Contribution of oak tree-ring width and stable isotopes to reconstruct hydroclimate variability in central France over the last millennium
Résumé
To better understand current global warming, it is crucial to document natural climate variability during the pre-industrial period. This helps to characterize the patterns of variability and identify the forcings driving it, assess the accuracy of the variability simulated by climate models, and determine the role of anthropogenic effects (IPCC 2021). In Europe, tree-ring width (TRW) and maximum latewood density (MXD) series have been widely used to reconstruct temperature variations over the past millennium, particularly in high-altitude or high-latitude regions (e.g. Briffa et al 1992; Corona et al 2010). At lower altitudes, tree growth is more controlled by drought and precipitation, making these records valuable for characterizing hydroclimate variability (e.g. Büntgen et al 2011, Cook et al 2016, Cooper et al 2013). However, these tree-ring series are affected by age-related effects and stand dynamics trends, requiring standardization procedures that can impact the low-frequency component of the signal (see Helama et al 2017, for a review). The dendroisotopic approach, which is based on the determination of the oxygen and carbon isotopic composition (δ18O and δ13C) in tree-ring cellulose, partly overcomes the issues associated with conventional tree-ring measurements. These isotopic proxies are highly sensitive to water stress, yet they are less affected, or even unaffected, by age-related growth effects. However, although numerous multi-century reconstructions exist across Europe (e.g. Büntgen et al 2021, Treydte et al 2024) and specifically in France (Etien et al 2008, Labuhn et al 2016), there are no millennial reconstructions based on stable isotopes or tree-ring widths at low altitudes in France. This is notable given that these areas have long been inhabited by human societies for centuries. Our goal is to deepen the understanding of past climate variability at the regional scale, where changes are more pronounced, by producing the first millennial scale tree-ring width chronology for central France. In parallel, we aim to construct a cellulose δ18O chronology covering the period 1200-1400 CE, which will help define the climatic transition between the relative warming of the Medieval Climate Anomaly (MCA; ∼950-1250 CE) and the cooling of the Little Ice Age (LIA; 1350-1850 CE).
We present here the methodological framework of this work along with the initial findings on growth-climate relationships during the instrumental period. The climate sensitivity of the 3 proxies (TRW, δ18O and δ13C) has been evaluated. Notably, the strong correlation between the June-July SPEI and cellulose δ18O (r = -0.75), combined with model verification, indicates that δ18O is the most reliable proxy for reconstructing past hydroclimatic variability (Fig. 3). In contrast, TRW seems to be controlled more by the March-July SPEI and precipitation (r = 0.61). The observed correlations between the proxies and instrumental data indicate the potential to reconstruct past hydroclimatic variability. Dry and wet periods can be reconstructed over the last millennium from TRW using the vast corpus of archaeological wood studied, as well as from cellulose δ18O during the period 1200-1400 CE. This will provide new data on both climatic long-term variability and the MCA-LIA transition at low altitude in France.
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