Anatomical acclimatization and physiological mechanisms of Robinia pseudoacacia dieback in the Loess Plateau.

Background and aims: Widespread dieback and mortality have occurred in mature artificial forests in recent decades. It is unclear how anatomical structures and physiological activities determine the dieback and even mortality of the canopy.

Methods: We analysed the variation in anatomical structure, hydraulic function and non-structural carbohydrates among canopy levels along the drought gradient.

Results: We found that, first, Robinia pseudoacacia coordinated hydraulic efficiency and safety by increasing the vessel diameter and vestured overlap and by decreasing the total pit membrane area in xylem with increasing drought stress. Second, the hydraulic conductivity within the canopy gradually decreased from the bottom to the upper canopy, whereas embolism increased. As precipitation decreases, the upper twigs might dieback first owing to hydraulic failure, accompanied by a substantial reduction (~2.20 % total dry mass) in soluble sugar and starch within these branches. Third, although the upper canopy branches did not reach the hydraulic safety margin, the soluble sugar and starch contents reduced significantly (~1.59 % total dry mass). Meanwhile, the content of non-structural carbohydrates in other canopy tissues increased to cope with drought stress.

Conclusions: In conclusion, dieback of upper canopy twigs might be a result of hydraulic failure, while the non-structural carbohydrates of the branches in the upper canopy decreased, which can be viewed as an adjustment of carbon allocation and avoidance of water loss at the whole-tree level, contributing to plant survival under drought stress.