Effects of environmental gradients on the performances of four dominant plants in a Chinese saltmarsh: implications for plant zonation

Physical conditions and biotic interactions are believed to be the determinants of plant zonation in saltmarshes. However, in rapidly developing estuarine marshes, succession is regarded as the primary process responsible for plant zonation and it is controlled mainly by environmental factors. Salinity and inundation are two important factors responsible for the distribution pattern of dominant plants in coastal saltmarshes. Here we conducted a common garden experiment as well as a field transplanting to examine the responses of four dominant saltmarsh plants (native Scirpus mariqueter, Scirpus triqueter and Phragmites australis, and exotic Spartina alterniflora) in the Yangtze River estuary to environmental gradients, which may help us understand their current and potential zonation. The results showed that Scirpus adapted to freshwater and less inundated habitats, Phragmites performed well in brackish or freshwater environments with less inundation, and Spartina tolerated the highest salinity and deepest inundation. In the harshest environments (the highest salinity and water level), only Spartina performed well. In the mild environments, however, there were only minor differences in the performances among the four species. The potential ranges of Phragmites and Spartina were predicted to be larger than their current ones, and their lower boundaries might be set by tidal scour rather than edaphic factors. With the saltmarsh succession, invasive Spartina in the Yangtze River estuary might ultimately replace Scirpus, and alter the zonal patterns of native saltmarsh plants, which will lead to severe ecosystem consequences. Thus, proper management measures (e.g., repeated mowing) need to be implemented to control this invasive exotic plant, and restore the vulnerable ecosystems invaded by Spartina in the Yangtze River estuary.