Seasonal non-structural carbohydrate dynamics differ between twig bark and xylem tissues

KEY MESSAGE: Bark was the major NSC component in twigs across species. Future research should increasingly follow a bark–xylem approach for a better understanding of NSC distribution and its function in the twig. Despite extensive research on non-structural carbohydrates (NSC), the distribution of total NSC (TNC) and its primary components (sugar and starch) to the bark and xylem of twigs remains poorly understood. We determined seasonal NSC dynamics in twig bark and xylem in seven temperate tree species exhibiting different xylem anatomies and leaf habits. Seasonal trends in sugar were similar across species with concentrations peaking at least 15 days earlier in the spring for xylem than bark. However, evergreens exhibited maximum bark starch concentrations in early spring, followed by declines throughout the growing season, whereas deciduous species exhibited early spring declines in bark starch, followed by late growing season increases. Evergreens exhibited limited seasonal variation in xylem starch concentrations, whereas deciduous species exhibited variation in xylem starch concentrations that was similar to variation in bark starch. With a few exceptions, concentrations and seasonal amplitudes for sugar, starch, and TNC were generally higher in bark than xylem. Sugar concentrations were generally higher than starch, especially in the bark, which resulted in variability of TNC concentrations in bark or xylem. NSC concentrations varied significantly between xylem and the entire twig when bark was not explicitly considered. Averaged across species, sugar, starch, and TNC content in bark accounted for 66%, 54%, and 61% of total twig content, respectively, even though bark only accounted for 53% of the total twig mass. We recommend quantifying bark sugar and starch separately from xylem when determining twig NSC concentration and content.