Mangrove conservation and restoration

Mangroves are trees or large shrubs which are salt-tolerant and grow in intertidal zones in tropical and subtropical regions (ref). They form dense forests along many tropical and subtropical coasts, are found in 123 countries and territories and are estimated to cover over 150,000 square kilometres globally (ref).


© Peter Prokosch / grida.no

Mangroves form two groups known as true mangroves and associate mangroves. True mangroves are highly adapted to the intertidal zone where all or part of them are regularly submerged in saltwater. The length of inundation tolerated varies between true mangrove species. Globally, there are 69 species in 27 genera, belonging to 20 families that are considered as true mangrove species (ref). Plant species other than the true mangroves are known as associate mangroves and include species such as Hibiscus tiliaceus (Var/Cotton Tree) or Acrostichum aureum (Fouzer Lanmar/Mangrove Fern). These are species found in mangrove forests and should be included as mangrove species for the purposes of conservation management and can greatly increase the potential of mangrove forests as an ecosystem-based adaptation (EBA) approach. They are also highly adapted to salty conditions, but less so than true mangroves, and will only tolerate infrequent inundation by saltwater during extremely high tides or wave run-up due to storm events.

Common on-site approaches to mangrove conservation and restoration

A common approach to mangrove conservation, as with conservation of many ecosystems, involves establishing protected areas that reduce anthropogenic pressures. Mangrove restoration often involves reforestation using appropriate species, for example red mangrove Rhizophora spp. (ref). It usually follows three main stages: sorting propagules, supplying propagules to planters and planting. More details on the broader range of issues that can affect success are presented below. 


©Ronny Vossen Rasmussen

Mangrove conservation and restoration as an EBA measure

There is good evidence that, in the right circumstances, mangroves can help to reduce vulnerability to climate-related coastal hazards. As a result, mangrove restoration has been used as an ecosystem-based disaster risk reduction and adaptation measure, particularly following the 2004 Asian Tsunami, when many affected countries embarked on ambitious replanting programmes (ref).  

Mangroves provide coastal protection

Mangrove forests can reduce wave energy, erosion (ref), and storm surge water levels (ref) and by doing so mitigate coastal flooding (ref). This is particularly relevant in the context of climate change, as coastal flooding and erosion are predicted to increase in severity as sea levels rise.

Mangroves may keep pace with sea level rise

There is evidence that mangroves support soil stabilization and sediment capture, and are able to build up soil levels vertically (accrete) through formation of layers of peat (ref). This ability means that mangroves are able, in the right conditions, to keep pace with sea level rise (ref). However, mangrove tree health and the supply of incoming sediment are both important determinants of the scale of the accretion process (ref). By comparing accretion rates with projected rates of sea level rise, it becomes clear that in some locations there will be a threshold beyond which mangroves will not be able to keep up with the rising sea levels (ref).


©Peter Prokosh / grida.no

Mangroves may provide a refuge from ocean acidification

It has been recently been found that mangroves may provide a refuge for coral reef species from climate change. Scleractinian corals were found growing in shaded colonies on mangrove prop roots, which provided a refuge from thermal and acidification stress (ref). It is uncertain whether mangroves themselves will be directly affected by ocean acidification (ref). However, there are potential negative indirect effects if coral reefs, which in some cases supply sediment to mangrove systems, are affected so that the sediment supply is reduced (ref).  

Mangroves serve as a habitat and nursery ground for fish and other marine species, supporting fisheries and livelihoods

Many commercially important marine species utilize mangroves for some, or all, parts of their lifecycle, for example species of snapper, mullet, shrimp, crab and sharks (ref). Mangroves therefore support the income of coastal communities and potentially reduce their vulnerability to climate change, by increasing access to sources of income and nutrition (ref). Mangroves have been found to be highly beneficial as nursery habitats (ref), however, this is not ubiquitous and a number of features have a bearing on the nursery value. Beneficial features include connectivity with adjacent habitats such as coral reef and seagrass beds, large spatial extent and high diversity of tree species (ref).

Mangroves support diversified livelihoods

Mangroves supply a range of products that support livelihoods (ref). Wood is a particularly important mangrove product, with many coastal and indigenous communities relying on mangroves for timber and construction material, as well as for fuel. In addition, non-timber mangrove forest products can provide significant revenue through the provisioning of, for example, honey, dye, fodder, herbal remedies and fruits (ref).

Additional benefits

There may be additional benefits for biodiversity

The structural diversity of mangrove roots and their position at the interface between land and sea gives mangroves an important role as habitats for numerous species (ref). Mangroves provide habitats for threatened species, including the endangered Bengal tiger which occurs in the Sundarban mangrove ecosystem shared by India and Bangladesh (ref) and the critically endangered hawksbill turtle (ref). Mangroves have also been found to act as a refuge for corals from ocean acidification (ref).

