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 (Delta C-13) reconstructed by tree rings at 12 sites in the United Kingdom over the period 1979-2016. Modeled and measured Delta C-13 time series were compared at each site and their relationships with local climate investigated. Modeled Delta C-13 time series were significantly correlated (p < 0.05) with tree-ring Delta C-13 at eight sites, but JULES underestimated mean Delta C-13 values at all sites, by up to 2.6. Differences in mean Delta C-13 may result from post-photosynthetic isotopic fractionations that were not considered in JULES. Inter-annual variability in Delta C-13 was also underestimated by JULES at all sites. While modeled Delta C-13 typically increased over time across the UK, tree-ring Delta C-13 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 Delta C-13 but failed to capture the direction of change in Delta C-13 caused by air temperature, atmospheric CO2 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 Delta C-13, and to reduce uncertainties in predicting vegetation-environment interactions. Plain Language Summary Carbon has two stable isotopes, C-12 and C-13. During the diffusion of carbon dioxide through the stomata of C-3 plants, the lighter carbon isotope (C-12) diffuses more easily resulting in leaf internal air that is depleted in C-13 compared to the ambient air. Tree rings record this discrimination against C-13 (noted Delta C-13) which is used to decipher information about past climates, including atmospheric CO2 concentration when the rings formed. The Joint UK Land Environment Simulator (JULES) is a land-surface model that predicts environmental processes (e.g., the carbon and water cycles). Comparing JULES estimates of Delta C-13 with tree-ring measurements helps to improve simulations, leading to more accurate projections of ecosystem functioning under different climates. We found that JULES accurately predicted tree-ring Delta C-13 in 8 out of 12 UK sites over a 38-year period, with offsets in mean values of up to 2.6, but underestimated interannual variations in Delta C-13 at all sites. JULES captured the influence of environmental changes on tree-ring Delta C-13 relatively well, although the trees' responses at individual sites varied. Results suggest that improvements are needed in JULES to reduce uncertainties in predicting the carbon and water cycles and therefore to produce more accurate projections of future climate-vegetation interactions.