Fig. 4

Black spruce ecophysiological response to rising c_i based on five-year resolution δ¹³C and δ¹⁸O analyses for the period 1919–2018. a Tree-ring δ¹³C and δ¹³C-derived ecophysiological parameter values (c_i, c_i/c_a, iWUE); b tree-ring δ¹⁸O values. Results from CAS0, CAS50, and CAS100 are shown in black, red, and blue, respectively. Boxplots show the median, quartile range, extreme values (dotted lines), and outliers (dots) of the distribution. Different letters above the boxes indicate significant differences between the sites based on Tukey’s HSD test (the absence of letters means no significant difference between the three sites). The grey dotted line represents a significant breakpoint (P <  0.01) in slopes based on Davies test