Ordered mortality of mangroves under extreme flooding in a tropical microtidal estuarine lagoon in China

Abstract

Tidal inundation is a key environmental factor in coastal wetland ecosystems, such as mangroves. Understanding the response of mangroves to increased flooding stress under rising sea levels is crucial in the context of rapid global climate change. While previous studies have focused on the mangrove seedlings in controlled experiments, field studies on the response of mature mangrove trees to rising sea levels remain limited. This study examines the impact of flooding stress caused by tidal inlet narrowing on mangroves in a tropical microtidal estuarine lagoon on Hainan Island, China. We assessed the mortality of four common mangrove species: Avicennia marina, Ceriops tagal, Rhizophora apiculata, and Scyphiphora hydrophyllacea, and conducted an elevation gradient survey of C. tagal in two transects to evaluate how different mangrove species respond to flooding stress. The average tree heights of these species were 1.79 m, 1.16 m, 3.14 m, and 2.38 m, respectively, with survival probability after the flooding event of 42%, 12%, 89%, and 95%, respectively. We found that survival probability was positively related to tree height across the four species (P < 0.001), and the site elevation of C. tagal is significantly positively associated with survival probability (P < 0.001). Additionally, higher elevations correspond to better health in surviving C. tagal, indicated by more live leaves per twig and a higher leaf Fv/Fm ratio (P < 0.001). These results suggest two mortality patterns in mangrove responses to the extreme flooding event: tree height-based and elevation-based mortality. The ordered mortality implies that mangrove vulnerability to sea-level rise is heterogeneous. These findings imply that future assessments of mangrove vulnerability should consider the fine-scale spatial distribution of species and the spatial heterogeneity of tree height.