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Harvard Forest Symposium Abstract 2008

  • Title: Multi-year measurements of carbon isotope ratios of respired CO2 suggest a transient pattern of 13C discrimination in US coniferous forests
  • Primary Author: ChunTa Lai (San Diego State University)
  • Additional Authors: Jim Ehleringer (University of Utah)
  • Abstract:

    Chun-Ta Lai1* and James Ehleringer2

    *Author of correspondence: Tel. 619-594-0678, Email:

    1Department of Biology, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182

    2Department of Biology, University of Utah, 257S, 1400E, Salt Lake City, UT 84112

    We present spatial patterns of the carbon isotope ratio measurements of atmospheric samples collected from a number of AmeriFlux sites that spanned the major biomes in the contiguous U.S.A. Measurements of carbon isotope ratios of ecosystem respiration (d13CR) indicated short-term changes in photosynthetic pathway and in the stomatal control of C3 gas-exchange activity of plants at the ecosystem scale. Here we present the strongest evidence yet available showing a transient signature in 13C discrimination in USA coniferous forests using multi-year d13CR observations. More negative d13CR values occurred in spring and fall that bracketed the most positive values observed in mid-summer. An average intra-seasonal variation of 2.3(0.5)‰ in the d13CR values was observed at the Wind River Canopy Crane in southern Washington over the period of 2001 - 2007. This seasonal pattern of d13CR was consistently observed in three coniferous forests in the Pacific Northwest (Wind River Canopy Crane, Mary-Fir, and Metolius). Stomatal response to summer drought common for this region is likely to have been the major control over these d13CR gas-exchange changes. A similar seasonal pattern was also observed in an eastern USA coniferous forest (Howland Forest). On the contrary, we observed no apparent seasonal differences in the d13CR value at Harvard Forest, a mixed deciduous forest. Three possible mechanisms perhaps contributed to the lack of a detectable d13CR variation in this forest: 1) a short duration of any strong seasonal drought stress, 2) differences in the timing of individual response to environmental stress, or 3) differences associated with xylem architecture and its apparent impacts on plant hydraulics and gas exchange. In mixed deciduous forests, ring-porous and diffuse-porous species compete for resources and are likely to have traits that act to offset one another. On a seasonal basis, the relationships between d13CR variations and environmental conditions are in general consistent with leaf-level physiological response, regardless of ecosystem types. Differences in the strength of the relationship were noted among functional groups.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
  • HV_HL_WR_d13Cr_season_1.pdf