Abstract: Characteristics of the tropical lowland secondary rain forest, an important forest type in Hainan Island, are different from that of the mountain rain forest. A 3000 m² permanent plot was established in the lowland secondary rain forest in Ganshiling Natural Reserve and species composition and geographic elements were studied. It was found that vegetation characteristics and distribution patterns could be showed well in the sample size of 2700-3000 m², which was proofed by the speciesarea curve. The vascular plant species belonged to 64 families, 128 genera and 163 species which included 6 families, 7 genera and 8 species of pteridophyte, 9 families, 16 genera and 17 species of monocotyledoneae, and 49 families, 105 genera and 138 species of dicotyledons. 87.50% of families included only one or 2-4 species and, predominant families were obvious. Moreover, the species numbers in a genus varied from one to five, and the number of those including one species was the biggest accounting for 82.03%. The species richness in the tropical lowland secondary rain forest of Ganshiling was obviously lower than in the mountain rain forest of Jianfengling and the fragment rain forest in Yunnan Province. The tropical element percentages of family, genera and seed plants in the lowland secondary rain forest of Ganshiling were 75.87%, 92.56% and 93.87%, respectively, quite different from those in the Jianfengling mountain rain forest transitioning from tropical to subtropical/warmtemperate types.

Key words: selenium, Xiangcheng, growth, AsA-GSH cycle