The large-scale conversion of natural mangroves to aquaculture reduces species richness and diversity. Large areas of abandoned aquaculture ponds in areas where mangroves formerly predominated in China and southeast Asia represent important potential effective targets for mangrove restoration. Here, we empirically assessed the α-diversity (species richness) and β-diversity (variation in community composition) of mangrove, macrobenthos, fish and waterbird in a tropical mangrove bay on Hainan Island, China. We compared sites subjected to different pond-to-mangrove restoration programs more than 20 years ago (passive restoration without planting and active restoration with planting) to nearby reference site with natural mangrove forests and mudflats. To better understand how β-diversity responds to restoration, we also distinguished between β-diversity turnover and nestedness (richness difference). In general, α-diversity values for both fish and waterbird communities and β-diversity values for the mangrove, macrobenthos and waterbird communities were lower at the restoration sites than at the reference site, suggesting that the strong homogenizing effects of anthropogenic habitat alternation were still apparent after more than 20 years since aquaculture ceased. In addition, spatial turnover, not nestedness, dominated total β-diversity both across the whole study area and at individual sites, suggesting that multiple processes, such as environmental filtering, helped to shape multi-taxa community structures. Moreover, we found no evidence that planting in the abandoned ponds, in addition to standard hydrological restoration, supported greater species diversity of taxa like macrobenthos and waterbird than the naturally regenerated site after more than 20 years' recovery. Our results underline the importance of avoiding the conversion of natural mangrove stands to aquaculture wherever possible and the urgent need to design effective mangrove restoration techniques in tropical Asia.

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Climate change is altering species’ range limits and transforming ecosystems. For example, warming temperatures are leading to the range expansion of tropical, cold-sensitive species at the expense of their cold-tolerant counterparts. In some temperate and subtropical coastal wetlands, warming winters are enabling mangrove forest encroachment into salt marsh, which is a major regime shift that has significant ecological and societal ramifications. Here, we synthesized existing data and expert knowledge to assess the distribution of mangroves near rapidly changing range limits in the southeastern USA. We used expert elicitation to identify data limitations and highlight knowledge gaps for advancing understanding of past, current, and future range dynamics. Mangroves near poleward range limits are often shorter, wider, and more shrublike compared to their tropical counterparts that grow as tall forests in freeze-free, resource-rich environments. The northern range limits of mangroves in the southeastern USA are particularly dynamic and climate sensitive due to abundance of suitable coastal wetland habitat and the exposure of mangroves to winter temperature extremes that are much colder than comparable range limits on other continents. Thus, there is need for methodological refinements and improved spatiotemporal data regarding changes in mangrove structure and abundance near northern range limits in the southeastern USA. Advancing understanding of rapidly changing range limits is critical for foundation plant species such as mangroves, as it provides a basis for anticipating and preparing for the cascading effects of climate-induced species redistribution on ecosystems and the human communities that depend on their ecosystem services.

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