Intra-annual tree-ring δ18O and δ13C reveal a trade-off between isotopic source and humidity in moist environments

Tree-ring intra-annual stable isotopes (delta C-13 and delta O-18) are powerful tools for revealing plant ecophysiological responses to climatic extremes. We analyzed interannual and fine-scale intra-annual variability of tree-ring delta C-13 and delta O-18 in Chinese red pine (Pinus massoniana) from southeastern China to explore environmental drivers and potential trade-offs between the main physiological controls. We show that wet season relative humidity (May-October RH) drove interannual variability of delta O-18 and intra-annual variability of tree-ring delta O-18. It also drove intra-annual variability of tree-ring delta C-13, whereas interannual variability was mainly controlled by February-May temperature and September-October RH. Furthermore, intra-annual tree-ring delta O-18 variability was larger during wet years compared with dry years, whereas delta C-13 variability was lower during wet years compared with dry years. As a result of these differences in intra-annual variability amplitude, process-based models (we used the Roden model for delta O-18 and the Farquhar model for delta C-13) captured the intra-annual delta O-18 pattern better in wet years compared with dry years, whereas intra-annual delta C-13 pattern was better simulated in dry years compared with wet years. This result suggests a potential asymmetric bias in process-based models in capturing the interplay of the different mechanistic processes (i.e., isotopic source and leaf-level enrichment) operating in dry versus wet years. We therefore propose an intra-annual conceptual model considering a dynamic trade-off between the isotopic source and leaf-level enrichment in different tree-ring parts to understand how climate and ecophysiological processes drive intra-annual tree-ring stable isotopic variability under humid climate conditions.

Automated summary

This study reveals the effects of seasonal humidity and temperature on the stable isotopic variability of tree rings. The intra-annual delta O-18 variability is greater in wet years compared to dry years, while delta C-13 variability is lower. Process-based models show different capabilities in simulating the intra-annual stable isotopic variability in wet and dry years.