Summer droughts in the western United States are expected to intensify with climate change. Thus, an adequate representation of ecosystem drought response in land models is critical for predicting carbon dynamics. The goal of this study was to assess the performance of the Community Land Model, Version 4.5 (CLM) for an old-growth coniferous forest in the Pacific Northwest region of the United States (Wind River AmeriFlux site), characterized by a climate that has heavy winter precipitation followed by summer drought. Particular attention was given to the model skill in the simulation of stomatal conductance and its response to drought stress. CLM was driven by site-observed meteorology and calibrated primarily using parameter values observed at the site or at similar stands in the region. Key model adjustments included parameters controlling specific leaf area and stomatal conductance. Default values of these parameters led to significant underestimation of gross primary production, overestimation of evapotranspiration, and consequently overestimation of photosynthetic 13 C discrimination, reflected on reduced 13 C: 12 C ratios of carbon fluxes and pools. Adjustments in soil hydraulic parameters within CLM were also critical, preventing significant underestimation of soil water content and unrealistic drought stress during summer. After calibration, CLM was able to simulate energy and carbon fluxes, leaf area index, biomass stocks, and carbon isotope ratios of carbon fluxes and pools in reasonable agreement with site observations. Overall, the calibrated CLM was able to simulate the observed response of canopy conductance to atmospheric vapor pressure deficit and soil water content, reasonably capturing the impact of drought stress on ecosystem functioning. The calibrated parameters may be of use for future modeling studies involving stands of similar age and composition under a similar climate regime. More broadly, the calibration of the Ball-Berry stomatal conductance model in CLM aligned with observations reported in the literature for coniferous trees, suggesting that a future release of CLM would benefit from using a distinct, lower slope value ( m bb =6) for conifers, rather than a unique value for all C3 plants ( m bb =9). Thanks to the recent implementation of photosynthetic 13C discrimination within CLM, the results of this study indicate that carbon isotope measurements can be used to constrain stomatal conductance and water use efficiency in CLM as an alternative to flux observations. They also have the potential to guide structural improvements in the model in respect to the representation of carbon storage pools.