In addition, mangroves provide a number of important benefits for surrounding habitats contributing to water quality and nutrient transfer. Mangroves filter and trap sediment from run-off and river water before it reaches adjacent ecosystems, reducing the turbidity of the water and allowing essential light to reach ecosystems (ref). Mangroves therefore contribute to the survival of these adjacent ecosystems and the species they support.


©Ronny Vossen Rasmussen

There may be additional benefits for carbon storage

On average, the carbon stock of one hectare of mangroves, including soil carbon, is approximately 1,000 tonnes, more than twice the carbon storage of upland forests and five times that of savannah (ref), meaning that mangroves are among the most carbon-rich forests in the tropics. Therefore, despite mangroves constituting less than 1% of the area of tropical forests globally, deforestation in these systems releases a disproportionate amount of carbon into the atmosphere as the carbon protected by mangroves is released. As a result, it is estimated that mangroves may be responsible for as much as 10% of all emissions from deforestation globally (ref). Conservation and restoration of mangroves can therefore contribute significantly to climate change mitigation. Their ability to trap organic sediment and thus store carbon is why mangroves, among other systems, are referred to as ‘blue carbon’ sinks. It should be recognised that older stands of mangroves generally have accumulated large amounts of peat below them and thus store more carbon, therefore providing greater climate change mitigation benefits. In addition, as mangroves age, they store proportionally more carbon in their biomass because of higher productivity (ref).  Protection of mangroves should, where possible, prioritise older stands. 


©Peter Prokosh / grida.no

Key issues that can affect success

Anthropogenic pressures

Approximately one quarter of the world’s mangrove cover has been destroyed (ref) and the rate of mangrove loss  is  still very high, estimated to be around 2 to 5 times higher than the average rate of loss for all forests (ref). Human activities, including conversion to aquaculture, coastal development, overexploitation of timber and pollution, have been the primary causes of mangrove loss (ref).

Climate change pressures

A variety of climate change related pressures are likely to affect mangroves, including increased storm frequency and severity, sea level rise and changes in species distributions (ref) with sea level rise potentially being one of the greatest climate related threats (ref). This threat is exacerbated if there is development directly inland of mangroves, as it is impossible for the system to move inland with sea level rise (ref).

Site and ecosystem characteristics

Mangrove forest width is an important determinant of the likely effectiveness of the system for EBA. While narrow bands of mangrove forests, between 40 and 80m, can slow storm surge water flows, relatively wide bands of mangroves (several hundred meters or wider) are needed to significantly reduce storm surge flooding. The protection received also depends on the structure of the mangrove forest with denser structures needed to support protection by these relatively narrow bands of mangroves (ref). In areas with gently sloping topography, even a small reduction in water level can result in a relatively large reduction in flood area. More information can be found in Spalding et al. 2014 in the Useful resources and materials section at the bottom of this page.


© Peter Prokosch / grida.no

Maximum biophysical thresholds

As with any ecosystem, there are tolerance thresholds beyond which mangroves will not survive.  These predominately relate to extreme storm events where mangroves can be destroyed or seriously degraded through defoliation, erosion, burial by sediment and uprooting by high winds (ref). In addition, exceeding water quality thresholds through high levels of, for example, salinity, heavy metals and chlorine can cause tree death (ref). Sea level rise and sediment availability vary by location and generalized tolerances are difficult to specify, however, there is a threshold of sea level rise beyond which the mangroves will no longer be able to keep pace. In some locations it has been estimated that a more than 5mm rise in sea level per year will mean that mangroves die off (ref). Globally, it is estimated that by 2100 there will be a loss of between 10 and 15% of mangrove habitat due to sea level rise (ref).

Timeframes

Wherever possible, preservation of existing mangroves is to be prioritised as biodiversity and ecosystem services values are generally lower in restored habitats when compared to intact areas. However, restoration can increase ecosystem services provision and therefore is worth considering where suitable locations are available (ref)

Planting is often used as a restoration technique, although, mangrove restoration can occur naturally in 15-30 years if the tidal hydrology of the site is not disrupted and if there is a good supply of waterborne seeds or seedlings. (ref). Successful restoration requires an understanding of the causes of mangrove loss. If the causes are not addressed, then re-establishment may not be effective. This is particularly important if one of the causes is hydrological change (ref).  Five critical steps for mangrove restoration can be found in UNEP-WCMC 2014 in the Useful resources and materials section at the bottom of this page.

Relevant policy context and developments

Common policy approaches for conserving mangroves include assessing existing policies and regulations for their adequacy in promoting sustainable management of coastal forests, bridging gaps between existing policies and implementation, development of forest rehabilitation plans, clarifying tenure and jurisdictions over mangrove areas, developing guidelines on incorporating forestry into coastal disaster management strategies and promoting best practices in collaborative coastal forest protection (ref). For more information on the importance of policy measures in adaptation, click here.

