PS 15-2 - Variations in the photosynthetic carbon assimilation and isoprene emission capacities of Texas oak species during the exceptional spring 2011 drought

Tuesday, August 9, 2011
Exhibit Hall 3, Austin Convention Center
Csengele Barta1, Jonathan H. Gramann2, Stephanie L. White3 and Gunnar W. Schade2, (1)Biology, Missouri Western State University, Saint Joseph, MO, (2)Department of Atmospheric Sciences, Texas A&M University, College Station, TX, (3)Department of Chemistry, Texas A&M University, College Station, TX

Biogenic isoprene emission from vegetation is an important biosphere-atmosphere interaction and a key constraint to chemical properties of the troposphere.  Isoprene reactions can contribute to the formation of ozone and indirectly affect the accumulation of greenhouse gases, shaping regional and global air quality. An intricate, yet not completely understood balance exists between the impact of isoprene on the atmosphere and the feedback of global change on the eco-physiology of vegetation that defines future emission potentials, affecting to a yet unknown extent future isoprene emissions. To address the nature of these interactions, we set up a field study that characterizes the effects of climate and pollution gradients on carbon assimilation and isoprene emission fluxes of dominant oak tree species in Texas. We follow these oak species throughout their growth season by comparing tree responses to their growth environment at three selected sites along an urban to rural transect from downtown Houston to Sam Houston National Forest. As urban areas are warmer, more polluted and often drier than rural regions, they are used in our study as an environment that mimics conditions expected from global climate change.


Photosynthesis of south-facing, sun exposed leaves at 2-4 m canopy height of each species was measured biweekly in a controlled-environment cuvette enclosure. In parallel, basal isoprene emission rates (at 30°C and 1000 µmol m-2 s-1 PAR) were sampled from the cuvette outflow concentrating the air onto adsorbents and analyzing by thermal-desorption GC-FID.  Photosynthesis and isoprene emission rates were correlated with local climate and pollution starting in April, 2011, after the onset of isoprene emission from the local vegetation. Microclimate data was collected by meteorological stations established at all study sites. Preliminary results show that the early onset of exceptional drought stress (soil moistures of less than 5-10%; SPI of -2 to -3), alongside high air temperatures and ozone, dominated the spring season and decreased photosynthetic carbon assimilation by 30-50% compared to expected optimum rates in all species at all three sites. Isoprene emission was however less affected than expected by the early phase of stress, measured rates ranging from 52-77 µg C g-1 h-1 in Q. stellata and 65- 102 µg C g-1 h-1 in Q. falcata. Data from ongoing measurements throughout the spring to summer and expected correlations of photosynthesis and isoprene emission with seasonal and environmental gradient variations will be presented on the poster.

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