| Riparian zones are narrow strips of land located along the banks of rivers,streams,and water networks,and play a very important role in purifying water quality and preventing soil erosion with unique spatial structure and ecological function.Riparian zone is one of the most complex ecosystems in the earth's biosphere.The maintenance of vegetation diversity and community reconstruction in the riparian zone is a foundation for maintaining the ecosystem functions.However,the increasingly serious external disturbance has changed the original water level change rhythm,hydrodynamics rule,and nutrient level of the natural river,thus having a significant influence on the distribution and transfer of plant propagators in the riparian ecosystem.As a typical open system,riparian ecosystems are at risk of ecological fragility due to frequent natural and anthropogenic disturbance,as well as dynamic nutrient levels and hydrological conditions.In this study,we surveyed the riparian zones in Wujiang catchment,and the upper Han River Basin,and investigated the difference in vegetation characteristics and the functional traits in different types of riparian zones and researched the relationship between community structure and environmental factors,spatial factors.Then,taking the Han River Basin as an example,the responses of riparian plant community structure to different levels of human disturbance were studied.We hope to provide certain ideas and supporting for the construction mechanism of riparian plant communities and ecological protection and restoration.We got the following results:(1)The riparian plant community composition was significantly different between the two study areas.Altitude(Alt)and soil total nitrogen(STN)were the common key factors affecting the plant communities in the riparian zones of Wujiang catchment and upper Han River Basin.We investigated the species composition of the two types of riparian zones,and explored the relationship between environmental factors and community composition.Herbaceous plants dominated the two study areas,among which 83.2% in the Wujiang catchment,and 93.5% in the upper Han River Basin,and the species similarity coefficient was 22.8%.The Compositae,Poaceae,Cyperaceae,Polygonaceae,Fabaceae and Lamiaceae were all preponderant families in the upper Han River Basin,and Poaceae in the Wujiang catchment.Simpson index,Shannon index,Pielou index and faith 'PD index were significantly higher in the riparian region of the upper Han River Basin,and the species diversity was consistent with the trend of phylogenetic diversity.NRI < 0 and NTI = 0 in Wujiang catchment showed an overall divergent trend,and the community assembly was more affected by interspecific competition,while those of plant communities in the riparian region of upper Han River Basin showed an aggregative trend(NRI > 0,NTI >0),and the community assembly was more influenced by the role of habitat filtration in niche theory.Redundancy analysis(RDA)showed that the plant communities in the riparian zone of Wujiang catchment were mainly affected by the type of reservoir regulation,altitude,and total soil nitrogen.Altitude,average annual temperature,water p H,and soil total nitrogen(STN)were the main factors affecting the vegetation composition of the riparian zone in the upper Han River Basin,while altitude and soil total nitrogen were the common key factors affecting the plant community structure of the riparian zone of Wujiang catchment and upper Han River Basin.Altitude might affect the community structure by changing the habitat climate and soil factors.(2)Stochastic processes structured riparian plant communities in Wujiang catchment,while both deterministic and stochastic processes drived community assembly in upper Han River Basin.The Mantel test indicated that the beta diversity was positively related to geographic distance in Wujiang catchment(r = 0.216,P < 0.05),and was significantly correlated with environmental change and geographic distance,respectively(r = 0.298,P <0.05;r = 0.270,P < 0.05).The community assembly mechanism in Wujiang catchment was the result of the action of diffusion restrictions mainly based on neutral theory,while that of riparian plants in upper Han River Basin was combined by environmental screening based on niche theory and diffusion restrictions based on neutral theory,and environmental screening action is dominant.Cascade reservoirs of Wujiang river largely blocked the spread of plant propagules,and species diffusion was limited.Besides,flooding and flushing of water to the riparian zone resulted in soil nutrient deprivation at the riparian zone,which restricted plant growth,so the community assembly process was mainly affected by diffusion restrictions based on neutral theory.Upstream tributaries in the upper Han River Basin covered a variety of human disturbance and had high environmental heterogeneity,which providing a large range for environmental filtration,additionally,tributaries are not connected to each other and species dispersal was also limited.(3)The functional traits of different types of riparian plants were obviously different,such as specific leaf area(SLA)and Leaf dry matter content(m LDMC).Leaf phosphorus content(LPC)was positively correlated with soil total phosphorus content(STP)in the Wujiang catchment.Water nutrition affected the community-weighted mean(CWM)traits in the riparian zone of the upper Han River Basin distinctly.The proportion of C4 plants was higher than the riparian zone of the upper Han River Basin.It is helpful to screen C4 herbs for vegetation restoration to maintain the plant diversity and ecosystem function in the riparian zone of the reservoir.Hydrophyte species and mesophytes species were the main groups in the two types of riparian zones,and the combined ratio exceeds 90 percent.The proportion of mesophytic species in the Wujiang catchment was 71%,which is obviously higher than that in the upper Han River Basin.The proportion of hydrophyte species in upper Han River Basin were slightly higher than that of mesophyte species,but with little difference.Leaf phosphorus content(LPC)was positively correlated with soil total phosphorus content(STP)in the Wujiang catchment(LPC=35.811+3.17*STP).The CWM traits were mainly affected by water nutrition,and environmental factors,which accounted for more than54.2% of the variation.Total nitrogen and total phosphorus in water were the main environmental factors that affect the riparian plant community characters.(4)Anthropogenic disturbances significantly affect plant community structure in the riparian region of the upper Han River Basin,and ecological factors(environmental and spatial)driving its species distribution and variation in plant traits also differ.There were 51 families,115 genera,and 135 species of plants in the mildly disturbed sites,50 families,91 genera,104 species in the moderately disturbed area,and 42 families,82 genera,92 species in the severely disturbed sites.The behavior of the dominant families in the community dependent on human-induced environmental gradients.Poaceae(19)and Asteraceae(17)were the dominant families in the good sites,while Poaceae(14),Asteraceae(9),and Fabaceae(7)were dominant in the fair group.Fabaceae(10),Asteraceae(9),and Poaceae(8)were dominant in the poor group.Our result demonstrated that Asteraceae was the common dominating family across the three groups and the proportion of Poaceae plants was 14.07%,13.46%,and 8.70% for the Good,Fair and Poor groups,respectively.The results showed that both the taxonomic characteristics(species composition)and functional characteristics(functional traits)of the riparian community could reflect the response of the community to anthropogenic disturbance,but the biodiversity was the highest under mild disturbance.The three levels of human interference we studied have taken various factors into consideration comprehensively.The degree of interference in the moderate interference group was not the absolute medium intensity,and the mildly disturbed group and the moderately disturbed group may be closer to the "medium" level.Forward selection showed that different key drivers affecting the taxonomic and functional composition in environment and space,respectively.For taxonomic composition,three environmental drivers(i.e.Alt,STN,and MAT)and three spatial drivers(i.e.PCNM 1,4,and 17)were selected after forward selection.For functional structure,two environmental drivers(i.e.STN and WTP)and three spatial drivers(i.e.PCNM 2,11,and 3)were used for the subsequent variation partitioning process,STN was the common pivotal environmental factor affecting both taxonomic and functional structure. |
PDF Full Text Request