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.