Abstract: With the characteristics of barren soil tolerance and fast growth, the mangrove species Sonneratia apetala has become the main pioneer species in the coastal beaches of Southeast China. As an alien species, however, the invasiveness of S. apetala has attracted wide attention. Sanya River is the southernmost region for the introduction of S. apetala in China, but no information is available on the invasion of S. apetala in this area. The population dynamics and spread characteristics of S. apetala in Sanya River were examined using comprehensive investigation and plot sampling method. The results showed that the average density of S. apetala in the upper layer of community was 4 ind·100 m^-2, with an importance value of 41.2%. Although S. apetala had obvious advantages in the upper layer of the community, there were no saplings and seedlings in the middle and lower layers, indicating difficulty in the regeneration of S. apetala. Shannon index of mangroves in the middle and lower layers of the community was 1.68 and 1.09, respectively, while Simpson index was 0.78 and 0.57, respectively. The community species diversity was high. The native mangrove species grew well, whose living space was hardly occupied by this alien species. Furthermore, no natural spread of S. apetala was observed in Sanya River. High salinity may be the key factor inhibiting its spread. Considering its excellent performance of growth and adaptability,S. apetala can be used as a pioneer mangrove species in ecological engineering of coastal wetland without fresh water inflow. Meanwhile, long-term monitoring and positioning research should be strengthened.

Key words: alien species, mangrove forest, biodiversity, ecological restoration, natural spread.