Disentangling the abiotic versus biotic controls on C3 plant leaf carbon isotopes: Inferences from a global review

The carbon isotopic composition of C3 plant leaf tissue (813CC3) provides insights into carbon cycling, climate, and vegetation at various spatiotemporal scales. By disentangling the competing influences of climatic and biological factors, modern 813CC3 calibrations can offer quantitative constraints on their applications. In this work, we examined the response of 813CC3 values to rainfall, temperature, and atmospheric CO2 concentration (pCO2) across wide range of ecosystem gradients. Towards this, we prepared a global database of bulk (813CC3Bulk, n = 737) and n-alkane (813CC3-C29, n = 831) 813C values from species-level C3 plants for various plant functional types (PFTs). The 813CC3-Bulk and 813CC3-C29 values refer to carbon isotopic composition of total organic carbon and the C29 homologue of n-alkanes in C3 plant leaves, respectively. The PFTs were classified according to their leaf senescence (deciduous versus evergreen), seed cover (angiosperm versus gymnosperm), and growth forms (tree, shrub, herb, grass, forb, etc.). Though rainfall is commonly believed to be the major driver of the 813CC3 variability, our work demonstrated that, at the species level, it only influences 813CC3-Bulk values of evergreen shrubs (R2 = 0.47, p < 0.05) and grasses (R2 = 0.26, p < 0.05). Across a gradient from 251 to 398 ppm, pCO2-effect is apparent only on the 813CC3-Bulk values of deciduous shrubs (R2 = 0.32, p < 0.05), while 813CC3 values of other plant types are insensitive to low pCO2 conditions. For the first time, we showed that both species-level 813CC3-Bulk and 813CC3-C29 values of many PFTs are affected by annual temperature. Our metaanalysis provided evidences for a weak yet significant temperature effect on the 813CC3 values of deciduous herbs (813CC3-Bulk: R2 = 0.18, p < 0.05; 813CC3-C29: R2 = 0.11, p < 0.05) and evergreen angiosperm trees (813CC3Bulk: R2 = 0.17, p < 0.05; 813CC3-C29: R2 = 0.17, p < 0.05). We also observed that combined increase in annual rainfall and temperature can negatively drive the 813CC3 values of evergreen shrubs (813CC3-Bulk: adjusted R2 = 0.61, p < 0.05; 813CC3-C29: adjusted R2 = 0.25, p < 0.05) and deciduous angiosperm trees (813CC3-Bulk: adjusted R2 = 0.32, p < 0.05; 813CC3-C29: adjusted R2 = 0.38, p < 0.05), and 813CC3-Bulk values of grasses (adjusted R2 = 0.61, p < 0.05). Overall, our results suggest the abiotic stressors (considered in this work) can explain 12-61% and 0-38% of the total variations in the 813CC3-Bulk and 813CC3-C29 values, respectively. We propose that using PFT-specific 813CC3 values-abiotic stressor relationship, instead of a generalized relationship that includes all types of C3 plants, will add more certainty in paleoenvironment reconstructions and isotope-based PFT model preparation.