Abstract
Understanding the recent changes in mangrove adjacent to mega-cities is critical for conservation, management, and policymaking in coastal zones with fast population growth and global change. Here we investigated mangrove area changes in one of the world’s largest urban areas near the main estuaries in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA). Mangrove area changes are quantified for the period 1990–2018 by analyzing multiple sources of satellite images by classification algorithms. We found that estuarine mangrove area dynamics are driven by human actions and are contrasting between these two periods. (1) During 1990–2000, the estuarine mangrove area approximately decreased from 11.5 to 6.9 km2, among which the bulk part was transformed into aquaculture ponds (41.1%) and built-up area (29.9%). (2) During 2000–2018, the estuarine mangrove area rapidly increased to 18.3 km2 resulting from the protection and restoration efforts. Proportions of mangrove occurring in nature reserves increased from 37.5% in 1990 to >80% in the 2000s. Two major mangrove expansion ways, natural establishment (NE, in protected areas without any human interference) and human afforestation (HA) accounted almost equally (53.1 and 46.9%) for the gained estuarine mangrove area during 2010–2018. A future projection according to the current mangrove increasing rate suggests that all the low-lying land that is theoretically suitable for mangrove afforestation would be used up by 2060. Although afforestation has contributed to important gains in mangrove quantity, we highlight that it may also imply decreased habitat quality. It has resulted in a great occupation of high tidal mudflats and a loss of their valuable ecosystem services, and it may lead to spreading of non-native species, e.g., Sonneratia apetala, used in afforestation programs. Future restoration approaches should adopt more eco-friendly strategies, like reversing (abandoned) aquaculture ponds to native mangrove forests. Knowledge obtained from the GBA in this study may be also instrumental to ecological restorations for mangrove forests in other urbanized estuaries.
Highlights
Mangroves are important ecosystems in tropical and subtropical coastal zones, providing invaluable ecosystem services, such as biodiversity, fishery, carbon storage/sequestration, and shoreline stabilization (Chen et al, 2012; Alongi, 2014; He et al, 2018; Ouyang and Lee, 2020; Wang et al, 2020)
The total area of mangrove decreased to 6.9 km2 in 2000, and it rebounded to 13.0 km2 in 2010 and kept increasing to 18.3 km2 by 2018
The total mangrove area had a net increase of 6.8 km2 (59%) during 1990–2018
Summary
Mangroves are important ecosystems in tropical and subtropical coastal zones, providing invaluable ecosystem services, such as biodiversity, fishery, carbon storage/sequestration, and shoreline stabilization (Chen et al, 2012; Alongi, 2014; He et al, 2018; Ouyang and Lee, 2020; Wang et al, 2020). Many mangrove habitats around mega-cities like Singapore (Lai et al, 2015), and densely populated deltas like the Mekong Delta, Vietnam (Veettil et al, 2019), Ganges Delta in India (Pramanik, 2014), and Pearl River Delta (PRD), China (Ai et al, 2019a), have been reported with rapid loss and degradation. Such rapid mangrove area loss and associated weakened functions have called for an urgent need for mangrove restoration. It is essential to understand the roles of human activities in mangrove dynamics, which may guide future policies on conservation and management of coastal wetland ecosystems
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