Mangrove Restoration for Coastal Protection
Authors:   , van Bijsterveldt Celine Euphemia Johanna 
Published: June 2023

The world’s population continues to grow, and at the same time, the oceans are rising, posing challenges for those living near coastal areas. Almost two-thirds of the global population resides within 50 kilometers of the coast. Mangrove forests offer solutions to mitigate the potential for flooding and provide natural coastal protection. Traditionally, mangrove restoration has been approached from the seaward edge through large-scale planting, with limited success. This dissertation describes how mangrove restoration can be approached from both the seaward and landward edges along degraded coastlines to create functional mangrove forests.
Chapter 2 examines how the natural factors drive mangrove dynamics along the landward and seaward edges of a mangrove greenbelt. This work reveals that landward mangrove expansion in old aquaculture ponds was slow because of long inundation times and soft sediment. In contrast, seaward mangrove expansion was associated with elevated mudflats and gentle slopes. Chapter 3 further investigates the effects of different foreshore morphologies on mangrove expansion. This work shows that cheniers, or elevated sand lenses atop muddy foreshores, significantly decrease waves that enter the mangrove fringe. A stable chenier can promote mangrove expansion, even in combination with a negligible small mudflat of 10 to 20 metres. Without a chenier, a 10 times wider mudflat was required to make mangrove expansion more likely than mangrove retreat.
Chapter 4 compares traditional mangrove planting with ecological restoration measures for mangrove restoration (EMR). This work shows that EMR measures were more effective at creating connected forest expansion in a landward direction. EMR in the landward direction could be achieved through smart sluice gate management in the right season. This work shows how mangrove cover in associated mangrove aquaculture ponds significantly increased over the project lifetime. Along the seaward edge, field experiments revealed that EMR with permeable brushwood dams aimed at expanding mangrove habitat could not sufficiently overcome subsidence rates, indicating that mangrove restoration in seaward direction under relative sea level rise will become increasingly difficult.
Chapter 5 investigates the potential threat of plastic waste to mangroves once they have been restored. It is revealed that aerial root suffocation by plastic caused immediate pneumatophore growth and leaf loss. Mangrove trees are relatively resilient to partial burial by plastic waste, but mangrove stands are likely to deteriorate if plastic continues to accumulate. Chapter 6 examines the long-term mangrove greenbelt resilience in the face of continued rapid relative sea level rise. The study quantified that while the villages closest to the subsidence epicenter sank with 8.2 cm per year, their fringing mangroves sank nearly twice as slow on average. Trees were able to adjust to the rapidly rising water level through pneumatophore extension and the creation of new root mats when sediment was locally available. However system wide sediment shortage ultimately leads to a retreating coastline, affecting both mangroves and people.
In conclusion, seaward mangrove restoration is challenging along coastlines that are subject to rapid relative sea level rise. Foreshore management aimed at creating artificial cheniers or wide mudflats might reduce lateral erosion and lead to limited seaward mangrove expansion in less rapidly subsiding areas. It is also important to consider the landward edge of a desired greenbelt when attempting mangrove restoration, as hectares of potential mangrove habitat still exist in the form of other land-uses. Effective socio-economic measures are required to support mangrove expansion; these may be challenging, but not impossible. Ultimately, interventions that halt regional and global stressors of mangrove establishment and survival will have the most significant impact and long-term effect on coastal protection.