| Wetland ecosystem is one of the most productive ecosystems in the world, and has important environmental functions and ecological benefits in many aspects, such as climate adjustment, water conservation, flooding and drought prevention, purifying water and biodiversity protection. Dongting Lake is the most important hub for adjustment of water flood and sedimentation in the middle reaches of Yangtze River. However, reclaming land from the lake and sediment deposition led to the lake area decreased significantly, and which then caused the wetland degraded seriously and induced lots of ecological problems, such as biodiversity reduction and water pollution. Environmental changes have a close relationship with the changes of vegetation patterns. Wetland plants, as the most important part of the wetland, play an important role in the maintenance of wetland structure and the displaying of wetland functions. Therefore, clarifying the distribution patterns within its forming mechanism of wetland plants in Dongting Lake can provide theory basis for the protection of wetland resources and the restoration of the degradation wetlands.Three communities (Triarrhena sacchariflorus, Polygonum hydropiper and Carex brevicuspis) were chosen in this paper, which display an zonation pattern along an elevation gradient in the East Dongting Lake. Vegetation characteristic, soil characteristic, and the dominant factors determining plant distribution patterns were studied based on the methods of field investigation and indoor experiments. The results are as follows:(1) The seasonal dynamic and difference of biological characteristics among the three communities were investigated in the East Dongting Lake. A total of28species, were found in the three communities. Gramineae and Compositae have the highest species number. The species number in the T. sacchariflorus community displays a "V" changing pattern. The biomass was highest in the November, while the species richness was highest in the March and Lowest in the May. For P. hydropiper community, species number has insignificant change among different seasons. While, the density, coverage and height were higher before flooding than that after flooding. The coverage of C. brevicuspis was higher before flooding than that after flooding. Higher coverage limited the growth of other species, then leading a lower biodiversity before flooding. Therefore, we concluded that the biological characteristics changed greatly among the three communities, and which also showed significant different response to flooding. (2) The soil physi-chemical characteristics with their response to flooding among three communities were clarified in this paper. Before flooding, soil water content was higher in the P. hydropiper community than that in the other two communities,while after flooding, water contents in C. brevicuspis and P. hydropiper communities were higher than that in the T. sacchariflorus community. Flooding lead to a increment of soil water content only in the C. brevicuspis community. Before flooding, soil bulk density was highest in the C. brevicuspis community and lowest in the P. hydropiper community, while which was highest in the T. Sacchariflorus community after flooding. Flooding lead to a decrement of soil bulk density only in the C. brevicuspis community. Soil conductivity was higher in T. sacchariflorus and P. hydropiper communities than that in the C. brevicuspis community before flooding, while Which had insignificant change among three communities after flooding. Flooding lead to a increment of conductivity only in the C. brevicuspis community. Fine particles accounted for the most part of the particle composition. Flooding had insignificant influence on the particle composition in the T. sacchariflorus community, while which lead to a increment of the ratio of0.02-0.002cm and a decrement of the ratio of<0.002cm in the P. hydropiper community. Moreover, flooding lead to a increment of the ratio of0.25-0.05cm but decrement of the ratio of<0.02cm in the C. brevicuspis community. The organic matter and total nitrogen of P. hydropiper community were higher than that in the other two communities before and after flooding, which were insignificant affected by flooding. Before flooding, total phosphorus were highest in the T. sacchariflorus community than that in the other two communities. While after flooding, the total phosphorus was highest in the P. hydropiper community. Flooding lead to a increment of total phosphorus in the P. hydropiper community, but a decrement of total phosphorus in the T. sacchariflorus community. Before flooding, ammonia nitrogen had insignificant change among three communities, while which displayed an decrement in the T. sacchariflorus and C. brevicuspis communities after flooding. Flooding lead to an increment of nitrate nitrogen in all the three communities, while which had insignificant change among the three communities before and after flooding. Available phosphorus of C. brevicuspis community was much higher than that in the other two communities before and after flooding. Soil pH of C. brevicuspis community was much lower than that in the other two communities, while which was lowest in the P. hydropiper community after flooding. Flooding lead to an increment of soil pH in the C. brevicuspis community, but a decrement in the P. hydropiper community. Therefore, we concluded that flooding had significant influence on the soil physi-chemical characteristics of the three communities, while the influence were different with variation of communities.(3) The influences of flooding to contents of heavy metals were studied in this paper. Results showed that content of arsenic in C. brevicuspis community was much higher than that in other two communities before flooding, while after flooding, P. hydropiper community has the highest content. Flooding lead to a higher arsenic content in the P. hydropiper community and a lower arsenic content in the C. brevicuspis community. Contents of lead, mercury, cadmium and selenium displayed similar changing patterns with arsenic. Flooding lead to a decrement of chromium content among the three communities, but which had insignificant change among the three communities. Before flooding, the iron content was highest in the P. hydropiper community and lowest in the C. brevicuspis community. While after flooding, the highest iron content occurred in the T. sacchariflorus community. Flooding lead to a lower iron content in the P. hydropiper community, but a higher iron content in other two communities. Before flooding, contents of manganese in the P. hydropiper and T. sacchariflorus communities were much higher than that in the C. brevicuspis community, while there was insignificant change of manganese content among the three communities after flooding. Flooding lead to a higher manganese content in the C. brevicuspis community. Before flooding, nickel contents were much higher in the P. hydropiper and T. sacchariflorus communities than that in the C. brevicuspis community. While after flooding, T. sacchariflorus community had the highest nickel content among the three communities. Flooding lead to a significant decrement of nickel content in the P. hydropiper community. Moreover, our results suggested that the East Dongting Lake was polluted seriously by heavy metals, and which also displayed an integrated pollution trends.(4) The dominant environmental factors that determining plant distribution patterns were investigated in this paper. The results of DCCA showed that Eigenvalues of canonical axis1(0.849) and axis2(0.678) explained66.0%of the total species-environment variance information. The first canonical axis represented a gradient of the soil water content (WC) and soil bulk density (BD). Along with changing of the first axis from right to left, soil bulk density decreased, but water content and soil nutrients (organic matter, total nitrogen and ammonia nitrogen) increased significantly. Moreover, the dominant species changed from P. hydropiper to C. brevicuspis and finally to T. sacchariflorus communites. The second canonical axis represented a gradient of the soil available phosphorus (AP) and nitrate content (NN). Therefore, we concluded that the plant distribution pattern was the integrated effect with many environmental factors, and soil physical characteristics, such as water content and soil bulk density, play the most important role in determining plant distribution patterns in the East Dongting Lake. |
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