| Dongtan wetland of Chongming is one of famous wetlands of the world. It provides a wintering place for some rare and endangered waterfowl. At the same time, it is easy to be invaded by alien species owing to its special location and habitat conditions. Spartina alterniflora Loisel (Smooth cordgrass) is one of the invaders spreading in this area. Smooth cordgrass is native to Atlantic coast, which belongs to Poaceae. It was introduced to Dongtan wetland for siltation and epeirogenic purpose, but then it spread rapidly. Smooth cordgrass continued to outcompete the native specie Scirpus mariqueter and occupied the whole low tidal zone and the majority of middle tidal zone, especially in the north and east parts of Dongtan wetland. At the meantime, reclamation activities resulted in disappearance of large area of vegetation at high tide habitat. In this process, the spreading of Phragmites australis (Cav.) Trin.ex Steud (Common reed) as a native grass with similar competitive ability to smooth cordgrass was seriously disturbed. Therefore, smooth cordgrass invaded Dongtan wetland easily and spread quickly in the decades, which influenced the biodiversity and balance of Dongtan native ecosystems. As a result, some waterfowl also lost their habitats.Common reed is a native dominant species in Dongtan wetland, and it has strong asexual reproductive capability and competitive ability. Common reed is adaptive to grow in the intertidal mudflat. Some results showed the competitive ability between common reed and smooth cordgrass is similar; once one species occupied a habitat, another one can’t intrude again. Thus, it might be an efficient biological method to control smooth cordgrass using common reed replacing smooth cordgrass. This way might have lots of advantages, for example, avoiding chemical pollution, unnecessary introduction of natural enemy and saving manpower and resources. More importantly, it would be helpful for improving biological diversity, optimizing intertidal vegetation pattern and forming stable ecosystem. Based on these backgrounds, this thesis studied the interspecific relationships between common reed and smooth cordgrass along tidal gradient in Dongtan wetland, and discussed some related factors. The main results were as follows:(1) Smooth cordgrass and common reed are both dominant species of Dongtan wetland presently. In general, the important values of the species were similar. Concretely, smooth cordgrass was advantageous in abundance, and common reed was advantageous in height. Common reed dominated in the high tidal zones, and smooth grass dominated in the low tidal zone. In the middle tidal zone, the two species formed a mosaic pattern. In general, there were four patches of vegetation in the study area:Reed patches, Spartina patches, blank patches (sometimes mixing with very few other plants) and two plants mixed patches. The types and areas of patches were different along with tidal gradients. Results showed that:the patch density and edge density of reed population were greater than those of Spartina, suggesting that the degree of fragmentation of reed population was greater than that of Spartina. The aggregation degrees of reed, Spartina and blank patches were larger, connectivity indices were close to100, indicating that the structures of three types of patches at1500×100m2scale were compact, and connectivity was strong. Landscape shape index was only6.59, indicating that the shape of the landscape level was not complicated. The aboveground age structures of common reed and smooth cordgrass showed increasing type in relatively high tidal zones, and a stable type in relatively low tidal zones.(2) Common reed and smooth cordgrass took different competitive strategies to adapt to the different habitats. Common reed and smooth cordgrass are both clonal plants. Clonal architectures of the two species were different:common reed tended to be guerilla type, and smooth cordgrass, the phalanx or mixed type, which determined the difference in acquisition of soil water and nutrient between the two species. This article compared the competitive abilities of smooth cordgrass and common reed from three aspects:the apparent growth capacity, reproductive capacity and the potential growth capacity. The mean relative growth rate of smooth cordgrass was much higher than that of commom reed. However the maximum height reached by common reed at the end of growing period was greater than that of smooth cordgrass, indicating that the height growth variability of common reed was greater than that of smooth cordgrass. The biomass of common reed was generally significantly higher than that of smooth cordgrass, because reed rhizome was coarse, accumulating a large amount of biomass. Comparing the propagation characteristics of common reed and smooth cordgrass, we found that common reed occupied habitats mainly through coarse rhizomes, while smooth cordgrasss mainly through anchoring more ramets by means of asexual reproduction, and increasing plant density and exploring new habitats by means of sexual reproduction. The net photosynthetic rate and the maximum photosynthetic efficiency of smooth cordgrass were higher than those of common reed. However, the photochemical efficiency of common reed was higher than that of smooth cordgrass. The soluble sugar contents or N and P contents of common reed were higher than those of smooth cordgrass.