| Spartina alterniflora (or S, for short in this paper) has assumed a growth pattern of burst mode and expanded rapidly since it was introduced into China. Meanwhile, it has been one of the most successful invasive plants in salt marsh ecosystem in China. The introduction of S. alterniflora harms natural habitats of the introducing area, causes subsidence and sedimentation of beach sediment, affects the mudflat aquaculture and channel blocking. It also destructs the diversity of wetland ecosystem, especially for the ecosystem structure and function of mangrove which is known as the sea forest. There is a competition situation between 5. alterniflora and the main edificators of mangrove, resulting in the severe recession of native mangrove species in the wetland. In this study, S. alterniflora and two main edificators of native mangrove Bruguiera gymnorrhiza (or B, for short) and A egiceras corniculatum (or A, for short) in Guangxi were selected to be the research objects, and the allelopathic potential of leaves which were collected in field were compared. Moreover, a multiple de Wit replacement series was used to compare four ecophysiological characteristics which inclued morphological characteristics, biomass allocation, leaf physiological characteristics and allelopathy among three plants in different plant density (the number of short letters standing for planting number of each plant in the same pot). Through these experiments, the competitive relation of S. alterniflora with the two native mangrove species, and the ecophysiologica mechanisms which S. alterniflora invade the mangrove were revealed so as to propose suggestions for assessing its ecological consequences and controlling its invasion in the mangrove ecosystem. The main results were as follows:1. The allelopathic potential of leaves in natural state:Effects of leaf extracts (four kinds of concentration:0.2 g·mL-1,0.1 g·mL-1,0.05 g·mL-1 and 0.025 g·mL-1) of S. alterniflora with two native mangrove species on four test indicators of receptor were quite different. The most serious inhibition rate on seed germination of radish was 34.3%, which appeared with the extract of B. gymnorrhiza with the level of 0.2g·mL-1. The seed germination for radish was inhibited significantly under treatments with high leaf extract concentration of S. alterniflora, and its inhibition effect was stronger than A. corniculatum. The root length of radish seedling was inhibited significantly under the high extract concentration of the three species, and the most serious inhibition rate on seed germination of radish was 76.4%, which appeared with the extract of A. corniculatum with level of 0.2 g·mL-1 However, the height of radish seedling was generally increased with the leaf extracts of each plant. For the fresh weight of radish seedlings, 5. altemiflora palyed a promoting role, while B. gymnorrhiza and A. corniculatum showed a high-minimizing and low-promoting law. It may be that the leaf allelopathic effect of the invasive plant S. alterniflora towards the same testing indexes was not always stronger than the native mangrove plants.2. Morphological characteristics:The leaf area and leaf weight of S. alterniflora and the two native mangrove plants were increased to different degrees in the mixed cropping, but the two indexes of B. gymnorrhiza showed an obvious declining trend. Under the same treatment, S. alterniflora significantly had larger specific leaf area than mangrove plants. This meant that the leaf area of its unit mass was bigger than the two mangrove species, and leaf can capture more light to improve the ability of the carbon assimilation. When it was in the mixture with mangrove species, specific leaf area of S. alterniflora decreased in a certain extent, but this index of the two mangrove species increased. Maybe this was one of the response mechanisms of the native mangrove species to the process of competing with S. alterniflora. The plant height and root length of S. alterniflora increased when it was in the mixture with the two native mangrove plants at low density, and the plant height of mangrove species both showed a downward trend in the single cropping and mixed cropping. The root length of A. corniculatum had an increasing trend when it was in the mixture, but the changes reduced with the proportion of mixture with S. altemiflora increase. The native mangrove species resisted harmful effect of S. alterniflora by using the mechanism of the certain internal change in the process of competition. The inhibition effect of S. alterniflora to the native mangrove species may be a certain threshold value, out of which the mangrove species would be seriously affected but it still depended on the other experimental results of the field trial.3. Biomass allocation:The total biomass of S. alterniflora and the two native mangrove species was significantly decreased under single cropping in high