Mangrove ecosystems play irreplaceable and important roles for the stabilization and equilibrium of coastal estuary. Mangroves are the unique intertidal plant formations growing in sheltered tropical and subtropical coastal areas. Due to increasing population and economic development, they had been severely damaged in the last several decades, facing degradation and loss. In recent years, many measures were undertaken to rehabilitate mangroves in different countries. However, followed by anthropologic disturbance and natural disasters, lower survival rate of afforestation, and careless management to slow enhancement of mangrove area. Then lower survival rate was the prmiary threshold to mangrove restoration.
Critical tidal elevation for mangrove species was focused in this paper, because it is one of the key factors determining the chance of successful mangrove afforestation in the Beibu Gulf and even the whole coast of China. Based on the field surveys on the growth and the distribution of natural trees, the critical tidal elevations for the major mangrove species along Beibu Gulf coast were proposed. The results would be helpful to improve the rationalized mangrove afforestation along Beibu Gulf coast.
Tidal elevation is one of the most important factors affecting the survival of mangrove seedlings. The present study aimed at finding critical tidal elevation for the selection of tidal flat for mangrove afforestation. In this paper, tidal elevation and mangrove community survey were conducted at three sites: Yuzhouping, Shijiao and Jiaodong in Fangchenggang, Guangxi. Eighteen transects parallel to the coast were set and the occurrence, growth condition and tidal elevation of 4 mangrove species Avicennia marina, Aegiceras corniculatum, Kandelia obovata and Bruguiera gymnorhiza were surveyed. The changes of slope in three natural forests were: from -0.37 to -0.05 in Yuzhouping, from -0.23 to -0.10 in Shijiao, and from -0.72 to -0.10 in Jiaodong, respectively. At Shijiao and Jiaodong,Aegiceras corniculatum appeared majorly in the tidal flat with an elevation from 15 to 40 cm and 33 to 36 cm, Kandelia obovata from 43 to 60 cm and 37 to 51 cm, Bruguiera gymnorhiza from 94 to 107 cm and 111 to 119 cm, respectively. The corresponding tidal elevations for the maximum heights of Av. Marina (220 cm), Ae. Corniculatum (200 cm), K. obovata (200 cm) and B. gymnorhiza (280 cm) were +60 cm to +80 cm, from +20 cm to +40 cm, from +40 cm to +80 cm and from +60 cm to +100 cm, respectively. According to the occurrence frequency and height distribution along the transects, the critical tidal elevations of the 4 mangrove species in Fangchenggang were -7 cm (Ae. Corniculatum), +33 cm (K. obovata), +23~26 cm (Av. marina) and +44 cm (B. gymnorrhiza), respectively. The corresponding flooding times in one tidal cycle for each species were 8.5h, 7.0h, 7.0h, and 6.0h, respectively.
At Shijiao and Jiaodong, 30.0% and 56.7% of Ae. corniculatum trees with an average height over 2m occurred on the tidal flat below the local mean sea level (MSL), respectively. These Ae. Corniculatum were accompanied by some K. obovata trees (average height 1.75 m). The results showed that Ae. Corniculatum and K. obovata can grow in the tidal flat below the MSL.