Whole-tree dynamics of non-structural carbohydrate and nitrogen pools across different seasons and in response to girdling in two temperate trees
Despite extensive studies on seasonal dynamics of non-structural carbohydrates (NSC) and nitrogen (N) concentrations, sizes of NSC and N pools among different organs and their relative contributions to tree growth and maintenance are not well understood. We measured the changes of NSC and N concentrations in leaves, branches, stems and all root branch orders at monthly intervals in control and girdled trees of larch (Larix gmelinii) and ash (Fraxinus mandshurica). Biomass of each plant compartment was also determined to calculate the sizes of NSC and N pools.
In the two species, 13-37% of NSC and N pools were mobilized at the beginning of the growing season. Among the mobilized pools, stems and non-absorptive roots (branch order 4-9) acted as the largest NSC sources in larch and ash, respectively, and branches as the largest N source in both species. After stem girdling, 22% and 50% of root NSC storages in larch and ash, respectively, were mobilized to maintain the root activities in the growing season. Tree mortality was observed one year after girdling when root NSC pool was abundant. We conclude that the seasonal dynamics of NSC and N pools in trees were similar among different storage organs, and storage organs holding higher fractions of NSC or N pool did not necessarily mobilize more NSC or N for new tissue growth at the beginning of the growing season. C storage in non-absorptive roots played an important role in maintaining tree survival after the termination of photosynthate flux from aboveground.