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Harvard Forest Symposium Abstract 2009

  • Title: Impacts of physiological response and species composition on ecosystem metabolism
  • Primary Author: ChunTa Lai (San Diego State University)
  • 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

    Stable isotope analyses provided insights into physiological controls of ecosystem metabolism. Here we present measurement updates of carbon-13 ratios of ecosystem respiration (d13CR) observed in Harvard Forest, as a part of a national effort to monitor changes in ecosystem metabolism under the influence of increasing atmospheric CO2 levels. In a synthesis study, these d13C measurements were compared to those observed in western and eastern coniferous forests, a tallgrass prairie in the Great Plains, an alpine forest in the Rocky Mountains and a savanna ecosystem in central Texas. Stands with a relatively uniform lifeform show a predictable seasonal pattern of d13CR that is consistent with known leaf physiological response to environmental stresses, such as drought and coldness. A pronounced summer peak of d13CR was consistently observed over a range of coniferous forests that differ in ages and their geographic locations. The major controls of the seasonal d13CR variation in coastal coniferous forests include soil moisture availability, soil temperature and to a less degree, vapor pressure deficit. A curvilinear, negative relationship exits between d13CR and soil moisture contents in the two coniferous forests (Wind River and Metolius). This relationship results from plants’ strong stomatal regulation under summertime water stress conditions in this region. Older, bigger trees that grow in wetter conditions appear to have a more modest response than younger, smaller trees that grow in more arid environment.

    The expected physiological response is weakened in mixed hardwood forests, such as the Harvard Forest. Moderate seasonal changes in the d13CR value were found and can be explained by three possible mechanisms: 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. We did not find any correlations between d13CR values and precipitation or vapor pressure deficit in Harvard Forests. A less conclusive, yet negative relationship between d13CR and water availability was found in the mixed hardwood (Harvard) forest. We suggest that more drought-resilient species may offset some of the canopy stomatal closure under dry conditions.

    When considering physiological response and its isotopic effects in global 13C inversion analyses, we suggest that ecosystems that consist of species with a wide range of ecological traits are likely to have species effects that offset one another and will require additional parameterization than coniferous forests of relatively uniform lifeforms.

  • Research Category: Forest-Atmosphere Exchange