Abstract: This paper is a part of studies of mangrove forest in Dongzhai harbour of Hainan Island in China. It deals mainly with the accumulation and biological cycle of elements Ca and Mg of 55-year-old Bruguiera sexangula community. The main results of the study are as fallows: 1) The total amounts of Ca and Mg in standing crop of this community were 3258.1kg·ha^-1 and 597.9kg·ha^-1 respectively among which, 1891.9kg·ha^-1 of Ca and 247.1kg·ha^-1 of Mg contained in the aerial part, and 1366.2kg·ha^-1 of Ca and 350.8kg·ha^-1 of Mg contained in the ground part. 2) In the biological cycles of Ca and Mg of this stand, 382.0kg·ha^-1 of Ca and 100.4kg·ha^-1 of Mg were annually uptook by the community, among which 250.7kg·ha^-1 of Ca and 80.4kg·ha^-1 of Mg were returned and 131.3kg·ha^-1 of Ca and 20.0kg·ha^-1 of Mg were retained. 3) The turnover period of Mg (8 yrs.) was faster than that of Ca (13 yrs.).

Key words: Mangrove, Biological cycle, Accumulation Ca and Mg