Can Fine-Root Non-Structural Carbohydrates Explain Seasonal Variations in the Respiration in Subalpine Forests?

Fine root respiration drives root growth and resource acquisition in cold, nutrient-poor ecosystems, yet its association with root phenology remains unclear. Non-structural carbohydrates (NSC), stored as soluble sugars and starch are primary metabolites that play key roles in physiological functions. Analyzing NSC pools and availability contributes to the understanding of the seasonality of root respiration. Here, we examined seasonal variations in fine-root respiration, sugars and starch for Abies mariesii and Betula ermanii, from spring to autumn at 2000 and 2500 m in subalpine forests. Additionally, specific root length and root nitrogen concentrations were evaluated. Root respiration rates became higher, mostly following soil temperature. Sugar concentrations were the highest in spring and the lowest in summer. In autumn, sugar accumulated earlier in the fine roots of A. mariesii than in B. ermanii. Starch concentrations and the root functional traits did not show significant seasonal variations. Significant relationships between root respiration and sugars were found in spring for B. ermanii and in autumn for A. mariesii. Furthermore, root respiration and sugars were correlated positively at 2000 m and negatively at 2500 m for both species. These differences explain that B. ermanii actively consumes stored sugars for root respiration to facilitate root elongation and resource acquisition associated with spring foliation, particularly in a shorter growing season. In contrast, A. mariesii suppresses sugar consumption via root respiration because improving cold tolerance before dormancy is critical for survival at the treeline. Therefore, NSCs play different roles in the seasonality of fine roots and determine the species-specific patterns of temperature-independent root respiration.