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.
Canopy transpiration and conductance during increasing soil water stress and cycles of VPD.
Grieve, Katie*,1, Barron-Gafford, Greg1, Bobich, Edward1, Patterson, Randy1, Marshall, John2, Murthy, Ramesh1, 1 Biosphere 2 Center, Oracle, AZ, USA2 University of Idaho, Moscow, ID, USA
ABSTRACT- In order to separate the individual effects of atmospheric vapor pressure deficit (VPD) and drought on leaf and stand level sap flux, conductance, and transpiration of eastern cottonwoods (Populus deltoides Bartr.) growing under ambient and elevated levels of CO2, cottonwoods were examined under increasing soil water stress and cycles of high and low atmospheric VPD. Rates of sap flux were measured on six trees within each of three stands of fast-growing cottonwoods established in 1998 in the controlled, sealed greenhouse system at the Biosphere 2 Center (Oracle, Arizona). Atmospheric pressure, VPD, relative humidity, and temperature were measured continuously and averages were stored every 15 minutes in data-loggers. Stand level (canopy) transpiration was calculated using measures of sapwood area index and sap flux. Stand-level conductance was calculated using a mass balance model for this particular closed system. Stand level transpiration rates for plants growing under 400, 800, and 1200 ppm CO2 averaged 0.562, 0.953 and 0.933 meters per day, respectively, under low VPD (1.0 kPa) and high soil water content (0.3 v/v). Stand level transpiration rates for plants growing under 400, 800, and 1200 ppm CO2 averaged 0.518, 0.661, and 0.848 meters per day, respectively, under high VPD (3.1 kPa) and high soil water content (0.3 v/v). After experiencing four weeks of drought, volumetric soil water content had declined to 0.1 v/v, resulting in a 57, 69, and 43% decrease in stand level transpiration under low VPD for the 400, 800, and 1200 ppm CO2 treatments, respectively. Under high VPD, stand level transpiration decreased by an average of 63, 57, and 46%, respectively. Stand level conductance and leaf level transpiration and conductance also decreased dramatically as soil water content declined for trees under all CO2 treatments. Results at leaf and stand levels will be compared and discussed.
Key words: drought, Biosphere 2 Center, sap flux, CO2