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.

Effect of CO₂ and VPD on isoprene emissions during drought in a Populus deltoides ecosystem.

Pegoraro, Emiliano¹, Murthy, Ramesh¹, Barron-Gafford, Greg¹, Grieve, Katie, VanHaren, Joost, ¹ Biosphere 2 Center, Oracle, AZ, USA

ABSTRACT- From July to December 2002 we studied the effect of CO₂ and vapour pressure deficit (VPD) on isoprene emission from a cottonwood forest during a drought experiment in the Intensive Forestry Mesocosm at the Biosphere 2 Center. We measured leaf and ecosystem isoprene emission rates. Photosynthesis, stomatal conductance, VPD, and soil water content were also monitored. Difference in isoprene emission rates between elevated CO₂ and ambient CO₂ grown trees decreased as the drought progressed. Although isoprene emission rates decreased when water stress was severe, they were considerably less sensitive to drought than photosynthesis and stomatal conductance. As a result, the percentage of carbon lost as isoprene increased as they became more water stressed reaching carbon losses of 11 %. Results also showed that during the drought isoprene biosynthesis used recently fixed carbon, and this decreased when it was limited. We also observed that the inhibitory effect of elevated CO₂ on isoprene emission was compensated by the decrease in intercellular CO₂ concentration (C_i) caused by the high VPD treatment. Results show the strong negative relationship between isoprene emission rate and C_i in all CO₂ treatments. The existence of a soil sink for isoprene was also investigated. Biosphere 2 Centre is a unique facility for the studying the response of entire ecosystems to selected environmental variables.

Key words: CO2 , VPD, Biosphere 2 Center, Isoprene