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Harvard Forest Symposium Abstract 2007

  • Title: Environmental controls on the carbon isotope composition of ecosystem respired CO2 in contrasting forest ecosystems in USA and Canada
  • Primary Author: ChunTa Lai (San Diego State University)
  • Additional Authors: Jim Ehleringer (University of Utah)
  • Abstract:

    Flask air samples from the Harvard Forest contribute to a synthesis study that compares the carbon isotope composition of respired CO2 (13CR) from eleven forest ecosystems in USA and Canada. We examine differences among forests in their response to seasonal variation in photosynthetic photon flux density (PPFD), soil and air temperature, vapor pressure deficit (VPD), precipitation, and soil moisture during May-October 2004. We hypothesized that variations in 13CR could be used as a proxy for short-term changes in photosynthetic discrimination and associated shifts in integrated ecosystem-level intercellular to ambient CO2 ratio, ci/ca. Our analyses included comparisons of 13CR responses for three functional groups: deciduous, boreal, and coastal forests. The 13CR values were well predicted for each group based on the six environmental variables considered, where the highest R2 values determined for the coastal, deciduous, and boreal groups were 0.81, 0.80, and 0.56, respectively. Consistent with several previous studies, the highest correlations between 13CR and shifts in environmental conditions were achieved when the environmental variables were averaged for 2-, 3- or 4-days previous to 13CR sample collection. The relationships we observed between 13CR and environmental factors were consistent with known leaf-level responses, and this was especially apparent within each functional group. However, there were differences among the functional groups for the strength and/or significance of the relationships between 13CR and particular environmental factors. For example, while VPD and soil temperature were significant determinants of variation in 13CR in the boreal group, PPFD was not a significant factor. By contrast, in the coastal group, variation in 13CR was very strongly correlated with changes in PPFD, and there was no significant relationship with VPD. At a single site comparisons made between years suggested the potential application of 13CR values to assess year-to-year variation in ecosystem physiological responses to changing environmental conditions, but also showed that all environmental factors influencing carbon isotope discrimination during photosynthetic gas exchange need to be considered in such an analysis.





    Reference:

    Alstad, KP, C.-T. Lai, L.B. Flanagan, J.R. Ehleringer, Environmental controls on the carbon isotope composition of ecosystem respired CO2 in contrasting forest ecosystems in Canada and USA, Tree Physiology, in press.

  • Research Category: Forest-Atmosphere Exchange