Abstract
Abstract. Climatic reconstructions based on tree-ring isotopic series convey substantial information about past conditions prevailing in forested regions of the globe. However, in some cases, the relations between isotopic and climatic records appear unstable over time, generating the “isotopic divergences”. Former reviews have thoroughly discussed the divergence concept for tree-ring physical properties but not for isotopes. Here we want to take stock of the isotopic divergence problem, express concerns and stimulate collaborative work for improving paleoclimatic reconstructions. There are five main causes for divergent parts in isotopic and climatic series: (1) artefacts due to sampling and data treatment, relevant for dealing with long series using sub-fossil stems; (2) stand dynamics, including juvenile effects mostly occurring in the early part of tree-ring series; (3) rise in atmospheric pCO2, which can directly influence the foliar behaviour; (4) change in climate, which may modify the isotope–climate causal links; and finally (5) atmospheric pollution, which may alter leaf and root functions. Future paleoclimate research would benefit from interdisciplinary efforts designed to develop further process-based models integrating multi-proxy inputs so as to help identify causes of isotopic divergences and circumvent some of them in inverse applications.
Highlights
Tree-ring isotopes can serve as proxies for climatic parameters for reconstructing past climate variability, which is useful for understanding regional and global climatic patterns (Treydte et al, 2007; Braconnot et al, 2012; Naulier et al, 2015a)
Many studies dealing with a variety of species and sites reported long increasing trends in the first 20–50 years of cellulose δ13C series (Bert et al, 1997; Duquesnay et al, 1998; Arneth et al, 2002; Li et al, 2005; Gagen, 2008; Labuhn et al, 2014), but no effect was detected in larches growing in open canopies (Daux et al, 2011; Kilroy et al, 2016)
The weakening of the early wood δ18O response to climate of Abies forrestii from southwestern China may be attributable to changes in atmospheric circulation patterns linked to Pacific sea surface temperatures (An et al, 2019)
Summary
Several post-photosynthetic processes can modify the final δ13C values fixed in tree rings through C exchange with stored carbohydrates (Gessler et al, 2014). Any direct or indirect climatic factors operating on the photosynthetic or respiratory functions of trees modify the A, g, A/g ratio or O fractionation factors (Eqs. 1, 2), and act upon the tree-ring δ13C and δ18O values. The sensitivity of these proxies to changes in temperature, precipitation, RH and light (cloudiness, radiation) vary with the species of trees, site conditions and regional climate.
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