4.2.3 | Phloem contents
We assumed that the phloem contents represented recent photosynthates. However, the phloem contents have to be used carefully before photosynthesis begins in spring. During this period, the phloem does not contain newly assimilated C (Dubbert, Rascher, & Werner, 2012; Gessler et al., 2004), but rather the mobilized C reserves that trees store in their tissues. This would lead to WUEi values that do not represent current photosynthates (Michelot, Eglin, Dufrêne, Lelarge-Trouverie, & Damesin, 2011). As soon as the environmental conditions improve in spring, stomatal conductance increases, the phloem fills with new photosynthates, and δ13Cp begins to fall. This process may explain why δ13Cp was the highest outside the thermal growing season and decreased when photosynthetic activity recovered. Without correcting the WUEi, the annual GPPiso/SF became too high for spring in a boreal forest (Saurer et al., 2014; Tang et al., 2014). The influence of these high values was reduced by the \(\hat{\alpha}\) correction. It is encouraging that the \(\hat{\alpha}\) correction had almost no effect during most of the summer (Figure 3); this suggests that we have limited the impact of non-photosynthetic periods.