Intertidal mangrove forests occur on tropical and subtropical shorelines, and support ecosystem goods and services. Cordgrass (Spartina alterniflora) was introduced to China from the USA in 1979. It grows vigorously in China and has spread over much of the Chinese coast, mixed with mangroves in the south. Sonneratia apetala (keora), was introduced to China in 1985 from Bangladesh. Because of its higher survival and growth rate compared to native mangrove species, S. apetala has been used for mangrove restoration and afforestation in China. However, the potential invasiveness of the exotic S. apetala is of concern. Plant-plant interactions are likely to be important in mediating the success and impact of both exotics, and to vary across the tidal elevation gradient. In Zhangjiang Estuary, the invasion of S. alterniflora has influenced the growth and regeneration of native mangroves over the past two decades. S. apetala has naturally dispersed into intertidal zones of this estuary within the past 3-4 years. We studied the growth of the exotic mangrove and a native mangrove (Kandelia obovata), and interactions between these two mangroves and the exotic grass, S. alterniflora, across intertidal elevation, using a ‘marsh organ’.
Results/Conclusions
Both native and exotic mangrove seedlings performed best at mid-tidal elevations, and grew and survived poorly in high- and low-marsh elevations. S. alterniflora suppressed the growth of both mangrove species across most of the elevational gradient. Competition from S. alterniflora was strongest at mid-intertidal elevations where S. alterniflora grows most vigorously. S. alterniflora facilitated growth of K. obovata at the lowest intertidal elevation. After two growing seasons, the exotic S.apetala, which grows more rapidly than the native K. obovata, had begun to emerge from the canopy of S. alterniflora. This suggests that the effect of S. alterniflora on S. apetala may switch from negative to neutral after two growing seasons. In contrast, the slow-growing K. obovata may rarely survive long enough to emerge from the canopy of S. alterniflora and escape competition. We suggest that the exotic grass S. alterniflora can slow down the early colonization of mangroves, especially at the intertidal elevations where they would perform best. Moreover, the negative effect of S. alterniflora is greatest on the native mangrove species. Without aggressive management intervention, estuaries in southern China are likely to transition to near complete dominance by exotic grass and mangrove species.