U.S. Department of Energy Office of Biological and Environmental Research

PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Evaluating the Community Land Model (CLM4.5) at a coniferous forest site in northwestern United States using flux and carbon-isotope measurements

James Ehleringer
University of Utah

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9 October 2017

Model ability to simulate the observed energy and carbon fluxes, carbon stocks, and ecosystem response to water stress is investigated.

The Science  
Droughts in the western United States are expected to intensify with climate change. Thus, an adequate representation of ecosystem response to water stress in land models is critical for predicting carbon dynamics. The study's goal was to evaluate the performance of CLM4.5 against observations at an old-growth coniferous forest site in the Pacific Northwest region of the United States (Wind River AmeriFlux site), characterized by a Mediterranean climate that subjects trees to water stress each summer.

The Impact
CLM4.5 was able to reasonably simulate the observations at Wind River after significant calibration of parameters. While most of the adjustments were site-specific, the adjustment of the slope of the leaf stomatal conductance equation aligned with results from other studies at different coniferous forest sites, suggesting that CLM4.5 could benefit from a revised default value. The results also demonstrate that carbon isotopes can expose structural weaknesses in CLM4.5 and provide a key constraint that may guide future model development.

Summary
DOE-supported scientists evaluated CLM4.5 against observations at an old-growth coniferous forest site that is subjected to water stress each summer. They found that, after calibration, CLM4.5 was able to reasonably simulate the observed fluxes of energy and carbon, carbon stocks, carbon isotope ratios, and ecosystem response to water stress (response of canopy conductance to atmospheric vapor pressure deficit and soil water content). The calibration of the slope parameter in the Ball-Berry leaf stomatal conductance model aligned with other studies, suggesting that CLM4.5 could benefit from a revised value of 6, rather than the default value of 9, for needleleaf evergreen temperate forests. This study demonstrates that carbon isotope data can be used to constrain stomatal conductance and intrinsic water use efficiency in CLM4.5, as an alternative to eddy covariance flux measurements. It also demonstrates that carbon isotopes can expose structural weaknesses in the model and provide a key constraint that may guide future model development.

Contacts (BER PM)
Daniel Stover
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289)

Dorothy Koch
SC-23.1
Dorothy.koch@science.doe.gov (301-903-0105)


(PI Contact)
James Ehleringer
University of Utah, Department of Biology
jim.ehleringer@utah.edu (801-581-7623)

Funding
This research was supported by the US Department of Energy’s Office of Science, Terrestrial Ecosystem Science Program, under award number DE-SC0010624. BMR and DRB were supported by the US Department of Energy’s Office of Science, Terrestrial Ecosystem Science Program, under award number DE-SC0010625. PET was supported by the US Department of Energy’s Office of Science, Biological and Environmental Research, Accelerated Climate Modeling for Energy project. We acknowledge the Wind River Field Station AmeriFlux site (US-Wrc, PIs: Kenneth Bible, Sonia Wharton) for its data records. Funding for AmeriFlux data resources was provided by the US Department of Energy’s Office of Science. Data and logistical support were also provided by the US Forest Service Pacific Northwest Research Station.

Publications
Duarte, H. F., Raczka, B. M., Ricciuto, D. M., Lin, J. C., Koven, C. D., Thornton, P. E., Bowling, D. R., Lai, C.-T., Bible, K. J. & Ehleringer, J. R. Evaluating the Community Land Model (CLM4.5) at a coniferous forest site in northwestern United States using flux and carbon-isotope measurements. Biogeosciences 14, 4315-4340 (2017). https://doi.org/10.5194/bg-14-4315-2017

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