Mangrove Response to the Continuous Disturbance by Multiple Typhoons on the Qi'ao Island, Zhuhai, Using Sentinel-2 Remote Sensing Images

Mangroves are typical ecosystems located in the intertidal zone of tropical and subtropical coasts that protect the area against wave action, promote accretion in river estuaries, and act as an important global carbon sink. However, several natural mangroves have recently degraded due to climate change and human activities, increasing the risk to coastal areas' ecological environments. The mangroves on Qi'ao Island in Zhuhai, China, have been destroyed by human activities and the invasion of the exotic species Spartina alterniflora. In 1999, the Zhuhai municipal government imported pioneer species, such as Sonneratia apetala, to Qi'ao Island to restore the degraded mangroves, which have now evolved into a sufficiently continuous community with a stable area since 2015 with S. apetala as the dominant species. However, because the intensity and frequency of typhoon landfalls are increasing under global warming, dominant species in the mangroves of Qi'ao Island, S. apetala, is more vulnerable to typhoon disturbance due to its height and low trunk density, leading to greater uncertainty regarding their ecological functions on Qi'ao Island. Existing studies have mainly focused on the damage and recovery patterns of mangroves by a single typhoon, having seldom analyzed the response process of mangroves to consecutive disturbances by multiple typhoons; thus, these studies cannot be used to predict the succession process of mangroves with increasing typhoon frequency. In this study, the damage and recovery patterns of mangroves on Qi'ao Island were studied under the disturbance of three consecutive typhoons (2016 NIDA, 2017 Hugo, and 2018 Mangkhut), and their influencing factors were discussed based on the Normalized Difference Vegetation Index (NDVI) using Sentinel-2 remote sensing images. The results showed that (1) the NDVI of mangroves on Qi'ao Island decreased after the typhoon, and the damage levels could be classified as mild or mild to moderate. Moreover, the severity of the damage was relatively small. (2) An annual legacy effect existed across multiple typhoons. The recovery process of mangroves generally followed a circulating succession mode after typhoon disturbances in the order of more severe damage→slow recovery→milder damage→rapid recovery→ more severe damage pattern. (3) The damage levels of mangroves on Qi'ao Island increased with typhoon intensity. However, they decreased if S. apetala recovered slowly before the typhoon. Thus, the recovery pattern was mainly influenced by the damage levels of S. apetala after the typhoon landing and was not largely dependent on the hydrothermal conditions at regional scales. The results suggest that mangroves on Qi'ao Island, mainly consisting of S. apetala, have evolved to adapt to the consecutive typhoon disturbances, which has promoted their succession from plantations to near natural forests and the formation of a stable typhoon-resistant ecosystem