Challenges and advances in intra-annual tree-ring stable isotope research, a systematic review

The intra-annual stable isotope approach emerged in the 1970s as a cornerstone in dendrochronology temporal resolution. Despite the recent progress, it still grapples with methodological and interpretation hurdles that limit its potential. Aiming at stepping forward and envisaging the systematization of the intra-annual approach, we systematically reviewed the literature using a search expression on SCOPUS that resulted in 132 eligible studies. From each study, we gathered data on sampling sites, stable isotopes, studies’ goals, sampling methods, and results interpretation. This review points to geographical biases reflecting the early dendrochronological development in temperate sites, disbelief in tropical tree rings, and the eventual limited access to high-end methods in developing countries. Although methods like laser microdissection and ablation-combustion open new research avenues, cheap razor blades are the primary sampling tool. If sampling is not the bottleneck, the number of analyses is because of the trade-off between the number of intra-annual samples and the chronology length observed in the studies. Dendroecological studies rely on dozens of intra-annual samples obtained across short tree-ring series, whereas dendroclimatological studies assess a few segments, often earlywood and latewood, over centuries-long series. We also identified two main approaches in the analyses of the intra-annual data, either the studies obtained intra-ring samples and analyzed them on an interannual basis using long-established dendrochronological methods to address common question in dendroclimatology and dendroecology, or they analyzed the seasonal variability of the intra-annual stable isotopes using novel statistical approaches addressing new research questions. These questions range from the potential to reconstruct short-term extreme climate events to a detailed evaluation of the responses of trees to environmental stress. Based on the analyses of these studies, we bring five propositions for methodological advances and discuss future research avenues. These prospects and propositions are a starting point for systmatizing the intra-annual stable isotope approach and fostering research.