Quantifying the Long-Term Expansion and Dieback of Spartina Alterniflora Using Google Earth Engine and Object-Based Hierarchical Random Forest Classification

Abstract

The overwhelming spread of Spartina alterniflora (smooth cordgrass) over recent decades has put many native plant communities and coastal environments at risk. Therefore, longterm monitoring of S. alterniflora dynamics is necessary to better understand and manage the invasion of the species. However, it is difficult to map Spartina saltmarshes in China on an annual or multiyear epoch basis. To address this issue, we developed a classification approach integrating Google Earth Engine (GEE) and object-based hierarchical random forest (RF) classification, and we applied this approach to quantify the expansion and dieback of S. alterniflora at Dafeng Milu National Nature Reserve, Jiangsu, China during 1993-2020. Results showed that the area of S. alterniflora expanded from 24.48 ha in 1993 to 1,564.96 ha in 2010. However, after ecological hydrological engineering and an increase in Elaphures davidianus (Pè re David's deer) numbers in 2011, the S. alterniflora area decreased significantly to 944.28 ha in 2020. During 2011-2020, the S. alterniflora area decreased substantially at a rate of 67 ha per year and by 86% in one area studied in Dafeng Milu National Nature Reserve. In 2020, the 944.28 ha of S. alterniflora in the reserve was mainly distributed in mudflats by the sea. Overall, these results show that it is feasible to identify S. alterniflora using the GEE platform and object-based hierarchical RF classification; moreover, this approach could improve understanding and management of this invasion species.

Examining the expansion of Spartina alterniflora in coastal wetlands using an MCE-CA-Markov model

“…The DMNNR was considered to be the first and largest wildlife nature reserve in the world for Elaphurus davidianus (Père David's deer). The Ramsar Wetland Convention Organization designated the DMNNR as a wetland of international importance, in 2002 (Lu et al, 2018;Yan et al, 2021). The present study took the third core area of DMNNR and its tidal flat as the research area (25 km 2 ), which silted up in the last 30 years (Figure 1).…”

“…A total of 2.5 km of small dykes and 10 km of artificial freshwater ditches were built to meet the needs of various wild animals and to control the spread of S. alterniflora (Figure 1). DMNNR reported that the number of wild E. davidianus individuals increased from 9 in 1999 to 1820 in 2020, which resulted in a decrease in S. alterniflora by foraging and trampling (Yan et al, 2021).…”

“…Because Spartina alterniflora (S. alterniflora) is adaptable and has a high diffusion capacity, it has expanded extensively along the coast of China. Indeed, it has been one of the most widespread invasive plants in coastal wetlands (Liu, 2018a;Liu et al, 2018b;Yan et al, 2021;Yan et al, 2022). Tremendous researches has demonstrated that the invasion of S. alterniflora has resulted in severe ecological crises through its reshaping of the structure and function of the original coastal wetland ecosystem (Grosholz et al, 2009;Page et al, 2010).…”

“…Consequently, accurate modelling and forecasting of the temporal and spatial dynamics of S. alterniflora expansion in the future is urgently needed. Moreover, anticipating the expansion of S. alterniflora in coastal wetlands will be a significant step towards the protection and ecological management of coastal ecosystems (Luan et al, 2020;Yan et al, 2021).…”

“…Numerous studies have shown that tidal inundation and soil salinity are the key factors affecting the ecological characteristics of S. alterniflora (Burns, 2011;Moffett and Gorelick, 2016;Luan et al, 2020). Yan et al (2022) showed that elevation and soil salinity are the main internal environmental drivers affecting the growth of S. alterniflora, and the number of elk and artificial ditches are the external factors affecting the growth of S. alterniflora (Yan et al, 2021). Therefore, fully considering the natural and human factors affecting the expansion of S. alterniflora is important for examining the expansion of S. alterniflora.…”