HR: 0830h
AN: B31E-0347    [PDF]
TI: A Non-steady-state Analytical Model to Compute the Stable Isotope Enrichment of Leaf Water at the Evaporative Sites Under Field Conditions
AU: * Ogee, J
EM: ogee@lsce.saclay.cea.fr
AF: LSCE, CEA-orme des merisiers, Gif sur Yvette, 91191 France
AU: * Ogee, J
EM: ogee@lsce.saclay.cea.fr
AF: Bioclimatologie, Inra-Bordeaux, Villenave d'ornon, 33883 France
AU: Bariac, T
EM: thierry.bariac@inra.fr
AF: BiomCo, Inra-grignon, Thiverval Grignon, 78850 France
AU: Peylin, P
EM: peylin@lsce.saclay.cea.fr
AF: BiomCo, Inra-grignon, Thiverval Grignon, 78850 France
AU: Cuntz, M
EM: cuntz@lsce.saclay.cea.fr
AF: LSCE, CEA-orme des merisiers, Gif sur Yvette, 91191 France
AB: The $^{18}$O/$^{16}$O ratio of leaf water is a useful signal, notably because it affects the oxygen isotopic composition of atmospheric CO$_{2}$ which thereby gives rise to the possibility of assessing terrestrial gross primary productivity at different spatial scales. It is now established that this ratio is governed by two competing effects which are the isotopic enrichment of leaf water during transpiration and the back-diffusion of the heavy water from the sites of evaporation into the xylem. In most studies, measurements are made in the laboratory with controlled environment and saturating light, so that leaf transpiration is at its maximum and steady-state conditions are reached. However, in a natural and varying environment, where leaves are not light-saturated, we expect non-steady-state effects to be strong. Indeed 2-3 h are usually needed for the steady-state to be reached while environment variables are known to change significantly over a time step of only 30 min. For this reason we developed a non-steady-state model that allows to compute the $^{18}$O/$^{16}$O ratio of leaf water between the xylem and the evaporative sites in a varying and not light-saturated environment. Under realistic field conditions the deviation from the steady-state values can be as great as 10-15\permil at the evaporative sites after 1 h. This demonstrates the importance of considering the non steady-state when studying the $^{18}$O/$^{16}$O ratio signal in the natural environment. When the initial conditions for the $^{18}$O/$^{16}$O ratio are assumed to vary exponentially from the leaf xylem to the evaporative sites, the model has an analytical solution which makes its use possible also for larger scale studies.
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 0614 Biological effects
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting