HR: 17:15h
AN: H54C-06    [Abstracts]
TI: Field observations of oxygen isotopes in two forest ecosystems - linking the water and carbon cycles at the ecosystem scale
AU: * Seibt, U
EM: useibt@stanford.edu
AF: Department of Global Ecology, Carnegie Institution of Washington, 260 Panama St, Stanford, CA 94305 United States
AU: Wingate, L
EM: lwingate@ed.ac.uk
AF: Institute of Atmospheric and Environmental Science, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JU United Kingdom
AU: Hemming, D
EM: debbie.hemming@metoffice.com
AF: Hadley Centre for Climate Prediction and Research, Fitzroy Road, Exeter, EX1 3PB United Kingdom
AU: Berry, J
EM: joeberry@GlobalEcology.stanford.edu
AF: Department of Global Ecology, Carnegie Institution of Washington, 260 Panama St, Stanford, CA 94305 United States
AB: Oxygen isotopes are valuable tools for studying the coupling of water and carbon cycles at the ecosystem scale. Our study focuses on plant foliage, where water and carbon are concurrently exchanged through the stomatal openings. We report photosynthetic $^{18}$O discrimination from branch bag experiments and $\delta$$^{18}$O signatures of plant water measured during field campaigns in a spruce plantation in Scotland and a beech forest in central Germany. The contrasting micro-climate and species characteristics at the two sites enabled us to investigate the response of plant gas exchange to fluctuating environmental conditions through their effects on foliage water $\delta$$^{18}$O signatures and photosynthetic $^{18}$O discrimination. The two sites show pronounced differences in magnitude and variability of $^{18}$O discimination. At both sites, the $\delta$$^{18}$O signatures of evaporating site foliage water played an important role in determining photosynthetic $^{18}$O discrimination. The extent of the $^{18}$O enrichment of evaporating site water, in turn, depended largely on foliage transpiration rates and their response to changes in environmental conditions. For example, the non steady state effects of transpiration limited foliage water turnover on evaporating site $\delta$$^{18}$O signatures were more apparent at the spruce site with smaller transpiration rates compared to the beech site. This resulted in 2 - 3 permil higher $^{18}$O discrimination on average for the spruce, but 1 - 2 permil lower for the beech branches. The results of our field measurements combined with a simple model of well mixed canopy air emphasize the importance of foliage transpiration in determining the $\delta$$^{18}$O signatures of water and CO$_{2}$ exchanged between ecosystems and the atmosphere.
DE: 1818 Evapotranspiration
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting