Soil moisture controls on the dynamics of nonstructural carbohydrate storage in Picea meyeri during the growing season

Nonstructural carbohydrate (NSC) dynamics have been suggested to be an important trait reflecting carbon balance under environmental changes. However, our understanding of NSC storage dynamics, their controls, and their responses to environmental factors remain unclear. In this study, we selected the evergreen conifer Picea meyeri at five altitudes (2040 m, 2260 m, 2440 m, 2600 m, and 2740 m a.s.l.) on Luya Mountain, North-central China. NSC concentrations in the needles, shoots, stems, and roots were measured, and the environmental variables were monitored during the growing seasons in 2018 and 2019. The results showed that the NSCs first decreased slightly and then increased gradually as altitude increased in the needles, shoots, stems, and roots. The NSC concentrations with the highest levels occurring in the needles and the lowest in the stems, whereas the shoots and roots showed intermediate concentrations. Moreover, soil moisture was the major factor influencing the dynamics of NSC storage in P. meyeri. The concentrations of soluble sugars and starch in all four organs increased with decreasing soil moisture, except for needle starch. In particular, the soluble sugars in the needles were significantly negatively correlated with air temperature, and those in the stems had a different response to soil moisture and vapor pressure deficit (VPD). Furthermore, root NSCs reserves may be important for P. meyeri to respond to low soil moisture. These findings provide new insights for understanding tree NSC allocation and the mechanism of stress resistance.