Abstract: Global warming has led to an increase in the frequency of strong typhoons, posing a challenge to the stability of lime-sand islands. The South China Sea(SCS) is located in the western Pacific Ocean, the most typhoon prone regions in the world, and the stability of the lime-sand islands in SCS will also be challenged by future typhoon intensification. However, the response characteristics of the SCS lime-sand islands to typhoons and the factors leading to the differences in their responses remain poorly understood. In this study, we explored the changing characteristics of spatial patterns of lime-sand islands in the Xisha Islands under the impacts of typhoons and their driving factors, which are of great significance to the ecological protection and sustainable development of lime-sand islands in the SCS. First, to extract the area of the BeiDao, ZhongDao and Nandao islands of Xisha Islands and their beaches, the toe of beach line (ToB) and the boundary of vegetation were extracted using the gradient vector flow-snake(GVF-snake) model based on Sentinel-2 remote sensing images. According to the geomorphological characteristics of the various areas of the islands and the distribution on the reef flats, the islands were divided into the north tip, the south tip and the oceanward section.Afterwards, the movement of boundary lines affected by typhoons was calculated using the digital shoreline analysis(DSAS) system. The monthly average area of the study islands from 2016 to 2022 was examined using the change point detection method, and the Typhoon Doksuri in 2017, Typhoon Vamco in 2020, and Typhoon Noru in 2022 were identified caused significant changes in the size of the study islands. Finally, the spatial and temporal characteristics of islands affected by typhoons were analyzed based on the changes in lime-sand islands area, vegetation area, beach area and normalized difference vegetation index(NDVI) during typhoon events. The results show that: 1) The vegetation of the islands was mainly slightly stressed by the typhoon, and the severely stressed areas were mainly located along the shorelines. The typhoons caused a significant retreat of the vegetation boundary of the islands, and the sediments were stripped off with the damaged vegetation and washed to the beach with the wind and waves, resulting in the seaward advancement of the ToB lines. The windward coasts were the first to be impacted by the direction of the typhoon path, and the vegetation along the windward side of islands was heavily damaged, resulting in more significant changes in vegetation, vegetation boundary, and ToB lines on the windward side. 2) As the vegetation along the lime-sand islands was damaged, the retreat of the vegetation boundary line and the seaward advancement of the ToB line resulted in the expansion of the beach area, whereas after the typhoon, due to the loss of vegetation fixation, the loose sediment accumulated on the beach was transported away from the islands to the reef flats by the wind and wave action, resulting in the reduction of the beach area. Therefore, coastal vegetation that buffers and fixes sand was critical to the stability of lime-sand islands. 3) Increased typhoon wind speeds and shorter distances between typhoon paths and islands can lead to an increase in the area of severely damaged vegetation on the lime-sand islands, resulting in greater shoreline movement and changes in beach area. In addition, typhoons can cause sediment transport between the lime-sand islands and their surroundings, which can directly change the beach area and morphology of the lime-sand islands. The loss of large amounts of unconsolidated sediments on the lime-sand islands after the typhoon, which led to an abrupt decrease in the size of the islands, together with the decrease in the rate of biogenic carbonate production due to global warming and the insufficient supply of sediments from the surrounding environment, may be the reasons for the long-term decreasing trend in the size of the lime-sand islands in the Xisha Islands.

Key words: lime-sand island, typhoon, remote sensing, Xisha Islands