Mangroves and salt marsh coexist and interact in many places in the world. In recent decades, mangroves have replaced salt marsh on shorelines of Australia and North America, possibly because warmer temperatures have enhanced the competitive advantage of mangroves. In contrast, the salt marsh grass Spartina alterniflora, introduced to China from the United States, has aggressively invaded stands of native mangroves in China over the last ten years. Few studies have explored the interactions between exotic S. alterniflora and native mangroves in China. We investigated the distribution and interactions of mangrove species and S. alterniflora across an estuarine salinity gradient in Zhangjiang Estuary Mangrove National Nature Reserve, Fujian Province, in the southeast of China. We asked: (1) What are the current distribution patterns of S. alterniflora and mangroves along the salinity gradient within the estuary? (2) How does S. alterniflora impact the establishment of mangrove seedlings across the salinity gradient? (3) Are mangrove forests likely to be replaced by S. alterniflora in the future?

Results/Conclusions

Spartina alterniflora established where soils were bare and overlapped in distribution with mangroves along the estuary. S. alterniflora occurred within mangrove woodlands with more open canopies at low-salinity sites, but only occurred on the margins of mangrove forests at the brackish sites. S. alterniflora occurred at saline sites where mangroves did not occur. S. alterniflora facilitated recruitment of mangrove propagules by trapping them and improving early establishment, but competitively inhibited growth of mangrove seedlings later in the growing season. The strongest relative interaction intensity between S. alterniflora and mangrove seedlings was found at the brackish site, where S. alterniflora had the highest stem density. Transplanted S. alterniflora seedlings had high survival and growth rates in unvegetated mudflat habitats, but failed to survive when planted in the mangrove understory. These results indicate that environmental variability can profoundly influence the distribution and competitive interactions between mangroves and S. alterniflora.  In particular, we predict that S. alterniflora will vigorously invade high salinity habitats where mangroves do not occur, and mudflats below the vertical distribution of mangroves in low-salinity habitats, but will not competitively replace Chinese mangroves from the low to moderate salinity habitats where they currently occur.