High-resolution diel dynamics of nonstructural carbohydrates in trees reveal organ-level coordination and trait–environment coupling

Nonstructural carbohydrates (NSC), comprising soluble sugars and starch, serve as critical metabolic reserves buffering trees against temporal carbon supply-demand imbalances. Despite their functional importance, diel NSC dynamics in trees remain elusive, which limits mechanistic understanding of carbon allocation strategies and also restricts the consistency in sampling protocols. Here, we conducted a high-resolution analysis of diel NSC dynamics in leaves and branches of 15 tree species across 4 climatically distinct sites in western China using generalized additive mixed models and a simplified proportional-integral-derivative (PID) control framework. We found that starch and soluble sugars exhibited consistent diel patterns, peaking in the late afternoon and declining to minima before dawn. Starch peaked earlier and exhibited greater amplitudes than soluble sugars, thereby dominating fluctuations of total NSC concentration. Mid-morning (09:00 to 11:00) concentration of NSC fractions closely approximated diel means, offering a practical benchmark for standardized sampling. Variations in diel means and amplitudes were explained more by environmental contexts, particularly vapor pressure deficit (VPD), than by species identity, while trait associations revealed that wood density correlated negatively with diel means and diameter at breast height (DBH) with diel amplitudes. Diel NSC dynamics were actively regulated and coordinated across tissues, with environmental conditions modulating both their magnitude and central tendency. This study highlights the importance of incorporating temporal resolution, environmental context, and trait-based trade-offs into assessments of plant carbon status, providing insights to refine process-based models of carbon allocation and resilience under climate change.