2. Materials and Methods

2.1 Study area

The Yellow River Delta (118°32.981′E—119°20.450′E and 37°34.768′N—38°12.310′N), which is located at the estuary of the Yellow River in Dongying City, Shandong Province, China (Fig. 1a-c), is one of China’s four largest river deltas. As a national nature reserve, it was officially listed on the international list of important wetlands in 2013. The delta, bounded by the Bohai Sea in the north and the Laizhou Bay in the east, has a typical temperate monsoon climate with four distinct seasons, and rain and heat over the same period. The average annual temperature is 12.1°C, the precipitation is 530–630 mm, and the evaporation is 1900–2400 mm in the study area (Liu et al. 2017; Shandong Yellow River Delta National Nature Reserve Administration 2016). Despite the flat topography, the ecological pattern varies with time and space, and the types of wetlands are diverse in this region.

2.2 Data acquisition

2.2.1 Remote sensing data and image preprocessing

Due to the complex habitat information and seasonal succession of estuary wetlands, three Sentinel-2 images and three Landsat-8 images taken under clear weather conditions were selected to test their ability in discriminating between native species and invasive species. The Sentinel-2 satellite images used in this study were recorded on May 27, 2018, September 09, 2018, and October 19, 2018. In order to reduce the differences caused by different times, Landsat-8 satellite images with times similar time to those of the Sentinel-2 images were selected, and images were used that were produced on May 28, 2018 (Sentinel-2A), September 17, 2018 (Sentinel-2B), and October 19, 2018 (Sentinel-2B). The data is introduced in Surppporting Imformation part2(S2).

2.2.2 Sample data and validation data

The quality of the sample data is directly related to the accuracy of the native and invasive species extraction (Liu et al. 2017). A typical and representative pure pixel was selected as the sample. The data in this study mainly come from on-site field surveys and visual interpretation based on high-resolution images such as those from Google Earth. From 2017 to 2018, the experimental group conducted a detailed survey of the Yellow River Delta Nature Reserve, using high-precision GPS to locate, record, and photograph different vegetation communities. At the same time, a part of the native and invasive species was visually interpreted indoors through Google Earth software and the MAP WORLD website. The above two parts of the data constitute the sample data and verification data for the establishment of the classifier and the accuracy verification. The distribution and specific sources of sample data and verification data are shown in Table 1.
*Methods are introduced in the S3.