Abstract: To clarify the water adaptation mechanisms of mangroves across vertical canopy strata, we selected four species (Aegiceras corniculatum, Kandelia obovata, Avicennia marina, Bruguiera gymnorrhiza) from Zhangjiangkou Mangrove National Nature Reserve, Fujian Province. We measured the hydraulic structure and photosynthetic characteristics across different vertical layers, and compared the differences between the upper and lower canopy levels. The results showed that net photosynthetic rate and carboxylation efficiency of four mangrove species in the upper leaves was significantly higher than that of the lower leaves, whereas intercellular CO₂ concentration of the lower leaves was significantly higher in upper leaves. Among the four species, A. marina exhibited the highest net photosynthetic rate, with a rate of 10.87 and 8.94 μmol·m^-2·s^-1 in its upper and lower leaves, respectively. Upper-canopy branches of the four species demonstrated significantly higher sapwood specific conductivity and leaf-specific conductivity compared to lower-canopy branches. A. corniculatum exhibited the highest sapwood specific conductivity in all layers. A. corniculatum consistently displayed the highest sapwood specific conductivity across strata, with values of 0.105 and 0.061 kg·s^-1·m^-1·MPa^-1 for upper and lower branches, respectively. The Huber value was significantly lower in the upper canopy. The upper canopy exhibited stronger coordination between hydraulic architecture and photosynthetic traits, indicating a greater demand for efficient water supply and possessing a more effective water transport system. In contrast, the lower canopy showed weaker hydraulic-photosynthetic trait coordination, invested more heavily in structural carbon, provided enhanced structural support, and displayed lower embolism vulnerability. The coupling mechanism between photosynthesis and hydraulics was relatively consistent across species within the upper canopy. However, there was significant interspecific variation in this coupling mechanism in the lower canopy. These results demonstrated that mangrove species employ distinct hydraulic strategies at different vertical canopy levels to optimize water use efficiency.

Key words: mangrove, hydraulic characteristics, vertical hierarchy, photosynthetic characteristics, adaptation strategy