| Affected by the topography,the riverside wetland along the Hanjiang River is distributed in a narrow strip,with a small area and serious patching.With the rapid development of social economy,the evolution of wetland internal landscape is relatively fierce,the anti-interference ability is weakened,and the ecological function is degraded.Wetland vegetation is one of the important components of wetland.It is the basis of wetland structural stability and functional integrity.It is also the most direct indicator of wetland degradation and succession.In different succession stages,the growth state and spatial distribution pattern of wetland vegetation are also different.Analyzing the change law of vegetation growth state and its driving factors,clarifying the temporal and spatial heterogeneity and stability of vegetation coverage and its environmental response mechanism,and exploring the response relationship between water quality and wetland vegetation plays an important role in quantitatively analyzing the impact of human activities on wetland vegetation,and is of great significance and value to the protection and restoration of wetland ecosystem.In this study,wetland vegetation is characterized by normalized vegetation index(NDVI)and vegetation coverage(FVC).Using MOD13Q1 data from 2000 to 2020,Landsat remote sensing images and field survey data,the vegetation change law is analyzed by Sen+MK trend analysis method,MK mutation test method,Two-way indicator species analysis method,Markov transfer matrix,coefficient of variation,Pearson correlation coefficient and other methods,This paper quantitatively analyzes the change characteristics of wetland vegetation in Hanjiang River Basin(Shaanxi section),explores the succession direction of wetland vegetation with different coverage types,and analyzes the relationship between wetland vegetation growth state,coverage heterogeneity and precipitation and water quality conditions.The results are as follows:(1)In terms of time,NDVI in the study area increased significantly from March to May and decreased significantly from September to November.The average annual NDVI shows an upward trend(R~2=0.753).On the spatial scale,significant degradation occurred in some parts of the study area from 2000 to 2020.The area with obvious decline of NDVI accounted for 7.70%and the area with slight decline accounted for7.84%.The maximum degradation area of wetland vegetation NDVI in the upper reaches of the basin reaches 73%of the total area.The maximum degraded area of wetland NDVI in the lower reaches of the basin is 61%of the area.(2)The vegetation coverage zoning map of the typical study area shows that the wetland vegetation is distributed in a gradient along the vertical direction of the river,and the vegetation coverage gradually increases with the increase of the distance from the river.There are various types of vegetation coverage along the curved river bank and the intersection of tributaries and trunk streams.(3)In the vegetation coverage zoning map of the typical study area in the upper reaches of the basin,the proportion of low and medium coverage vegetation showed a gentle upward trend,increasing from 12.99%to 18.86%.The low vegetation coverage area is in severe fluctuation,and the coefficient of variation is 0.34.The net change of vegetation with medium low and medium high coverage in the typical study area at the upper reaches of the basin is-4.60%and-3.42%of the regional area respectively.Therefore,the specific performance of degradation from 2000 to 2020 may be the reduction of medium low and medium high coverage vegetation such as Polygonum lapathifolium community,Cynodon dactylon community and Phragmites australis community,accounting for 8.02%of the total area of the study area.(4)From 2000 to 2005,the low coverage area and high coverage vegetation area in the typical study area at the lower reaches of the basin increased,and the change of medium and low coverage vegetation was severe fluctuation(r=0.43).From 2000 to2005,the wetland vegetation area decreased by 4.01%of the downstream typical study area.From 2000 to 2020,the net change of medium and high coverage vegetation is-2.82%of the regional area.The vegetation communities dominated by degradation may be medium and high coverage vegetation such as such as Conyza canadensis community and Phragmites australis community.(5)Climate and human activities will have a significant impact on the monthly growth of vegetation in the upper reaches of the basin.Agricultural activities can inhibit the growth of wetland vegetation in the study area in the upper reaches of the basin,and the increase of rainfall and temperature can promote the growth of wetland vegetation.Human activities are the main driving force for the inter annual change of wetland vegetation growth state and the change of wetland vegetation structure.The enhancement of man-made pollution discharge activities has an inhibitory effect on the growth of medium and low coverage vegetation(such as Polygonum lapathifolium community,Cynodon dactylon+Aster subulatus Michx community)and medium and high coverage vegetation(Phragmites australis community).(6)The monthly variation of NDVI of wetland vegetation in the lower reaches of the basin has a significant positive correlation with rainfall and fecal Escherichia coli(P≤0.05).The increase of rainfall can promote the growth of vegetation,and human activities that produce organic and inorganic salt pollution can significantly inhibit the wetland vegetation.Human activities are also the main driving force for the annual change of wetland vegetation growth status and the change of wetland vegetation structure.The emission of organic pollutants can promote the growth of medium and high coverage vegetation such as Conyza canadensis community,and inhibit other plant communities.The research shows that the wetland vegetation in Hanjiang River Basin(Shaanxi section)is distributed in a gradient along the vertical direction of the river,and the vegetation diversity along the curved bank and the intersection of tributaries and trunk streams is high.The vegetation degradation dominated in the typical study area at the upper reaches of the basin may be medium and high coverage vegetation such as Polygonum lapathifolium community,Cynodon dactylon+Aster subulatus Michx community and Phragmites australis community,while the vegetation degradation dominated in the typical study area at the lower reaches of the basin may be medium and high coverage vegetation such as Conyza canadensis community and Phragmites australis community.Climate and human activities will have a significant impact on the monthly growth of vegetation in the typical study area.Agricultural activities can inhibit the growth of wetland vegetation in the typical study area in the upper reaches of the basin,and the increase of rainfall and temperature can promote the growth of wetland vegetation.Human activities are the main driving force for the annual change of wetland vegetation growth state and the change of vegetation structure.The discharge of organic pollutants can promote the growth of medium and high coverage vegetation such as Conyza canadensis community,and inhibit other plant communities. |
PDF Full Text Request