Studies On Ecology Of Plant Communities In The Drawdown Zone Of The Three Gorges Reservoir

The Three Gorges Reservoir starts from Hubei Zigui(San Douping)in the east and Chongqing Jiangjin(Hong Huabao)in the west,involving 22 districts(counties)in Hubei Province and Chongqing.In addition to the main stream of the Yangtze River,the Three Gorges Reservoir also includes more than 400 secondary rivers,such as Xiangxi River,Pengxi River,Daning River,Wujiang River and Jialing River.After the completion of the Three Gorges Dam,the river's hydrological form changed significantly,forming a drawdown zone of"submerged in winter,exposed in summer",the drawdown zone is 30 m wide and covers an area of about 348.93 km~2.The impoundment of the Three Gorges Reservoir has inundated the original terrestrial ecosystem,changed the original biomes,and reduced the biodiversity,especially the plant community structure and distribution pattern.The flood storage in winter and the short-term flood in summer have a great impact on the plant community of the drawdown zone.By 2018,the Three Gorges Reservoir has been stored for a high water level for 10 years.Under the 10-year seasonal water level change and repeated flooding stress,what is the current status of the plant community in the drawdown zone?What happened?What is the law of change and the adjustment mechanism?Therefore,it is important to study the status quo and change laws of plant communities in the drawdown zone after high water level storage for ten years.And it has great theoretical significance and application value to understand the ecological evolution of large-scale water storage reservoirs and their driving mechanisms,as well as the ecological protection of the Three Gorges Reservoir.In this paper,the plant communities in the drawdown zone of the Three Gorges Reservoir are taken as research objects.Based on the field investigation,the statistical and quantitative ecological methods are used to systematically study the community of the drawdown zone.The species composition and floristic characteristics of plant communities,plant diversity and spatial distribution pattern,niche and interspecific relationship of dominant species,relationship between plant communities and environmental factors,and dynamic changes of plant communities in the past 10 years after high water level impoundment are also discussed.The main research results are as follows:(1)There are 150 species of vascular plants belonging to 125 genera and 44families in the drawdown zone of the Three Gorges Reservoir.The composition of plant species is concentrated in Gramineae,Compositae,Cyperaceae and other families,while dispersed to few families and few families.The life style is dominated by annual herbaceous plants.The species of woody plants are few and all exist in the form of seedlings near 175 meters above sea level.The floristic composition of the drawdown zone is mainly tropical and temperate.Compared with the initial stage of water storage,the floristic types decrease and the proportion of tropical elements increases.(2)Herbaceous plants are the main building species of plant community in the drawdown zone.The species richness of the community was larger in the middle part of the reservoir,and the trend along the vertical gradient increased first and then decreased,while it increased along the elevation gradient.The Shannon-Wiener index shows a decreasing trend along the longitudinal gradient,and increases along the elevation gradient.The total coverage showed a certain decreasing trend along the longitudinal gradient and the elevation gradient direction,but the change was not significant.The aboveground biomass along the vertical gradient shows lower at the head and tail of the reservoir,while the middle is higher.And the aboveground biomass along the elevation gradient is higher in the middle altitude(160-170m),lower altitude(<155m)and high altitude(170-175m)are lower.(3)According to the frequency and average coverage of each species in the sample,15 dominant species of the drawdown zone such as Cynodon dactylon,Xanthium sibiricum,Bidens tripartite,Echinochloa crusgalli,Polygonum lapathifolium,Cyperus rotundus,Setaria viridis,Digitaria sanguinalis,Eclipta prostrata,Abutilon theophrasti,Melilotus officinalis,Conyza Canadensis,Physalis minima,Alternanthera philoxeroides and Hemarthria altissima were screened.It is found through calculation that the niche breadth and niche overlap value of the dominant population are large,indicating that under the long-term repeated flooding environment of the drawdown zone,various resources are scarce and the competition between species is fierce.At the same time,the chi-square test and the Spearman rank correlation analysis were used to obtain 105 pairs,and the positive correlation logarithm was slightly more than the negative correlation logarithm.The 15 dominant species were divided into three ecological species groups,which basically reflected the distribution of dominant species with altitude gradient.In summary,it can be concluded that after 10 years of repeated flooding of the Three Gorges Reservoir,the plant community structure has initially stabilized.(4)Herbaceous plants are the main groups of plant community construction in the drawdown zone,and the number of species and community types in the drawdown zone gradually increase with the increasing elevation gradient.TWINSPAN classification classifies plant communities in the drawdown zone into 8 community types.CCA ordination analysis shows that along the axis 1 from left to right,the depth of flooding gradually decreased from left to right,along with the elevation gradually increased and the time of flooding gradually shortened.The plants are transferred from the low-altitude areas of the Cynodon dactylon,Echinochloa crusgalli and Cyperus iria to the Conyza Canadensis,Artemisia lavandulaefoliaandArtemisia annuadistributed in the high-altitude areas.The community type was changed from the Cynodon dactylon community and Cynodon dactylon+Echinochloa crusgalli community to the Bidens tripartite+Melilotus officinalis community and Bidens tripartite+Conyza Canadensis community near 175m above sea level.The diversity and distribution pattern of the community in the drawdown zone are the result of a combination of various factors,but mainly affected by the flooding disturbance(flooding depth).(5)Comparing with the investigation results in the early stage of high water storage,it was found that:compared with the early stage of high water storage,plant species in the drawdown zone decreased by 14 families and 25 species;The dominant families were Gramineae,Compositae and Cyperaceae;The therophyte is the main life type,and woody plants decreased sharply.The species richness and shannon-wiener index of the plant community increased with the elevation;As the elevation increased,the total community coverage gradually decreased,which was larger in the areas below165m,with no significant difference,and gradually decreased from above 165m;The aboveground biomass first increased and then decreased,reaching the maximum at the altitude of 160-170m.