The salinity gradient influences on the inundation tolerance thresholds of mangrove forests

Abstract

In order to test whether salinity gradient influences inundation tolerance thresholds of mangrove forests, we used validated hydraulic and dispersion model to characterize these two abiotic environmental factors on four riparian mangrove forests, namely Wazwei, Zhuwei, Guandu, and Shezi, with only one mangrove species (Kandelia obovata) in the Tanshui River system. The results showed that the inundations of the highest and the lowest elevations for mangrove growth both increase while salinity decreases. In other words, mangrove inundation tolerance is found to be inversely dependent on salinity concentration. The inundations of K. obovata growth were 0.35–22.11% (0.08–5.31h/day), 1.82–24.99% (0.44–6.00h/day), 3.75–39.99% (0.90–9.60h/day), and 5.73–38.25% (1.38–9.18h/day) on the Wazwei, Zhuwei, Guandu, and Shezi wetlands respectively due to different annual mean salinity (28.57, 21.30, 11.98 and 10.01 ppt for the Wazwei, Zhuwei, Guandu, and Shezi wetlands respectively). Mangrove forests with lower salinity condition, such as the Guandu wetland, have higher potential to invade into river and to occupy flow area, and thus damage flood prevention project. These mangrove forests need not only protection projects but also further management projects, such as regular artificial forest thinning. In addition, under consideration of salinity and inundation frequency alteration, the K. obovata forests on the Zhuwei wetland and the Guandu wetland would vanish around 63% and 35% by 2100, because landward migration would be limited by levees. The difference of mangrove loss rate is due to different local topography. A binomial distribution representing the relationship of annual mean salinity and inundation frequency was developed for evaluating appropriate hydrological regimes of mangrove growth as well. It is also suggested to guide a suitable restoration project and predict mangrove forest loss due to sea level rise.