3.3 | Isotopic data
The isotopic data from the atmosphere and from the phloem were also used to infer WUEi. We observed strong, but different, patterns of seasonal variation for atmospheric δ13Ca and for phloem contents (δ13Cp). From January to the beginning of February, δ13Ca decreased to a minimum of -9.2‰ (Figure 3-A). Then it increased rapidly, by about 1‰, during the initial weeks of high photosynthesis in late June and early July. The main peaks of δ13Ca occurred during the thermal growing season, when canopy conductance was also the highest. It then stabilised until late September, when it again began to fall (Figure 3-A). In contrast, the phloem data (Figure. 3-B) did not simply track the atmosphere. Instead they showed a steep drop only at the beginning of the thermal growing season. The δ13Cp value depended significantly on the date (p-value < 0.0001, df = 1, F = 53.09, Figure 3-B). It was significantly higher in the F plot (-27.5‰) than in the R plot (-28.0‰) (p-value < 0.0001, df = 1, F = 76.96) as well.