Holocene-long climate signals in tree-ring stable isotopes from the European Alps

It has recently been argued that tree-ring stable isotopes (TRSI) can reveal persistent long-term hydroclimate trends that are usually not captured by more traditional dendroclimatic studies using tree-ring width or density (Büntgen 2022). Since the putative long-term discrepancy between ‘growth-dependent’ ring width and density versus ‘growth-independent’ TRSI proxies is likely unrelated to biases from age-trend removal (Büntgen et al. 2021; Yang et al. 2021), we propose a re-evaluation of the predictive power of various tree-ring parameters for reconstructing climate variability at interannual to multimillennial timescales. We analyse 7800 high-resolution δ¹⁸O, δ¹³C, δD measurements from about 200 high-elevation conifers of the Alpine Holocene Triple Tree Ring Isotope Record (AHTTRIR; Arosio et al. 2022) to assess ultra-long climate trends well beyond the segment length of individual tree-ring samples. Despite the spatiotemporal data heterogeneity, and associated signal complexity of AHTTRIR, we show that δ¹⁸O values contain a reasonable level of coherency with summer hydroclimate variability. In line with two independent TRSI studies from central Europe and monsoon Asia (Büntgen et al. 2021; Yang et al. 2021), our new δ¹⁸O Alpine chronology reveals a significant long-term drying trend over the past 6000 years. We interpret this multimillennial hydroclimate trajectory as a response to long-term negative orbital forcing (i.e., insolation changes due to the Earth’s axial precession). Our findings advise caution when applying corrections to TRSI data in order to preserve Holocene long trends. Considering the unique paleoclimatic values of TRSI, more such records are needed from a wide range of species and regions in both hemispheres.

 

Arosio Tito, Malin Ziehmer, Kurt Nicolussi, Christian Schluechter, Andrea Thurner, Andreas Österreicher, Peter Nyfeler, and Markus Christian Leuenberger,. 2022. “Alpine Holocene Triple Tree Ring Isotope Record.” PANGAEA, 2022. https://doi.pangaea.de/10.1594/PANGAEA.941604.

Büntgen Ulf. 2022. “Scrutinizing Tree-Ring Parameters for Holocene Climate Reconstructions.” Wiley Interdisciplinary Reviews: Climate Change, e778.

Yang Bao, Chun Qin, Achim Bräuning, Timothy J. Osborn, Valerie Trouet, Fredrik Charpentier Ljungqvist, Jan Esper, Lea Schneider, Jussi Grießinger, and Ulf Büntgen. 2021. “Long-Term Decrease in Asian Monsoon Rainfall and Abrupt Climate Change Events over the Past 6,700 Years.” Proceedings of the National Academy of Sciences 118 (30): e2102007118.