Fig. 2.

c_i/c_a variation during the last century. c_i/c_a variation for 15-year periods at the beginning (1917–1931) and the end (2004–2018) of the last century (a) and its relationship with δ¹⁸O_cellulose variation (b). c_i/c_a, the ratio of substomatal to atmospheric CO₂ concentration, was calculated from the C isotope composition of hay samples using the model of Ma et al. [28] (the “Methods” section). Values represent the yearly averages of samples during the two periods of analysis (1917–1931, filled symbols; 2004–2018, empty symbols). The lines in b (1917–1931, continuous line; 2004–2018, dashed line) and the shaded areas indicate the regression line for the relationship ± CI95%