Abstract
This study mapped the areal extent, identified the species composition, and analyzed the changes of salt marshes in the intertidal zone of China during the period 1985–2019. With the aid of the cloud platform of the Google Earth Engine, we selected Landsat 5/8 and Sentinel-2 images and used the support vector machine classification method to extract salt marsh information for the years of 1985, 1990, 1995, 2000, 2005, 2010, 2015, and 2019. Seven major species of salt marshes: Phragmites australis , Suaeda spp., Spartina alterniflora , Scirpus mariqueter , Tamarix chinensis , Cyperus malaccensis , and Sesuvium portulacastrum were identified. Our results showed that salt marshes are mainly distributed in Liaoning, Shandong, Jiangsu, Shanghai, and Zhejiang, with varying patterns of shrinking, expansion, or wavering in different places. The distribution of salt marshes has declined considerably from 151,324 ha in 1985 to 115,397 ha in 2019, a drop of 23.7%. During the same period, the area of native species has dropped 95.4% from 77,741 ha to 3,563 ha for Suaeda spp. and 45.1% from 60,511 ha to 33,193 ha for P. australis ; on the contrary, the area of exotic species, S. alterniflora , has exhibited a sharp rise from just 99 ha to 67,527 ha. For the past 35 years, the driving factors causing salt marsh changes are mainly land reclamation, variations in water and sand fluxes, and interspecific competition and succession of salt marsh vegetation. These results provide fundamental reference information and could form the scientific basis for formulating policies for the conservation and utilization of salt marsh resources in China.
Highlights
Salt marshes are often found in the transitional zone between terrestrial and marine environment, which are inhabited by halophyte communities [1], and they grow on a variety of substrates, ranging from silt to gravel [2]
Information on the spatial distribution, areal extent, and species composition of salt marshes across a wide range of time is of critical importance to governmental agencies, science communities, and citizens, whereas despite national and international concerns, few data of salt marshes in China have been publicly available at national scale [12, 13]
The resulting information deficiency has limited decision-making regarding the sustainable ecosystem management of coastal wetlands and the socioeconomic development of coastal regions. In view of these considerations, the goal of the present study is to generate the detailed information on salt marshes, including native and exotic species during 1985– 2019 in the intertidal zone of China by using a quantitative approach based on Landsat, Sentinel-2, and unmanned airborne vehicle (UAV) imagery
Summary
Salt marshes are often found in the transitional zone between terrestrial and marine environment, which are inhabited by halophyte communities [1], and they grow on a variety of substrates, ranging from silt to gravel [2]. As one of the important Blue carbon ecosystems, salt marsh has great capacity in carbon storage and sequestration [3], playing a critical role in global change mitigation/adaptation. Despite such important functions and benefits, global salt marshes have been declining due to land development, reduced sediment supply, and/or sea-level rise, etc. Information on the spatial distribution, areal extent, and species composition of salt marshes across a wide range of time is of critical importance to governmental agencies, science communities, and citizens, whereas despite national and international concerns, few data of salt marshes in China have been publicly available at national scale [12, 13]
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