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Using a marsh organ to predict future plant communities in a Chinese estuary invaded by an exotic grass and mangrove

Exotic species vary in how harmful they are. The earlier we can predict the outcome of an invasion, the more time is available for management. We conducted a “marsh organ” experiment with six elevation treatments to evaluate how the exotic grass Spartina alterniflora, which has invaded almost the entire coastline of China, affects a native (Kandelia obovata) and an exotic (Sonneratia apetala) mangrove in Zhangjiang Estuary, southern China. Survival of transplants in monoculture over two growing seasons in the marsh organ was high except at extreme high and low elevations. All three plants displayed hump‐shaped patterns of biomass across elevation in monoculture, but K. obovata performed best at slightly higher and S. alterniflora at slightly lower elevations, whereas S. apetala performed well across all four intermediate elevations and grew larger than the other two plants. In mixtures, S. alterniflora suppressed growth of mangrove seedlings of both species by up to 90%. The competitive effect of S. alterniflora was greatest at intermediate elevations where its biomass was greatest. When transplanted into adjacent stands of S. alterniflora, K. obovata remained stunted, but S. apetala grew quickly and began to emerge from the canopy of S. alterniflora by the end of the second growing season. At this site, past and ongoing disturbances to mature stands of K. obovata are likely to allow both exotics to increase in abundance. Aggressive management intervention will be needed to prevent estuaries in southern China from transitioning to near complete dominance by the exotic mangrove S. apetala.