UC Berkeley

Abstract Uncertainty in land model projections remains high and the roles of parametric and structural uncertainty are difficult to disentangle. To compare parametric sensitivity across model structures we present two parameter perturbation ensembles using the Community Land Model (CLM) operating in satellite phenology mode. The ensembles contrast two vegetation modules: (a) the default CLM vegetation module and (b) the Functionally Assembled Terrestrial Ecosystem Simulator (CLM‐FATES). We perturbed over 300 parameters and quantified their effects on biophysical fluxes globally and across biomes. Most parameters have minimal impact on biophysical fluxes, with only a few substantially influencing results. While both models exhibit similar parameter sensitivity for some fluxes, CLM‐FATES shows larger spread in gross primary productivity (GPP), driven by strong sensitivity to carboxylation rate. CLM‐FATES also shows a weaker GPP response to soil hydrology parameters and exhibits higher water use efficiency (WUE). Cross‐model comparisons reveal similar sensitivities for some parameters (e.g., leaf dimension) but divergent responses to others (e.g., stomatal intercept), highlighting underlying structural differences. Differences in WUE and sensitivity to hydrology and stomatal conductance parameters underscore how model structure fundamentally alters parametric sensitivity. The data sets generated from these ensembles can be used to identify influential parameters and guide future calibration efforts. Plain Language Summary In the study of land models (computer models that simulate how the Earth's surface interacts with the atmosphere), a key concern is why different land models give different predictions for the future of global vegetation and ecosystems. This uncertainty comes from many sources, including that associated with input numbers (parametric uncertainty), and that associated with the fundamental design of the model (structural uncertainty). To understand these effects, we compared how parameter sensitivity changed between the Community Land Model (CLM) with its original vegetation configuration, and CLM connected to the Functionally Assembled Terrestrial Ecosystem Simulator (CLM‐FATES), systematically testing over 300 parameters. We found that CLM‐FATES showed a much larger range in carbon uptake, driven by strong sensitivity to maximum rate of photosynthesis. CLM‐FATES also had higher water use efficiency (WUE) and exhibited lower sensitivity to hydrology parameters. This difference in how carbon and water fluxes are simulated highlights the importance of model structure in shaping model behavior and sensitivity to parameters. These findings, along with the extensive set of outputs associated with our simulations, can be used to ask more questions about the two models, how they respond to changing parameters, and how we might improve them in comparison to observations. Key Points We constructed two parameter ensembles of Community Land Model using its default vegetation model and connected to FATES in satellite phenology mode Structural and parametric differences between the two models result in differing parameter sensitivity and impacts on biophysical fluxes The data sets produced in this study can be used for further parameter sensitivity analyses and to guide calibration efforts with both models