Native Phragmites dieback reduced its dominance in the salt marshes invaded by exotic Spartina in the Yangtze River estuary, China

Vegetation dieback occurs frequently in various ecosystems and causes tremendous consequences. We here examined the impact of native reed (Phragmites australis) dieback on the salt marsh communities invaded by exotic Spartina alterniflora through monitoring dynamics of component species of Phragmites-Spartina mixture, and comparing their performances in monoculture, dieback mixture and healthy mixture in Dongtan wetland of Yangtze River estuary, China. Phragmites showed poorer performance in dieback mixture compared with other communities. Survival rate (30 ± 4.08%), ramet density (37 ± 9.15 plants/m2), plant height (130.71 ± 20.39 cm) and aboveground biomass (222.64 ± 5.66 g/m2) of Phragmites in dieback mixture were all significantly lower than those in healthy mixture (68 ± 4.79%; 99 ± 17.24 plants/m2; 185.06 ± 17.75 cm; 837.07 ± 205.13 g/m2, respectively) (for all P < 0.05). By contrast, Spartina resprouted well with survival rate of 100% over the course of the experiment. Both ramet density and plant height of Spartina tended to be higher in dieback mixture than in healthy mixture, and its aboveground biomass (1042.19 ± 156.46 g/m2) was significantly higher than that in healthy mixture (618.76 ± 129.50 g/m2) (P < 0.05). Our results suggest that reduced dominance of native Phragmites due to its own dieback weakens its competition with exotic Spartina, which favors Spartina invasion in the salt marsh community.