Reconstruction of summer droughts using tree-ring cellulose isotopes: a calibration study with living oaks from Brittany (western France)

The aim of this study is to establish a calibration of the late wood cellulose carbon and oxygen isotopic inter-annual variability measured on four living oaks (1879–1998) in the Atlantic area (Rennes Forest, Brittany, western France) to meteorological (beginning in 1885) and hydrological (beginning in 1951) data. We find a better tree-to-tree consistency of the δ¹⁸O ratio, compared with that of the tree-to-tree variability of the ring width and the δ¹³C possibly affected by individual competition effects.

On a century-long time scale, the δ¹³C ratio in the cellulose reflects the globally decreasing trend of δ¹³C in atmospheric CO₂, which is mainly due to fossil fuel burning. In contrast with the ring width, which here shows a weak and complex dependence on meteorological parameters, the isotopic composition of the cellulose enables a reliable reconstruction (R² > 0.45), mainly due to the δ¹⁸O signal, of selected summer climatic parameters: relative humidity, soil moisture deficit and temperature. The reconstructed parameters capture both low-frequency variations and extreme dry years (summer droughts). While both summer temperature and annual mean precipitation have a long-term increasing trend, the reconstructed water stress indicators do not show a significant trend during the 20th century. On average one summer drought occurs every seven summers, but this frequency varies in parallel to decadal changes in mean summer temperature, with fewer droughts in the 1930s and 1960s–1970s and more droughts in the 1900s, 1940s and 1990s.