Changes in the leaf functional traits of mangrove plant assemblages along an intertidal gradient in typical mangrove wetlands in Hainan, China

The understanding of biodiversity-ecosystem functioning linkages under environment change has been advanced immensely by the development and application of the functional trait theory. Intertidal zones, characterized by significant environmental gradient differences, are severely threatened by sea-level rises caused by global warming. The functional trait distribution pattern for mangrove communities along the intertidal gradient and underlying mechanisms remain unclear. We investigated differences in the leaf functional traits of mangrove plant assemblages among the high, middle, and low intertidal zones, and their relationships to sediment physicochemical properties, in Dongzhaigang and Qinglan Harbor reserves, Hainan, China. In both reserves, most sediment physicochemical properties (pH; salinity; total potassium, carbon, nitrogen, and sulfur concentrations; organic carbon content; ammonium nitrogen and available phosphorus concentrations) differed significantly among gradient zones. The sediment pH, total phosphorus, and available potassium concentrations differed significantly between the dry and wet seasons. From the low to the high intertidal zone, the mangrove specific leaf area and leaf nitrogen and phosphorus concentrations increased significantly and the leaf thickness decreased significantly. The leaf tannin and lignin concentrations were significantly lower and the leaf phosphorus concentration, nitrogen concentration and hardness were significantly higher in the dry season than in the wet season. The sediment salinity, pH, and total sulfur and potassium contents were key factors affecting the distribution of mangrove leaf functional traits along the intertidal gradient. The changes in mangrove leaf functional traits reflect gradient-level responses to environmental stress and the sediment nutrient status. Our findings improve the understanding of the functional trait diversity and maintenance mechanisms of mangrove communities under environmental change and provide a new perspective for the prediction of how coastal mangroves will respond to future global warming.