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 .