Water-carbon coordination in Taxodium distichum but not T. ascendens under prolonged off-season submergence in the Three Gorges Reservoir riparian zone.

Abstract

Prolonged winter submergence induced by reservoir impoundment poses severe physiological challenges to riparian trees, yet post-submergence recovery dynamics remain poorly understood. In this study, we investigated hydraulic function, xylem anatomy and non-structural carbohydrate (NSC) dynamics in two congeneric species, Taxodium ascendens and T. distichum, following a winter impoundment in the Three Gorges Reservoir. During the initial post-submergence exposure phase, we comparatively analysed stem xylem micro-anatomy, percentage loss of conductivity (PLC) and NSC allocation across the xylem and phloem in branches collected from the flooded water-level fluctuation zone and the adjacent unflooded upland zone. T. ascendens exhibited reduced tracheid diameter and increased cell wall reinforcement, yet PLC remained elevated. This hydraulic impairment was associated with phloem starch retention, suggesting a possible tissue-level carbon allocation constraint; this association does not indicate that hydraulic impairment caused carbon retention. By contrast, T. distichum maintained stable xylem morphology and low PLC, with coordinated NSC depletion across the xylem and phloem. Trait-network analysis suggested that T. ascendens responses exhibited strong associations between PLC and carbon retention, whereas T. distichum was focused on micro-anatomical homeostasis. Prolonged counter-seasonal submergence was associated with distinct initial physiological patterns in congeneric flood-tolerant species. The elevated PLC of T. ascendens was associated with phloem starch retention, but this relationship does not establish a hydraulic constraint on carbon allocation. These observations document early-phase physiological differences and cannot predict long-term survival, recovery trajectories or interspecific competitive performance. Sequential seasonal sampling and field survival monitoring are required for further validation.