Ongoing management

Management of mangroves that have previously demonstrated resilience to climate stressors, and/or are naturally situated to survive global threats, will increase the resilience of mangroves as an EBA measure. Climate resilient management also includes controlling human stresses on mangroves by establishing inshore buffer zones to reduce impacts from adjacent land use and allow mangroves to migrate in response to sea level rise (ref) (for more details see McLeod 2006 in the Useful resources and materials section at the bottom of this page. For mangroves to contribute optimally to risk reduction, their conservation needs to be incorporated into broader coastal zone management and planning, including the protection of intact habitat in protected areas (ref). Community involvement is also key to successful mangrove management, protection and restoration (ref). 

On the ground implementation

Restoration efforts need to first consider the hydrology, nature of the substratum, planting period (e.g. middle of the rainy season for Rhizophora) and seedling quality before undertaking re-planting. Reforestation then involves three main stages, including sorting propagula, supplying propagules to planters, and planting. Subsequently, monitoring is required to evaluate success rates – see Sow 2012 in the Useful resources and materials section at the bottom of this page for helpful guidelines on reforesting mangroves using Rhizophora. For mangrove ecosystems, local conditions such as the availability of sediment and freshwater to compensate for increased salinity, will also aid mangrove survival and increase their resilience to sea-level rise.


©Sura Nualpradid / Shutterstock.com

Local community involvement

Although to some extent mangrove management requires specialised knowledge and equipment, many aspects of conservation and restoration can be implemented at the community level (ref), for example in re-planting activities and – following some training – post-planting monitoring, (ref). Community-led vegetation planting has had varying success, depending on local commitment, so awareness-raising campaigns may assist in promoting local efforts to protect mangroves (ref). Community ownership and sense of responsibility is important in long-term successful conservation and restoration efforts (ref). Creating policy measures which enhance the awareness, capacity and engagement across relevant stakeholders will support effective EBA outcomes. 

Useful resources and materials

Spalding et al. 2014. This practical guidebook summarises the findings of the reviews and provides practical management recommendations to coastal zone managers and policymakers. It includes three key elements: Understanding of the risk to the shoreline, the role of mangroves in risk reduction, and how to manage mangroves for coastal defence (including integrated solutions with other problems). Mangroves for Coastal Defence

Baba et al. 2013The report discussing different products that can be gathered from mangroves. Wood and non-wood products are examined through case studies. The different species of mangroves are referenced with their uses. There is a Pacific and South East Asia focus for the material but some examples are also highlighted from Africa and the Caribbean. Useful products from mangrove and other coastal plants

Primavera et al. 2012. Expertise and support for community base mangrove rehabilitation. It is focused on the Philippines but is applicable to other locations. If used in other locations, the native mangrove species should be used. More information on mangrove species native to different areas can be found: World Mangrove Atlas. Manual on Community-Based Mangrove Rehabilitation

McLeod. 2006. This report provides some considerations for conservation practitioners as they design conservation strategies for mangroves. These ideas build upon the concept of resilience and includes strategies, tools and methods for managers to promote resilience. Managing Mangroves for Resilience to Climate Change

Sow, 2012. Factsheet 6 provides guidance on reforesting mangroves using Rhizophora. Guide on adaptation options in coastal areas for local decision-makers: Guidance for decision-making to cope with coastal changes in West Africa.​

Wetlands International provides some good resources on mangrove restoration including the following two reports helping understand storm management by mangroves by McIvor et al. 2012: Storm Surge Reduction by Mangroves and Reduction of wind and swell waves by mangroves. In Natural Coastal Protection Series. 

UNEP-WCMC. 2014. Provides an overview of the Ecosystem Services available from mangroves, the threats faced and reviews conservation measures. The Importance of Mangroves to People: A Call to Action

Short introduction to mangroves and links to sources of data. Biodiversity A-Z – Mangrove Page

UNEP-WCMC. An online tool providing access to a range of datasets that are useful for informing decisions on conserving marine and coastal biodiversity. Users are able to view and download a range of data, including data on: the global distribution of coral reefs, seagrasses and mangroves, global seagrass species richness and data from the World Mangrove Atlas. Ocean Data Viewer

Mangrove Action Project. Illustrates five important steps that should be tailored to each unique situation and coastal region where mangrove restoration is being attempted. Mangroves Five Steps to the Successful Ecological Restoration of Mangroves

MS Swaminathan Research Foundation, India. Reflects the process and results of restoration activities carried out over seven years by the project Coastal Wetlands: Mangrove Conservation and Management and is meant for foresters, field technicians, researchers and others interested in restoration of degraded mangroves. Mangrove Forest Restoration in Andhra Pradesh, India