1. Introduction
Invasive species are considered to be the second largest factor in
global biodiversity reduction after habitat destruction, have
significant advantages in terms of growth, reproduction, and
competition, and can pose a great threat to local ecosystems (Liu et
al.2015). The estuary wetland, located at the sea-land junction of the
river and the sea, is an ecological marginal zone formed by two distinct
ecosystems. This ecosystem is so sensitive and fragile that it is a
frequent area for biological invasion (Wang 2005). The Yellow River
Delta Wetland, as a typical estuary wetland in northern China, appeared
in the “List of Ramsar wetlands of international importance” by the
Ramsar Convention on Wetlands in 2013. It is not only one of the most
representative estuarine wetlands in the world, but it is also the most
complete and the vastest, youngest wetland ecosystem in the warm
temperate zone of China (Gong et al.2016; Wen et al.2011).
Smooth cordgrass (Spartina alterniflora ) is an invasive plant
that was artificially introduced. It exerts a certain degree of wind
protection, berm protection, beach preservation, and siltation in the
estuary wetland in the early stage. S. alterniflora has been
introduced into many countries throughout history, including the west
coast of North America, Europe, New Zealand, and China, and it has
spread rapidly at all these introduction sites(Chen, et al., 2004; Deng
et al., 2006). S. alterniflora was introduced into China as an
ecological project in 1979. However, S. alterniflora has strong
salt and reproductive capacity. At the same time, it lacks competing
species and natural enemies along the coast of China, which has resulted
in a great expansion of S. alterniflora in China’s coastal
areas.. It has gradually encroached on the living space of the native
marsh plant species, which consists of seepweed (Suaeda salsa )
and the common reed (Phragmites australis ), and it has caused
serious damage to the original ecosystem of the Yellow River Delta (Zhou
et al.2017). Therefore, it is urgent to use advanced technology to
timely monitor the distribution patterns and trends of the native and
invasive plant species in the Yellow River Delta, which will provide a
scientific basis for the protection and rational use of wetland
resources in the future.
Compared with field research, remote sensing (RS) technology has the
advantages of wide observation range, flexible monitoring period, and
large amount of information. These technologies were widely used in many
studies in China. For example, saltmarsh vegetation types were mapped
and analyzed based on remote sensing images with an ideal precision
(Huang et al. 2007; Sun et al. 2016;Ai et al. 2016). Combined with
vegetation phenological characteristics, Wu et al. (2012) used Landsat
and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data
to obtain vegetation indexes to track the expansion of S.
alterniflora in the coastal area of the southern Yellow Sea. Wan et al.
(2010) collected HJ-1 charge-coupled device (CCD) images and
high-resolution unmanned aerial vehicle (UAV) images in the Guangxi
Beihai area, and extracted the distribution pattern of S.
alterniflora . The results indicated that the UAV image can provide
detailed information on the distribution range and expansion process ofS. alterniflora . Under the support of 3S (GIS, GPS, and RS)
technology, Wang et al. (2018) analyzed the spatial distribution
characteristics of S. alterniflora in Jiangsu Yancheng National
Nature Reserve from 2006 to 2015 through the landscape index and
centroid change, and then they used Gaofen-2 (GF-2) satellite images to
identify the expansion pattern of S. alterniflora and predict the
expansion trend. The above research on S. alterniflora has been
conducted mainly in southern China. Since 2010, S. alterniflorahas invaded beaches and expanded its range, but there are few studies on
the growth scale, habitat structural changes, and expansion pattern ofS. alterniflora in the Yellow River Delta.
With the improvement in Earth observation systems in the
21st century in various countries and regions, an
increasing number of remote sensing images have surpassed the limitation
of the number of bands, spatial resolution, and revisiting cycles (Zheng
et al. 2017). The long-sequence terrestrial satellite Landsat has become
one of the common sources of multi-source imagery, and the twin
Sentinel-2 satellites launched by the European Space Agency (ESA) are
also becoming widely used in research. Tesfamichael et al. (2017)
compared multi-band remote sensing images (Landsat-8, Sentinel-2A,
Spot-6, Pleiades-1B, and WorldView-3) to distinguish three intrusions of
shrub plants from native plants that are symbiotic with shrub plants.
Compared to the Landsat series of data, the two Sentinel-2 satellites
are equipped with a Multispectral Instrument (MSI), which provides
remote sensing images with multi-scale, medium-high spatial resolution
(10 m, 20 m, and 60 m) from visible and near-infrared to short-wave
infrared (13 bands). Moreover, the Sentinel-2 satellites specifically
monitor three bands in the red edge region (670-760 nm) of the
vegetation spectrum, which is very effective for monitoring vegetation
growth information and its health status (Shoko et al. 2017; Delegido et
al. 2011; Korhonen et al. 2017). At the same time, the revisiting cycle
of the two Sentinel-2 satellites was shortened to 5 days, which greatly
enhances the capability for Earth observation (Zheng et al. 2017).
Therefore, how to exploit the advantages of the two Sentinel-2
satellites in monitoring the surface of the Earth is bound to become the
focus of future research in global scientific research.
In view of this, it is imperative that we build a capacity to monitor
timely the spatial and temporal characteristics of the invasive plant
species and their dynamic changes in order to restore ecological
conditions and protect the environment in the Yellow River Delta. The
main objectives of this study are to characterize the spatial
distribution and spatiotemporal variation of native/invasive species via
remote sensing and to understand the expansion mechanism of S.
alterniflora in the Yellow River Delta, which are the key point in the
restoration and reconstruction of the estuary wetland ecosystem.
The following analyses were conducted to meet the objectives of this
study:
(1) Conducting spectral feature selection using Sentinel-2 images and
Landsat-8 images acquired at different phenological stages of the native
and invasive species in 2018 in the Yellow River Delta. The selection
process was accomplished by using a random forest algorithm.
(2) Based on selected spectral bands of Sentinel-2 and Landsat-8 images
of 2018, the native and invasive species of the Yellow River Delta were
extracted and mapped using a random forest classifier and results were
validated using reference data.
(3) Based on results from step (1) and (2), the temporal and spatial
variations of native and invasive species were quantified and analyzed,
and the main expansion directions and expansion patterns of the S.
alterniflora community were examined from the perspective of landscape
ecology.