Spatio‐Temporal Variations in Carbon Isotope Discrimination Predicted by the JULES Land Surface Model

Stable carbon isotopes in plants can help evaluate and improve the representation of carbon and water cycles in land‐surface models, increasing confidence in projections of vegetation response to climate change. Here, we evaluated the predictive skills of the Joint UK Land Environmental Simulator (JULES) to capture spatio‐temporal variations in carbon isotope discrimination (Δ(13)C) reconstructed by tree rings at 12 sites in the United Kingdom over the period 1979–2016. Modeled and measured Δ(13)C time series were compared at each site and their relationships with local climate investigated. Modeled Δ(13)C time series were significantly correlated (p < 0.05) with tree‐ring Δ(13)C at eight sites, but JULES underestimated mean Δ(13)C values at all sites, by up to 2.6‰. Differences in mean Δ(13)C may result from post‐photosynthetic isotopic fractionations that were not considered in JULES. Inter‐annual variability in Δ(13)C was also underestimated by JULES at all sites. While modeled Δ(13)C typically increased over time across the UK, tree‐ring Δ(13)C values increased only at five sites located in the northern regions but decreased at the southern‐most sites. Considering all sites together, JULES captured the overall influence of environmental drivers on Δ(13)C but failed to capture the direction of change in Δ(13)C caused by air temperature, atmospheric CO(2) and vapor pressure deficit at some sites. Results indicate that the representation of carbon‐water coupling in JULES could be improved to reproduce both the trend and magnitude of interannual variability in isotopic records, the influence of local climate on Δ(13)C, and to reduce uncertainties in predicting vegetation‐environment interactions.