© 2007 Heron Publishing—Victoria, Canada
Environmental controls on the carbon isotope composition of ecosystem-respired CO2 in contrasting forest ecosystems in Canada and the USA
Karrin P. Alstad (1, 2), Chun-Ta Lai (3, 4), Lawrence B. Flanagan (1, 5) and James R. Ehleringer (3)
1. Department of Biological Sciences, University of Lethbridge, 4401 University Drive, Lethbridge, AB T1K 3M4, Canada / 2. Present address: Department of Environmental Sciences, Bowman-Oddy Laboratories, Mail Stop 604, University of Toledo, Toledo,
OH 43606, USA / 3. Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112-0840, USA / 4. Present address: Department of Biology, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-4614, USA / 5. Corresponding author (larry.flanagan@uleth.ca) / Received October 30, 2006; accepted February 13, 2007; published online July 3, 2007
Summary
We compared the carbon isotope composition of ecosystem-respired CO2 (δ13CR) from 11 forest ecosystems in Canada and the USA and examined differences among forest δ13CR responses to seasonal variations in environmental conditions from May to October 2004. Our experimental approach was based
on the assumption that variation in δ13CR is a good proxy for short-term changes in photosynthetic discrimination and associated shifts in the integrated ecosystem-level
intercellular to ambient CO2 ratio (ci/ca). We compared δ13CR responses for three functional groups: deciduous, boreal and coastal forests. The δ13CR values were well predicted for each group and the highest R2 values determined for the coastal, deciduous and boreal groups were 0.81, 0.80 and 0.56, respectively. Consistent with previous
studies, the highest correlations between δ13CR and changes in environmental conditions were achieved when the environmental variables were averaged for 2, 3 or 4 days before
δ13CR sample collection. The relationships between δ13CR and environmental conditions were consistent with leaf-level responses, and were most apparent within functional groups,
providing support for our approach. However, there were differences among groups in the strength or significance, or both,
of the relationships between δ13CR and some environmental factors. For example, vapor pressure deficit (VPD) and soil temperature were significant determinants
of variation in δ13CR in the boreal group, whereas photosynthetic photon flux (PPF) was not; however, in the coastal group, variation in δ13CR was strongly correlated with changes in PPF, and there was no significant relationship with VPD. At a single site, comparisons
between our δ13CR measurements in 2004 and published values suggested the potential application of δ13CR measurements to assess year-to-year variation in ecosystem physiological responses to changing environmental conditions,
but showed that, in such an analysis, all environmental factors influencing carbon isotope discrimination during photosynthetic
gas exchange must be considered.
Keywords:
AmeriFlux, boreal forest, Fluxnet-Canada, Keeling plot, stable isotopes.
