Decadal Changes in Mangrove Extent, Structure, and Anthropogenic Pressures at a Southern Range Limit

Abstract

Understanding how mangroves respond to natural and anthropogenic pressures at global distributional range limits informs protection and restoration strategies. Mangroves along the South African coastline are at one of the most southerly global distribution limits. These endangered ecosystems have been studied for more than 25 years to determine their vulnerability and responses to global climate change. This study assessed the drivers of change in mangrove area cover, species composition and population structure between 2011 and 2021 in 17 estuaries. There was a net increase in mangrove cover over this 10-year period of 3 ha and this was attributed to natural regeneration along tidal sand banks and into areas previously covered by salt marsh. Bruguiera gymnorrhiza was the only mangrove tree species found in all the estuaries, Avicennia marina occurred in 71% and Rhizophora mucronata in 47% of all estuaries. Major anthropogenic pressures that reduced mangrove cover and caused shifts in population structure were wood harvesting, livestock browsing and trampling. Anthropogenic pressures have persisted since 2011 resulting in structural mangrove degradation indicated by a decrease in seedling and sapling density and increase in canopy gaps. These results provide input to the National Biodiversity Assessment and are relevant to the implementation of the Global Biodiversity Framework informing site specific restoration strategies. The research also informs global studies on climate-driven range shifts; mangrove expansion is occurring, but this is slow and influenced by local pressures. The study recommended that adaptive management and monitoring frameworks are used to track mangrove changes.