density. Under the mixed cropping with S. altemiflora, some indexes of the two mangrove species were obviously reduced, which included the total fresh weight of individual, the relative growth rate, the aboveground and belowground biomass. The total fresh weight of A. corniculatum obviously decreased under the SSA treatment than that under the A treatment, and the significant reduction ratio reached to 66.48%. To compare the total biomass before and after cultivation, the treatments which were in the mixture with S. alterniflora at high density significantly restrained the growth rate of mangrove species, and it was significantly reduced by 71.75% under the SSB treatment than that under the B treatment. When it was planted at high density, the total fresh weight and aboveground and underground biomass of S. alterniflora had more increase in the mixture with the mangrove species than that of the single cropping in the same number. The increasement was more obviously in the mixture with A. corniculatum than B. gymnorrhiza. Under the same treatment, S. alterniflora had higher root top ratio than the two mangrove species. Both under the single cropping or mixed cropping, S. alterniflora had higher growth rate of biomass than the two native mangrove species, and it kept high proportion allocation of belowground biomass. This was a favourable way to help S. alterniflora adapt to the new environment and occupy the dominant position.4. Leaf physiological characteristics:The chlorophyll content of S. alterniflora were all higher than single cropping condition when mangrove plants were planted, and the chlorophyll content reached the highest point when the mixture proportion was 1:1. It was of benefit to improve the photosynthetic capacity of S. alterniflora. The chlorophyll content of the mangrove species gradually decreased as the mixture proportion of S. alterniflora increasing in the mixed cropping. Different planting ways all had highly significant effect on the leaf MDA contents of S. alterniflora and B. gymnorrhiza. The MDA contents of S. alterniflora at the same density in mixed cropping were always lower than those in single cropping. The MDA contents of mangrove species at the same density in the mixed cropping with S. alterniflora were always higher than those in the single cropping, which was increased with of the mixed cropping proportion S. alterniflora. Thus, it indicated that the mangrove species got the stressing effect from S. alterniflora in the mixed cropping. Among the different planting treatment, the SOD activity of the two mangrove species had significant differences. This index in mixed cropping with S. alterniflora was always higher than that in the single cropping, but it was decreased with the mixed cropping proportion of S. alterniflora increasing. On the other hand, different planting treatment had no significant effect on the SOD activity of S. alterniflora.5. Allelopathy of plant roots:The root extracts of S. alterniflora and the two native mangrove plants (four kinds of concentration:0.4 g·mL-1,0.2 g·mL-1,0.1 g·mL-1 and 0.05 g·mL-1) under different planting ways showed different allelopathic effect. S. altemiflora and the two mangrove species all had an inhibition effect on the radish seeds in the mixed cropping. As the mixed-density of B. gymnorrhiza increasing, S. alterniflora had a stronger inhibition effect on the seed germination of radish, which was stronger as the concentration of the root extract was higher. B. gymnorrhiza had the strongest promoting effect when it was in the mixture with the biggest proportion of S. alterniflora, but the effect was not significant. Under the SA treatment, S. altemiflora and A. corniculatum both had the strongest inhibition effect on the seed germination of radish. Whether in single cropping or mixed cropping, S. alterniflora had a promoting effect on the radish seedling height and fresh weight. At the same time, the promoting effect under the mixed cropping condition was decreased with its mixture proportion declining. The two native mangrove species showed a similar effect to the radish seedlings height and fresh weight as S. alterniflora in the mixed cropping. S. alterniflora only had an inhibition effect on the root length of radish seedling under the SSB treatment, and it showed a "high-minimizing and low-promoting" law under each treatment in the mixture with A. corniculatum, while the mangrove species both showed a "high-minimizing and low-promoting" law in the mixed cropping. It showed that the allelopathy of the root for S. alterniflora didn’t demonstrate the advantage in the the process of competing with the native mangrove species. As an invasive species with strong and deep roots, maybe there were other important mechanisms of S. alterniflora root in improving the invasive ability of S. alterniflora, for example, the diversity of endophytic bacteria. |
PDF Full Text Request