| The Hanjiang River,the largest tributary of the Yangtze River,is the main water source for the middle route of South-to-north Water Diversion Project(SNWDP)in China.Water diversion in the middle route of the SNWDP and the construction of cascade dams,on one hand,has alleviated the problems of insufficient water resources and deteriorating ecological environment in North China,especially in Beijing and Tianjin,on the other hand,it also has a profound and irreversible impact on the ecological environment of the Hanjiang River.In recent years,the deterioration of water quality,frequent bloom,vegetation degradation and other problems have posed a serious threat to the safety of drinking water and the sustainable development of social economy in the middle and lower reaches of the Hanjiang River.Therefore,it is urgent to explore the influence and evolution process of water diversion and large-scale water conservancy project construction,so as to maintain the ecological security of the Hanjiang River.In the river ecosystem,the riparian vegetation is an important primary producer in the whole river ecosystem.It provides food source and habitat for other organisms,and plays an important role in the logistics,energy flow and biological flow between the river and land ecosystem.In this study,the vegetation of the riparian zone was taken as the research object,and the riparian zone of the middle and lower Reaches of the Hanjiang River(including the riparian beach and the central bank beach)was taken as the research area.Based on the comprehensive analysis of the effects of water diversion and large-scale water conservancy project construction,we conducted four field surveys on plant communities and environmental factors in the middle and lower reaches of the Hanjiang River in June 2018,September 2018,June 2019 and October 2019,respectively,to study the competition mechanism of herbaceous plant dominance and its response to environmental factors such as soil,water diversion and water conservancy construction.It is not only helpful to understand the basic characteristics of riparian vegetation after the construction of water diversion and large-scale water conservancy projects,but also to put forward countermeasures and suggestions for the protection of riparian vegetation in the middle and lower reaches of the Hanjiang River and the ecological regulation of the dams and river management.The main research results are as follows:(1)Due to the impact of water diversion and the construction and operation of cascade dams,the distribution characteristics of herbaceous plants in the riparian zone of the middle and lower reaches of the Hanjiang River may change.Based on the field vegetation survey data of June 2019,the niche width,niche similarity ratio,χ2 test,Pearson and Spearman rank correlation test were used to analyze the niche and interspecific relationships of dominant herbaceous plants in the middle and lower reaches of the Hanjiang River.Results show that the important value and ecological niche is not completely consistent,the riparian vegetation has strong ecological adaptability to the environment,and the dominant plant niche varies greatly among different riparian zones,indicating that the riparian plant community is in the early stage of unstable succession.The results of χ2 test,Pearson and Spearman rank correlation test indicated that after water diversion and large-scale water conservancy project construction,the distribution characteristics of vegetation have changed significantly and the herb community in the middle and lower reaches of the Hanjiang River was in the initial stage of unstable succession.Our study also found that species with extremely significant positive correlations,such as:Paspalum distichum-Polypogon fugax,Phragmites australis-Typha Angustifolia,Cynodon dactylon-Erigeron canadensis,Phalaris arundinacea-Polygonum aviculare,Polypogon fugax-Beckmannia syzigachne,etc.Therefore,it can be regarded as a pioneer species in the protection and restoration of the coastal communities in the middle and lower reaches of the Hanjiang River.(2)After the water diversion and the construction of cascade dams,the vegetation habitat in the riparian zone of the middle and lower reaches of the Hanjiang River has changed significantly.As an important environmental factor,soil plays a very important role in the vegetation community,the effects of soil factors on hydrophilicity,life history types and species diversity of riparian plants were analyzed using Redundancy analysis(RDA)based on the vegetation survey and environmental data of field samples in September 2018 and June 2019.The results showed that the dominant species were Paspalum distichum and Cynodon dactylon,accounting for 22.7%and 12.5%of the total samples,respectively.The first and second transects of the riparian zone are dominated by aquatic plants and hygrophytes,while the third and fourth transects are dominated by mesophytes.The first and second transects above Zhong Xiang(ZX)are dominated by perennials,while the lower transects are dominated by annuals.The third and fourth transects are dominated by perennials.Margalef species richness index,Simpson diversity index,Shannon-Wiener diversity index and Pielou evenness index were different in different transects,but the differences at some points were not significant.RDA results showed that the higher soil moisture content was,the higher the proportion of hydrophytes and hygrophytes was.The proportion of perennial plants and species diversity index were higher in areas with higher nutrient content.(3)After the water diversion and the construction of cascade dams,the hydrological regime of the middle and lower reaches of the Hanjiang River will change significantly,based on the analysis of the vegetation survey and environmental data of the field samples in June and October 2019,it was found that after the water diversion and the operation of the cascade dams in the middle and lower reaches of the Hanjiang River,the vegetation was divided into two habitats:upstream and downstream of the dam.In the upstream of the dam,due to the periodic fluctuation of water level,the water level is always in a high state.The transect closest to the water is mainly dominated by large aquatic plants,such as Typha Angustifolia,while the upland far from the water,due to less flood disturbance,is dominated by meserophytes,such as Imperata cylindrica and Artemisia argyi.In the downstream of the dam,due to upstream water diversion and dam interception,the river flow decreased sharply,except for periodic dam release,water area decreased,beach area increased,perennial herbaceous plants,such as Cynodon dactylon and Paspalum distichum,can have enough time to reproduce and become the dominant population.However,the upland far from the water is less disturbed by flood,and its habitat and vegetation type are similar to that of the upland upstream of the dam,which is also dominated by meso-xerophytes,and the community type gradually changes to meso-xerophytes.Therefore,in order to minimize the impact of water diversion and the construction and operation of large-scale water conservancy projects on the vegetation in the riparian zone,it is necessary to ensure the continuity of rivers and the pulse effect of floods.With reference to historical hydrological data,artificial flood peaks should be made according to local conditions to ensure the propagation efficiency of propagators and maintain the integrity and connectivity of rivers.Secondly,on the premise of ensuring the normal operation of the dam,ensuring the supply of flood in the riparian zone with low soil moisture content and nutrient content,which has a serious impact on water transfer and dam interception,is also an important way to maintain and improve the riparian plants in the middle and lower reaches of the Hanjiang River.Finally,longterm,large-scale dynamic monitoring should be carried out to consider the interaction between soil and other environmental factors and adapt to local conditions to ensure species diversity and ecosystem function. |
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