4. Discussion

4.1 Image Analyses and Species Classification
Based on the multi-temporal Sentinel-2 imagery and Landsat imagery, the extraction of the dominant species in the Yellow River Delta can be achieved by the more important bands optimized by the random forest algorithm. There were significantly more bands with more importance from Sentinel-2 images than those from Landsat images, which is mainly due to the fact that the three unique red-edge bands of Sentinel-2 images provide more data support for the spectral separability of different species. This resulted in an overall accuracy of the Sentinel-2 image extraction results that was slightly higher than that for the Landsat images. Both types of data have advantages and disadvantages when extracting native and invasive species. Sentinel-2 data can improve the classification accuracy by the advantages of band and spatial resolution. The Landsat data can make up for the shortcomings of Sentinel-2 data in long-term sequences. The combination of the two images enabled us to analyze the temporal and spatial variation of native and invasive species in the study area.
For extracting S. salsa in current study area, remote sensing image of September was selected because of the following reasons: (1) Studies have shown that the key substance for the discoloration ofS. salsa is salt. When S. salsa grows in a low salt environment (0.3–1%), the vacuole tissue of its leaf cells is mainly chlorophyll. However, in strong salinity (1–1.6%), betaine becomes the main vacuole structure, which results in leaves that appear red. (2) In September, it is in the middle growth stage ofS. salsa , with high biomass and a wide distribution area. At the beginning of September, the Chinese rain belt begins to rapidly withdraw south, with cold air remaining in the north. The rainy season in North China ends, the amount of water coming from the Yellow River decreases, the groundwater depth subsequently decreases, and the soil salinity increases. The various factors mentioned above lead to the change of color of S. salsafrom reddish to fuchsia in September, which can better highlight the difference in remote sensing images between S. salsa and other vegetation.
As can be seen from Figure 7 that the distribution pattern of native and invasive species has obvious gradients. The main driving forces for this distribution and succession are water (groundwater depth), salt (soil salinity), and human activities. The study area is located where river freshwater and seawater interact. From the river bank highland to the intertidal zone, the freshwater wetland is gradually transformed into saltwater wetland, and the water and salt distributions have obvious gradient differentiation. Since the S. salsa , P. australis , and S. alterniflora communities can only survive under certain water and salt conditions, the existence of water and salt gradients leads to the distribution and succession of the three communities with obvious zonal distribution. At the same time, the ecological restoration projects of the constructed wetland in the protected area will funnel the freshwater of the Yellow River into the ecological restoration area, which will increase the groundwater depth in this area. The salt washing and salt discharging actions by fresh water will reduce the degree of soil salinization. These conditions were suitable for the reed community, and promoted its growth so that it became the dominant species in the ecological restoration area.
Regardless of the landscape index or expansion analysis, the expansion of the S. alterniflora community has become an indisputable fact. According to the current research results, the expansion of the north shore is mainly manifested as expansion to the sea. This is because rivers flowing eastward in the northern hemisphere are affected by the Earth’s rotation, which will cause washing of the south bank of a river. However, sediment will be deposited due to the slower water flow in the north shore. It is the sedimentation on the north shore that provides habitat conditions for the rooting and wild growth of S. alterniflora . The data provided by the Lijin hydrological station (the nearest hydrological station in the Yellow River Delta) show that the artificial water and sediment adjustment occurring in the upper reaches of the Yellow River in 2010 and 2013 resulted in a large amount of sediment deposited on the north bank of the Yellow River estuary, which led to a significant increase in the area of the S. alternifloracommunity on the north shore in 2013 and 2016.
The data show that water and sediment adjustment has increased in 2018, and sedimentation in the northern part of the Yellow River estuary can be visualized from remote sensing images. It is foreseeable that in the next two years, the expansion of S. alterniflora on the north bank of the Yellow River estuary will still be considered to be on the sea side. There is less sedimentation in the south bank, and therefore, the expansion of S. alterniflora occurred mainly towards the land. The expansion pattern indicates that the S. alternifloracommunity is dominated by marginal expansion. This expansion relies mainly on underground roots for tillering, while the highly developed and aerated tissue of S. alterniflora provides sufficient oxygen to its roots to facilitate the growth of adjacent S. alternifloraplants. The external expansion area is small, but the number of patches is high. This type of expansion is very important for the development of plant growth in a new environment.
4.2 Biological and Ecological Impacts
The reproductive ability, high tolerance and adaptability to tidal flat environmental stress of S. alterniflora determine its competitive advantage in the intertidal zone. Due to its high tolerance to flooding and salinity, S. alterniflora has an absolute competitive advantage on the coastal side. When external factors such as tide carry the seeds of S. alterniflora to a new habitat, S. alterniflora colonizes in a new habitat, depending on sexual reproduction, and begins the external expansion. Because of its low productivity in the early stage of colonization, its patch area is relatively small and scattered. After the successful establishment ofS. alterniflora , the seedlings are difficult to survive due to the low light intensity under the canopy of the community. Therefore,S. alterniflora starts a rapid marginal expansion relying on asexual reproduction, forming a large area of single-spices community with high density and productivity, which prevents other vegetation from growing in its distribution area (Wang Q, 2006).
The external expansion patches in the southernmost tidal flat may be caused by the spread of seawater or ships and birds. It is expected thatS. alterniflora will gradually occupy the southern light beach in the next few years. The tide is one of the carriers of S. alterniflora seeds. The development of tidal creek can promote the external expansion of S. alterniflora . S. alternifloraseeds drift with the tide and germinate and settle on the edge of the tidal creek, and invades along the tide to the land side. In addition, due to the strong shore-fixing ability of S. alterniflora , the erosion of the tide creek by the tide has changed from horizontal to vertical, forming a narrow and deep tide creek (Shen Y M, 2003;Zhaoning Gong, 2019). S. alterniflora has a certain wave-slowing effect, which can promote bank consolidation, but it also affects the development of tidal creek and is not conducive to hydrological connectivity. In addition, the invasion of S. alterniflora has reduced the distribution of native species, endangered biodiversity, occupied the light beach, and caused the loss of bird habitats and food sources, such as rare birds — red-crowned cranes and black-billed gulls that depend on the habitat of Suaeda salsa(Gallardo B,2016; Callaway J C,1992; Guy-Haim T,2018). Prevention and management measures should be taken in time to suppress the invasion of S. alterniflora .