Triple-Isotope Evidence for Distinct Physiological Strategies of Larch and Cembra Pine Across the Holocene

Abstract Stable isotopes of carbon, oxygen and hydrogen in tree rings provide a record of plant physiological processes and environmental variability. Although an increasing number of studies now apply triple-isotope approaches, no investigation has yet tested their temporal stability over millennial timescales or assessed the relative impacts of physiology versus climate on long-term isotopic signals. Here, we used 9000 years of multi-isotope records from co-occurring deciduous larch (Larix decidua) and evergreen cembra pine (Pinus cembra) at the Alpine treeline. We found a high interspecies coherence for δ18O throughout the Holocene with a robust summer hydroclimate sensitivity, confirming its dominance by environmental drivers. In contrast, δ13C and δ2H show weaker and less stable coherence, reflecting species-specific physiology. Larch exhibits tight δ2H–δ18O and δ2H–δ13C correlations and stronger climate sensitivity, consistent with its reliance on freshly assimilated carbon. Pine, by contrast, shows weaker δ2H–climate relationships and frequent decoupling from δ13C and δ18O, reflecting potential storage use and metabolic fractionations. Thus, inter-isotope relationships reveal that δ18O is a robust long-term climate proxy, while δ13C and δ2H encode contrasting carbon-use strategies and metabolic processes across species that may vary over time. Together, these findings demonstrate that multi-isotope, multi-species approaches not only strengthen climate reconstructions but also provide a physiological dimension to long-term isotope records.