Global decadal variability of plant carbon isotope discrimination and its link to gross primary production

Carbon isotope discrimination (Delta C-13) in C-3 woody plants is a key variable for the study of photosynthesis. Yet how Delta C-13 varies at decadal scales, and across regions, and how it is related to gross primary production (GPP), are still incompletely understood. Here we address these questions by implementing a new Delta C-13 modelling capability in the land-surface model JULES incorporating both photorespiratory and mesophyll-conductance fractionations. We test the ability of four leaf-internal CO2 concentration models embedded in JULES to reproduce leaf and tree-ring (TR) carbon isotopic data. We show that all the tested models tend to overestimate average Delta C-13 values, and to underestimate interannual variability in Delta C-13. This is likely because they ignore the effects of soil water stress on stomatal behavior. Variations in post-photosynthetic isotopic fractionations across species, sites and years, may also partly explain the discrepancies between predicted and TR-derived Delta C-13 values. Nonetheless, the "least-cost" (Prentice) model shows the lowest biases with the isotopic measurements, and lead to improved predictions of canopy-level carbon and water fluxes. Overall, modelled Delta C-13 trends vary strongly between regions during the recent (1979-2016) historical period but stay nearly constant when averaged over the globe. Photorespiratory and mesophyll effects modulate the simulated global Delta C-13 trend by 0.0015 +/- 0.005 parts per thousand and -0.0006 +/- 0.001 parts per thousand ppm(-1), respectively. These predictions contrast with previous findings based on atmospheric carbon isotope measurements. Predicted Delta C-13 and GPP tend to be negatively correlated in wet-humid and cold regions, and in tropical African forests, but positively related elsewhere. The negative correlation between Delta C-13 and GPP is partly due to the strong dominant influences of temperature on GPP and vapor pressure deficit on Delta C-13 in those forests. Our results demonstrate that the combined analysis of Delta C-13 and GPP can help understand the drivers of photosynthesis changes in different climatic regions.