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.