Mechanisms And Consequences Of Reed Dieback In A Spartina Alterniflora-Invaded Salt Marsh

As one of the five major global environmental problems, invasive species can direcetly or indirectly influence the performance of native species in the invaded ecosystems, and even cause the extinction of native species. Recently, the native plant Phragmites australis was found to die back in the Spartina alterniflora-invaded Yangtze River estuarine salt marshes. Whether the dieback of native Phragmites (i.e., reed dieback) is related to the invasion of Spartina is still unknown. This thesis focused on the mechanisms and ecological consequences of reed dieback in a Spartina-invaded salt marsh to explore the relationships between Spartina invasion and reed dieback.Since 2010, the representative salt marshes from north to south along the east coast of China were selected to investigate the distribution pattern and characteristics of reed dieback. The results showed that reed dieback occurred only in the Dongtan wetland on Chongming Island in the Yangtze estuary in the late of vegetation growing season (from mid and late July to mid August). Reed dieback occurred in the pure stands of native Phragmites surrounded by exotic Spartina, or in their mixtures. So far, reed dieback has not been found in the pure stands of native Phragmites without Spartina invasion in other investigated sites.To explore the mechanisms of reed dieback, healthy pure Phragmites community, healthy pure Spartina community, their mixtures that were healthy or experiencing dieback were selected in the Dongtan wetland to measure the main environmental parameters. Plant and soil samples were collected in all communities to identify pathogenic Fusarium that might be responsible for the dieback. Meanwhile, plant and soil samples were also collected from the pure stands of Phragmites without Spartina invasion along the east coast of China to gain the pathogenic Fusarium. The roles of abiotic and biotic factors in reed dieback were tested through field and controlled experiments. The results showed that a pathogenic endophytic fungus Fusarium palustre was found only in the pure stands of Spartina and/or dieback Phragmites communities invaded by Spartina in our sampling sites. The strain is the one which was firstly isolated from dieback Spartina in the North American salt marshes according to the morphological and molecular characteristics, indicating that F. palustre was likely to be transported to the Dongtan wetland with Spartina invasion. F. palustre tended to be more abundant in dieback mixture (18.83 ± 4.00%) than in other communities (8.33 ± 5.90% in healthy mixture; 4.17 ± 4.15% in healthy pure Spartina community; 0% in healthy pure Phragmites community). Moreover, the abundance of F. palustre in the dieback Phragmites was significantly higher than those in other plants before dieback occurred, suggesting that F. palustre might be associated with reed dieback. After inoculation, both of Phragmites and Spartina were infected and their growth performance was affected. The disease symptoms on Phragmites were similar to those observed in the field. The disease incidence and disease index of Phragmites were higher than those of Spartina, indicating that the native Phragmites was more sensitive to F. palustre, compared with the exotic Spartina. The results from the field and controlled experiments also showed that abiotic factors did not directly contribute to reed dieback.The effects of reed dieback on plant dynamics were monitored in the above-mentioned four communities. The results showed that ramet density and plant height of Phragmites in dieback community were significantly lower than those in pure Phragmites community and healthy mixture during the growing season. After reed dieback occurred, survival rate of Phragmites in dieback community declined to 30 ± 4.08% and aboveground biomass was only 222.64 ± 5.66g/m2, which were significantly lower than those in pure Phragmites community and healthy mixture. However, ramet density, plant height and survival rate of Spartina were not affected by reed dieback, while its aboveground biomass in dieback community (1042.19 ± 156.46g/m2) was significantly higher than that in healthy mixture (618.76 ± 129.50g/m2). These results demonstrated that reed dieback altered the relative dominance and competition of the native Phragmites and the exotic Spartina, which may indirectly facilitate Spartina invasion.To explore the effects of reed dieback on the litter decomposition, the aboveground tissues of dieback and healthy Phragmites were collected in November and December,2011 respectively, and the litterbag method was used to study the differences of decomposition between the two Phragmites in the air and on the soil surface. The results showed that the C and N contents in the leaf, sheath and stem of dieback Phragmites litters were significantly higher and the C:N ratios in the three tissues of dieback Phragmites litters were significantly lower, compared with those of healthy Phragmites litters. The aerial stem of dieback Phragmites (0.566 ± 0.043/year) decomposed faster than that of healthy Phragmites (0.428 ± 0.052/year), while there was no significant difference of decomposition rates of leaf and sheath in the air or on the soil surface between dieback and healthy Phragmites. The aerial and surficial leaf litters of dieback Phragmites showed the net N release. Both the sheath and stem of dieback Phragmites, and the leaf, sheath and stem of healthy Phragmites showed the N immobilization. These results suggested that reed dieback might affect the native Phragmites litter decomposition.In conclusion, this study suggests that invasive Spartina is likely to transport its endophytic fungus F. palustre from their native North America to the introduced range, indirectly contributes to reed dieback in the Dongtan wetland, and may eventually facilitate its own invasion and affect the ecological processes in the invaded ecosystem. This thesis demonstrates that the indirect effects of exotic species play an important role in biological invasions; and the results obtained in this study improve our understanding on the mechanisms and consequences of exotic species invasion. Meanwhile, this case study enriches the research on the mechanisms and ramifications of vegetation dieback. It also provides theoretical directions for managing exotic speices and restorating the ecosystems.