Spatiotemporal Dynamic Remote Sensing Monitoring Of Typical Wetland Vegetation In The Yellow River Delta

The tidal flat vegetation is an important part of the coastal wetland,and its dynamic changes affect the structure and function of the wetland ecosystem.Affected by the combined action of freshwater sediment and ocean dynamics,the coastal wetlands in the Yellow River Delta are in dynamic changes,and the ecosystem is sensitive and fragile.In addition,with the rise of sea level and the frequent development of regional human activities,the coastal wetlands of the Yellow River Delta are facing many problems such as alien species invasion,biodiversity reduction,and landscape fragmentation.Therefore,achieving the accurate identification of wetland types based on remote sensing technology,monitoring and evaluation of long-term dynamic changes and landscape evolution characteristics of wetland vegetation is of great scientific significance for coastal resource management and ecological environmental protection.In this paper,the time series Sentinel-2 MSI data and multi-temporal Landsat series images are used as data sources,combined with the phenological characteristics of vegetation,based on the combination of object-oriented and random forest models to achieve accurate classification of typical coastal wetland vegetation in the Yellow River Delta,revealing the temporal and spatial distribution characteristics of Phragmites australis,Suaeda salsa and Spartina alterniflora in the study area from 2000 to 2020;By calculating the landscape index,the long-term landscape pattern evolution characteristics and driving mechanisms of coastal wetlands were deeply analyzed;On this basis,the impact of the invasive species Spartina alterniflora on the ecology of local coastal wetlands and tidal flat landforms was discussed.The main research results are as follows:(1)The overall accuracy of wetland vegetation mapping in the Yellow River Delta from 2000 to 2020 was 84.74%-92.39%,and the Kappa coefficient is between0.81-0.91.Therefore,on the basis of determining the optimal phase of vegetation phenological differences,the object-oriented random forest remote sensing recognition model based on multi-temporal Landsat series images can realize the accurate extraction of wetland vegetation information.(2)The long-term classification results show that the wetland vegetation in the Current Yellow River Estuary Reserve is distributed along the altitudinal gradient.Phragmites australis is the dominant species in the reserve,and its distribution area has maintained a steady growth in general;the area of Suaeda salsa has been decreasing since 2006,with an average annual loss of 75.86hm~2;The area of Spartina alterniflora increased year by year from 211.85 hm~2in 2002 to 5267.79 hm~2in 2020.The growth of Suaeda salsa in Diaokou Estuary Reserve was unstable,and its area fluctuated;the Phragmites australis community was severely degraded before 2010,and gradually recovered after 2010.(3)Combined with the knowledge of landscape ecology,it is found that the southern and northern reserve have different characteristics of landscape pattern evolution.In the southern reserve,the Phragmites australis landscape as a whole tends to be connected and clustered;Suaeda salsa community is severely fragmented and its development is unstable;The dominance of the Spartina alterniflora community in the study area continues to increase,with many new patches and more complex patche shapes.The degree of fragmentation of Suaeda salsa marshes in the northern reserve was significantly higher than that in the southern reserve,and the dominance of the Phragmites australis landscape continued to increase,but the overall landscape tended to be fragmented and the patch shape was complex.Among them,the change of wetland landscape in the southern reserve is mainly attributable to natural factors,and the change of water and sediment from the Yellow River is the most critical driving factor;while the evolution of wetland landscape in the northern reserve is mainly controlled by human factors such as artificial diversion and ecological water replenishment projects.(4)Combining the evolution characteristics of Spartina alterniflora in different regions and the expansion rates of the seaward and landward boundaries,the expansion process of Spartina alterniflora in the current Yellow River Estuary Reserve can be divided into three stages:in the initial expansion before 2008,affected by storm surge and other factors,it was difficult for Spartina alterniflora to survive;From 2008 to 2014,it was a rapid expansion stage,with an average annual expansion rate of 54%,which showed that the seaward expansion invaded the plain and the landward expansion invaded the habitat of Suaeda salsa in space;During the slow growth stage from 2014 to 2020,the average annual expansion rate was only 9%.The seaward expansion rate of Spartina alterniflora salt marshes in this stage was significantly lower than that of the previous stage,while the landward expansion was more significant.On the whole,Spartina alterniflora salt marshes have entered a stable period of growth in recent years.(5)The invasion of Spartina alterniflora has brought a series of problems to the local ecosystem,among which the impact on the Suaeda salsa is the most obvious.It not only encroached on its living space,forcing the Suaeda salsa community to change from seaward growth to inland shrinkage and retreat,but also changed the seaward succession sequence of wetland vegetation,gradually forming a Suaeda salsa-Spartina alterniflora staggered distribution zone.In terms of preventing coastal erosion,only the Spartina alterniflora salt marsh zone that expands to a sufficient width can play a certain role in promoting sedimentation and protecting the beach.