(3) The competitive abilities of common reed and smooth cordgrass were different. Through surveying single-or mix-species community and field removal experiment, we found that the relationship between common reed and smooth cordgrass were facilitative or competitive. Surveying single-or mix-species community showed that:for the average height and biomass, the reaction of the target species to the presence of neighbors differed with tidal gradient. In the high and low tidal zones, the presence of neighbors stimulated the growth of target species, while in the middle tidal zones; the presence of the neighbor species inhibited the growth performance of the target species. Competition was also conducive to the reproduction of plants. Germination rates of common reed and smooth cordgrass were greater in neighbors presence quadrats. In the field removal experiments, we selected relative growth rate (RGR) and the number of seedlings daily produced (TNT) as response parameters to measure the competitive intensity index. In the high and middle tidal zones, the presence of smooth cordgrass facilitated the growth of common reed, but the presence of common reed inhabited the growth of smooth cordgrass when RGR as a parameter, but in the low tidal zone, the absolute competitive intensity (ACI) and relative neighborhood effect index (RNE) of common reed and smooth cordgrass were positive, which represented both competing with each other. Along the tidal gradients (from high to low tidal zone), competitiveness index (Ⅰ) values of smooth cordgrass were from negative to positive; competitiveness index (Ⅰ) values of common reed were positive and decrease with tidal gradient. When taking TNT as a parameter, the existence of smooth cordgrass facilitated the growth of common reed in all three tidal zones, and the presence of common reed inhabited the growth of smooth cordgrass. With tidal gradient decreased (from the high to low tidal zone), I values of common reed were from negative to positive. I values of smooth cordgrass was negative, and the absolute value of I for smooth cordgrass was lowest in the middle tidal zone, In all the three tidal zones, there are crowding effects (Dr) between smooth cordgrass and common reed. In high and middle tidal zone, the crowding effects of common reed on smooth cordgrass were higher than that of smooth cordgrass on common reed. In low tidal zone, the crowding effects of smooth cordgrass on common reed were relatively higher. Overall, interspecific interactions of the two species were mutually beneficial in most cases, and when the relationship is competitive, the total competitive effects of smooth cordgrass were higher than the overall competitive effects of common reed. The direction of the RNE of smooth cordgrass and common reed was consistent with the direction of the I value, the Dr can only affect the size of the RNE.(4) Anthropogenic interference strongly influenced on the growth of common reed and smooth cordgrass. We analyzed the compensatory growth effects of the two species after mowing, and found that:the compensatory growth capacity of common reed afte r severe mowing was not as good as that of smooth cordgrass. While being mowed entirely or at1/2height, the height, density and biomass of common reed reduced to0, indicating that three months after mowing, common reeds did not regerminate and grow. However, in all of the treatments, smooth cordgrass regerminated and grew, but for1/2density-mowing treatment, the compensative ability of common reed was stronger than that of smooth cordgrass, in which it not only recovered from single plant level, but also from population level.(5) Common reed and smooth cordgrass coexisted in Dongtan wetland for many reasons, such as different regeneration niche, but ultimately it is because that they are mutually beneficial to each other to some extent and they played different roles in the habitat with different adaptation strategies. According to the general characteristics of common reed and smooth cordgrass, smooth cordgrass tended to be a r-strategist, and common reed, a K-strategist. According to the Grime classification on the plant life history strategies, in Dongtan wetland, common reed tended to be a competitor, while smooth cordgrass, a weed. In most cases, smooth cordgrass showed the facilitation effects to common reed to some extend, while the latter showed competitive effects to the former. If without the anthropogenic disturbance, along process of the tidal siltation, the area of common reed would gradually increase, while the position of smooth cordgrass would move downward. They would still coexist in the middle intertidal zone. |
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