Control Of Spartina Alterniflora By An Integrated Approach Of Clipping, Waterlogging And Ecological Replacement With Reed: An Experimental Study Of Ecological Mechanisms

Biological invasion is one of the most serious environmental problems of the world. Exotic plant invasions have caused severe ecological, economical and social consequences. China is heavily invaded by an array of exotic plants, so to effectively control invasive exotic plants is now an urgent task. This study used invasive Spatina alterniflora as the model plant and clipping as control method to experimentally examine habitat dependence of clipping control efficiency of invasive Spartina alterniflora. The major aim of this study was to illustrate how the control efficiency of Spartina through clipping can be enhanced through altering flooding and salinity in the environments, i.e. the habitat-dependence of the control efficiency. As an important component of the Spartina control, resotring native plant, Phragmites australis, was also experimentally explored in relation to the habitat characteristics. In so doing, field manipulative experiments and common garden experiments were performed in this study; and the main results obtained are summarized as follows:(1) There were significant differences in persistent flooding time and soil pore water salinity among the different sites in Chongming Dongtan estuary salt marsh, which caused varying control efficiency of Spartina alterniflora and differing recovery efficiency of Phragmites australis under the same experimental treatment. The control efficiency of Spartina alterniflora through clipping in the first year and the persistent efficiency in the following year increased with the increase of persistent flooding time, while the efficiency in the second year increased with the decrease of salinity. The growth of transplanted Phragmites australis in clipped Spartina alterniflora patches was promoted by lower salinity in both the first year and the second year, while the growth in the second year was stimulated by prolonged flooding time. The results showed that the control efficiency of invasive plants was habitat dependant.(2) Persistent flooding time was the major determinant of the control efficiency of Spartina alterniflora through clipping. The regrowth of clipped Spartina alterniflora decreased with the increasing flooding time, and Spartina alterniflora eventually died when the flooding time exceeded the threshold. Thus clipping control could be applicable to the low marsh or areas that are persistently flooded. The mechanism of this damage caused by flooding is that the oxygen availability, and the activity of key enzyme Glucose-6-phosphate dehydrogenase of Spartina alterniflora aerobic respiration decreased with the increasing flooding time until complete inactivation, i.e., inactivation of Spartina alterniflora roots. Thus, it was the response of Spartina alterniflora to the flooding time that caused the dependence of the control efficiency on the habitat properties. Therefore, it is practical to apply clipping in persistent flooded areas while other control methods should be considered in other areas with no persistent flooding. Clipping regime needs to be desgned in relation to to the flooding time of invaded areas so as to improve the whole control efficiency.(3) Substrate salinity is the physical determinant of recovery efficiency of ecological replacement. When salinity exceeded 15ppt, ramet mortality rate of tranplanted Phragmites australis reached about 50%, making its population establishment unlikely. When salinity exceeded 10ppt, Phragmites australis could not complete its sexual reproduction, making the transplanted population unable to expand rapidly. When salinity exceeded 8ppt, Phragmites australis was competitively inferior to Spartina alterniflora, which made the transplanted Phragmites australis subject to reinvasion by Spartina alterniflora. On the other hand, the target species Spartina alterniflora did not considerably change its growth, reproduction and storage under the experimental salinity range. This resulted in the decrease of recovery efficiency of native plant and the increase of reinvasion probability of Spartina alterniflora with the increasing salinity. Thus it can be seen that the development of transplanted Phragmites australis population would reach the expected goal only if the habitat salinity was lower than 8ppt. The effects of salinity on recovery efficiency are the result of the difference in tolerance to salt stress between Phragmites australis and Spartina alterniflora. Net photosynthetic rate, transpiration rate and stoma conductivity of Phragmites australis were sensitive to salinity, decreasing with the increasing salinity. Net photosynthetic rate and transpiration rate of Spartina alterniflora were maintained at a high level during its growing period. The reason was that Spartina alterniflora could exclude the salt ions from its main physiological processes, and maintain Mg²⁺ (which affects photosynthesis and transpiration) and K⁺ (which promotes photosynthesis and transpiration) content at a relatively stable level although the salinity increased. This not only reduced the damage by Na+ but also ensured the basic physiological processes such as photosynthesis and transportation within a wide salinity range. On the contrary, Na+ affected transpiration of Phragmites australis directly; and Mg²⁺ and K⁺ contents decreased with the increasing salinity. This not only caused the damage by salt ions, but also led to the reduction of photosynthesis and transportation with the increasing salinity. As a result, Phragmites australis could be used for restoration of fresh and brackish areas, whereas other native halophytes should be considered in saline habitats.This study explored the possibility of using physical control and ecological replacement methods to mange Spartina alterniflora at Chongming Dongtan in the Yangtze River estuary. In order to design sound control regime for invasive plants, habitat dependence of the efficiency of chemical, biological and integrated control methods needs to be studied further.