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 Community Land Model (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

U.S. Department of Energy (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 (i.e., 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.

Principal Investigator

James Ehleringer
University of Utah
jim.ehleringer@utah.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

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

References