| Species distribution and their coexistence mechanism have long been the focus of ecological research. Some previous studies considered elevation as the main factor that resulted in zonal distribution of salt marsh species at large scale. While in the low-middle tidal flat with quite low topographical gradients, zonal differentiation pattern also exists between species. Therefore, study of the mechanism of species spatial distribution will help to understand the colonization process of estuarine salt marsh vegetation and provide theoretical basis for salt marsh wetland management, protection and restoration. We investigated the transects across the tidal flat, Scirpus mariqueter front zone, Scirpus zone, mixed zone of Spartina alterniflora and Scirpus, and Spartina zone, measured biological and environmental factors, and analyzed the data with statistical methods. Based on our results, some main conclusions can be drawn:1. Adaptation and feed backs of Spartina and Scirpus to the soil environmental factors are important factors that hampered the distribution of ScirpusIn the area with similar elevation at eastern Chongming Island, soil salinity expressed as conductivity in the Spartina alterniflora zone was0.41±0.01S/m, which was significantly higher than that in the Scirpus mariqueter zone with the value of0.35±0.01S/m. Soil moisture in the Scirpus zone was53%, which was significantly higher than that in the Spartina zone with the value of45%.The flooding frequency in the Scirpus zone and its frontier was higher than that in the Spartina zone. The Eh (redox potential) in the Spartina zone was-21±2mV, and was-27±3mV in the Scirpus zone. It was-65±7mV in the mudflat, which was significantly lower than that in the two vegetated zones. Therefore, at the area without significant elevation difference, soil salinity and moisture can be significantly different.In the intertidal zone, the invasion of Spartina directly led to the significant increase of soil salinity in their growing zone. One of the important reason was that Spartina secreted salt out through its salinity glands during its respiration. Meanwhile, the temperature in the Spartina zone during its growing period was higher than that in the other zones. As a result, the high temperature increased the water evaporation of sediment’s surface and further increased the surface soil salinity. The increased soil salinity in the Spartina zone inhibited the landward growth of Scirpus. However, at the front of the Scirpus zone, the Eh was extremely low because of the frequent flooding, and it restrained the seaward establishment of Scirpus. This conclusion testified the environmental screening hypothesis in the community establishment theory.2. Different reproduction strategy and growth characteristics result in the better competitiveness of Spartina than ScirpusThe Scirpus and Spartina had different reproductive strategy in different zones, and there were also different responses to environmental factors. The number of vegetative growing individuals was1-2times that of the sexual reproduction growing individuals in the Spartina zone, while it was2-3times in the mixed zone. The number of vegetative growing individuals and sexual reproductive growing individuals of the Scirpus was significantly different among the mixed zone, Scirpus zone and front zone. The number of sexual reproductive growing individuals was3-4times than that vegetative growing individualsin the Scirpus zone,2-3times in the mixed zone, and1:8-1:10at the frontier of the Scirpus zone.The hundred grain weight (HGW) of Spartina was0.45±0.01g, while the HGW of Scirpus was0.80±0.01g. For Spartina, the biomass allocation to the production of rhizomes was2-3times that to the production of seeds. For Scirpus, the biomass investment to the seeds was10-17times compared to that to the rhizomes. The result showed that population regeneration of Spartina depended mainly on the asexual reproduction. For Scirpus, sexual reproduction probably was the main strategy when confronted with the interspecific competition. However, the asexual reproduction will increase with the increase of physical stress.From April to June, the relative growth rate (RGR) of Spartina was0.025±0.003g/g·day, and the RGR of Scirpus was0.030±0.001g/(g·day). From July to September, the RGR of Spartina was up to0.034±0.003g/(g·day), which was much greater than that of Scirpus (0.012±0.001g/(g·day)). The RGR of Scirpus from July to September was also significantly lower than that during the early growth period.Consequently, Spartina took over Scirpus with rapid RGR and shading to Scirpus. Under harsh conditions, the intraspecific competition of Spartina increased. However, facilitation was found among individuals of Scirpus. As a result, when environmental factors restrain the growth of both Spartina and Scirpus, coexistence of Spartina and Scirpus will be promoted; but if environmental factors just restrain the growth of Scirpus, Spartina will take over Scirpus rapidly.3. Vegetation difference result in different nutrient pools in the soil, with N and C storage in the Spartina community much higher than that in the Scirpus communityThe contents of carbon and nitrogen in soil were different among different zones. In vegetation zones, they were higher in the sub-surface layer than that in the surface layer. However, for mudflat, they were lower in subsurface layer than that in the surface layer. Carbon and nitrogen pools were1129±76g/m2and145±5g/m2respectively in the0-30cm layer of Spartina zone. In the Scirpus zone, carbon and nitrogen pools were640±63g/m2and99±6g/m2respectively.The accumulation rates of carbon and nitrogen were significantly greater in the Spartina zone than that in the Scirpus zone (P<0.05), and it was intermediate in the mixed zone compared to that in the Spartina and Scirpus zones. The accumulation rate of carbon (ARC) was35.8±8.1g/(m2·yr), and the accumulation rate of nitrogen (ARN) was5.3±0.5g/(m2·yr) in the Spartina zone. In the Scirpus zone, the ARC was6.6±5.9g/(m2·yr), and the ARN was1.2±0.4g/(m2·yr).Feedback existed between vegetation zonation and the accumulations of carbon and nitrogen. At eastern Chongming Island, the ages of different zones were very close with each other. There was significantly positive linear relationship between nutrient pools and biomass (P<0.05). Consequently, the different vegetation structure affected the accumulation of nutrient pools. The plants with high productivity facilitated nutrient retention in the soil. However, the increase of nutrient pools in soil also affected the vegetation zonation by modifying micro-topography, salinity, redox potential, and particle size, and finally affects vegetation zonation together with feedbacks in the sedimentation process.In conclusion, in the intertidal zone of eastern Chongming Island, the surface elevation was very homogeneous. We believed that the zonation between Spartina and Scirpus probably depended on the stress factors such as salinity, redox potential, and sediment and so on. The overlap of niches will result in competition between them. As a result, physical stress and species competition play interactively during vegetation zonation in the salt marsh. Since determination factors for dominant species are different across scales, the main factors controlling vegetation difference derived from this study at cohort and patch boundary scale are not controdictary with the conclusion that "elevation" is the determination factor of salt marsh vegetation zonation at larger scales. |
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