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Droughts in the western United States are expected to intensify with climate\udchange. Thus, an adequate representation of ecosystem response to water\udstress in land models is critical for predicting carbon dynamics. The goal of\udthis study was to evaluate the performance of the Community Land Model (CLM)\udversion 4.5 against observations at an old-growth coniferous forest site in\udthe Pacific Northwest region of the United States (Wind River AmeriFlux\udsite), characterized by a Mediterranean climate that subjects trees to water\udstress each summer. CLM was driven by site-observed meteorology and\udcalibrated primarily using parameter values observed at the site or at\udsimilar stands in the region. Key model adjustments included parameters\udcontrolling specific leaf area and stomatal conductance. Default values of\udthese parameters led to significant underestimation of gross primary\udproduction, overestimation of evapotranspiration, and consequently\udoverestimation of photosynthetic 13C discrimination, reflected in\udreduced 13C : 12C ratios of carbon fluxes and pools. Adjustments\udin soil hydraulic parameters within CLM were also critical, preventing\udsignificant underestimation of soil water content and unrealistic soil\udmoisture stress during summer. After calibration, CLM was able to simulate\udenergy and carbon fluxes, leaf area index, biomass stocks, and carbon isotope\udratios of carbon fluxes and pools in reasonable agreement with site\udobservations. Overall, the calibrated CLM was able to simulate the observed\udresponse of canopy conductance to atmospheric vapor pressure deficit (VPD)\udand soil water content, reasonably capturing the impact of water stress on\udecosystem functioning. Both simulations and observations indicate that\udstomatal response from water stress at Wind River was primarily driven by VPD\udand not soil moisture. The calibration of the Ball–Berry stomatal\udconductance slope (mbb) at Wind River aligned with findings from recent CLM experiments at sites characterized by the same plant functional\udtype (needleleaf evergreen temperate forest), despite significant differences\udin stand composition and age and climatology, suggesting that CLM could\udbenefit from a revised mbb value of 6, rather than the default\udvalue of 9, for this plant functional type. Conversely, Wind River required a\udunique calibration of the hydrology submodel to simulate soil moisture,\udsuggesting that the default hydrology has a more limited applicability. This\udstudy demonstrates that carbon isotope data can be used to constrain stomatal\udconductance and intrinsic water use efficiency in CLM, as an alternative to\udeddy covariance flux measurements. It also demonstrates that carbon isotopes\udcan expose structural weaknesses in the model and provide a key constraint\udthat may guide future model development

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