Elevation-Driven Adaptive Shifts in Phloem Anatomy and Non-Structural Carbohydrates: Differential Responses of Pinus koraiensis and Acer mono.

Phloem is crucial for long-distance transport, but its coordinated phenological, anatomical, and nonstructural carbohydrate (NSC) responses to environmental changes remain unclear. Along a 750-1150 m elevational gradient on the Changbai Mountains of China, we monitored phloem growth, anatomy, and total NSC (soluble sugars and starch) in Pinus koraiensis and Acer mono during the 2024 growing season. Elevation delayed phloem formation and compressed the growing season in both species. However, the two wood types diverged in adaptive strategies. P. koraiensis employed an active structural compensation under cold stress: thickened sieve cell walls to resist freeze-thaw, enlarged axial parenchyma for enhanced storage, and increased sieve cell density to compensate for reduced lumen area, while operating under a sink limitation regime where low temperatures suppress growth and surplus carbon is channeled into starch. By contrast, A. mono adopted a conservative anatomical response: sieve tube dimensions and wall thickness remained stable across elevations, while lignified fibers increased at mid elevation to maintain structural integrity, with phloem growth governed by developmental duration under a more source-sink balanced dynamic. Elevational stress tightened the functional coupling between phloem structure and NSC storage in P. koraiensis, whereas structural and carbon dynamics remained decoupled in A. mono. Thus, conifer and angiosperm phloems respond to elevational stress through different coordination regimes-coupling in gymnosperms versus decoupling in angiosperms-which challenges the view of a uniform phloem response and provides a mechanistic framework for understanding wood-type-specific carbon allocation under cold stress.