calculated using a mass balance model within this particular closed system. Results at leaf and stand levels will be compared and discussed.

Key words: Photosynthesis, Biosphere 2 Center, Drought, CO2