We compared the effects of hydrophilic polymer amendments on drought and salt tolerance of Metasequoia glyptostroboides Hu and W.C.Cheng seedlings using commercially available Stockosorb and Luquasorb synthetic hydrogels and a biopolymer, Konjac glucomannan (KGM). Drought, salinity, or the combined stress of both drought and salinity caused growth retardation and leaf injury in M. glyptostroboides. Under a range of simulated stress conditions, biopolymers and synthetic hydrogels alleviated growth inhibition and leaf injury, improved photosynthesis, and enhanced whole-plant and unit transpiration. For plants subjected to drought conditions, Stockosorb hydrogel amendment specifically caused a remarkable increase in water supply to roots due to the water retention capacity of the granular polymer. Under saline stress, hydrophilic polymers restricted Na+ and Cl− concentrations in roots and leaves. Moreover, root K+ uptake resulted from K+ enrichment in Stockosorb and Luquasorb granules. Synthetic polymers and biopolymers increased the ability of M. glyptostroboides to tolerate combined impacts of drought and salt stress due to their water- and salt-bearing capacities. Similar to the synthetic polymers, the biopolymer also enhanced M. glyptostroboides drought and salt stress tolerance.
N addition decreased stand structure diversity in young but increased in middle-aged Metasequoia glyptostroboides plantations