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Land surface models are useful tools to quantify contemporary and futureclimate impact on terrestrial carbon cycle processes, provided they can beappropriately constrained and tested with observations. Stable carbonisotopes of CO2 offer the potential to improve model representation ofthe coupled carbon and water cycles because they are strongly influenced bystomatal function. Recently, a representation of stable carbon isotopediscrimination was incorporated into the Community Land Model component ofthe Community Earth System Model. Here, we tested the model's capability tosimulate whole-forest isotope discrimination in a subalpine conifer forest atNiwot Ridge, Colorado, USA. We distinguished between isotopic behavior inresponse to a decrease of δ13C within atmospheric CO2 (Suesseffect) vs. photosynthetic discrimination (Δcanopy), bycreating a site-customized atmospheric CO2 and δ13C ofCO2 time series. We implemented a seasonally varying Vcmaxmodel calibration that best matched site observations of net CO2 carbonexchange, latent heat exchange, and biomass. The model accurately simulatedobserved δ13C of needle and stem tissue, but underestimated theδ13C of bulk soil carbon by 1–2 ‰. The model overestimatedthe multiyear (2006–2012) average Δcanopy relative to priordata-based estimates by 2–4 ‰. The amplitude of the averageseasonal cycle of Δcanopy (i.e., higher in spring/fall ascompared to summer) was correctly modeled but only when using a revised,fully coupled An − gs (net assimilation rate, stomatalconductance) version of the model in contrast to the partially coupledAn − gs version used in the default model. The modelattributed most of the seasonal variation in discrimination to An,whereas interannual variation in simulated Δcanopy during thesummer months was driven by stomatal response to vapor pressure deficit(VPD). The model simulated a 10 % increase in both photosyntheticdiscrimination and water-use efficiency (WUE) since 1850 which is counter toestablished relationships between discrimination and WUE. The isotopeobservations used here to constrain CLM suggest (1) the model overestimatedstomatal conductance and (2) the default CLM approach to representingnitrogen limitation (partially coupled model) was not capable of reproducingobserved trends in discrimination. These findings demonstrate that isotopeobservations can provide important information related to stomatal functiondriven by environmental stress from VPD and nitrogen limitation. Futureversions of CLM that incorporate carbon isotope discrimination are likely tobenefit from explicit inclusion of mesophyll conductance

biogeosciences

An observational constraint on stomatal function in forests: evaluating coupled carbon and water vapor exchange with carbon isotopes in the Community Land Model (CLM4.5)