Mangrove restoration is the regeneration of mangrove forest ecosystems in areas where they have previously existed. The practice of mangrove restoration is grounded in the discipline of restoration ecology, which aims to “ the recovery of resilience and adaptive capacity of ecosystems that have been degraded, damaged, or destroyed”. Since environmental impacts are an ongoing threat, to successfully restore an ecosystem implies not merely to recreate its former condition, but to strengthen its capacity to adapt to change over time.
Environmental context
Mangrove forests, along with the animal species they shelter, represent globally significant sources of biodiversity and provide humanity with valuable ecosystem services. They are used by mammals, reptiles and migratory birds as feeding and breeding grounds, and provide crucial habitats for fish and crustacean species of commercial importance. The roots of the mangrove physically buffer shorelines from the erosive impacts of ocean waves and storms. Additionally, they protect riparian zones by absorbing floodwaters and slowing down the flow of sediment-loaded river water. This allows sediments to drop to the bottom where they are held in place, thus containing potentially toxic waste products and improving the quality of water and sanitation in coastal communities. To the human communities who rely on them, mangrove forests represent local sources of sustainable income from the harvest of fish and timber, as well as non-timber forest products such as medicinal plants, palm leaves and honey. On a global scale, they have been shown to sequester carbon in quantities comparable to higher-canopy terrestrial rainforests, which means that they may play a role in climate change mitigation, in addition to physically protecting coastlines from the projected sea-level rise associated with climate change. However, there are limits to the capacity of mangroves to adapt to climate change. It is projected that a 1-meter rise in sea level could inundate and destroy mangrove forests in many regions around the globe, which would leave coastal communities vulnerable to the risks of flooding, shoreline erosion, saline intrusion and increased storm activity.
Mangrove loss and degradation
The issue of restoration is critical today since mangrove forests are being lost very quickly – at an even faster rate than tropical rainforests inland. A recent estimate puts the total mangrove area worldwide in 2005 at 152,000 km2 – down from 188,000 km2 in 1980. In other words, some 36,000 km2, or nearly 20% of the world's mangroves, were lost over a period of twenty-five years. Other estimates of loss may differ due to having been drawn from a smaller pool of data. The Millennium Ecosystem Assessment estimates the total loss worldwide at 35% between 1980 and 2000, but this result was drawn from data on only slightly more than half of the total mangrove area. Much of this lost mangrove area was destroyed to make room for industry, housing and tourism development; for aquaculture, primarily shrimp farms; and for agriculture, such as rice paddies, livestockpasture and salt production. Other drivers of mangrove forest destruction include activities that divert their sources of freshwater, such as groundwater withdrawals, the building of dams, and the building of roads and drainage canals across tidal flats.
Mangrove restoration
Mangroves are sensitive ecosystems, changing dynamically in response to storms, sediment blockage, and fluctuations in sea level and present a “moving target” for restoration efforts. Different restoration approaches face this challenge in different ways. The most common method simply consists in planting single-species stands of mangroves in areas thought to be suitable, without consideration of whether or not they supported mangroves in the past. This approach usually fails over the long term because the underlying soil and hydrological requirements of the mangroves are not being met. More informed methods aim to bring a damaged mangrove area back into its preexisting condition, taking into account not only ecosystem factors but also social, cultural and political perspectives. These approaches begin with the understanding that a damaged mangrove area may be able to repair itself through the natural processes of secondary succession, without being physically planted, provided that its tidal and freshwater hydrology is functioning normally and there is an adequate supply of seedlings. Taking this into account, it becomes crucial to the success of a restoration project to evaluate what the hydrology of a disturbed mangrove site should look like under normal conditions, and the ways in which it has been modified. One example of this approach is the Ecological Mangrove Restoration method which recommends the following steps, to be undertaken using healthy mangroves of the surrounding area as a reference:
Assess the ecology, especially reproduction and distribution patterns, of the mangrove species at the disturbed site;
Map the topographical elevations and hydrological patterns that determine how seedlings should establish themselves at the site;
Assess the changes made to the site that currently prevent the site from recovering by itself;
Design a restoration plan that begins by restoring the normal range of elevations and tidal hydrology at the site; and
Monitor the site to determine if the restoration has been successful in light of the original objectives.
The actual planting of seedlings is a last resort, since it fails in many cases; it should be considered only if natural recruitment of seedlings fails to reach the restoration objective.
Mangroves as climate change mitigation
Mangrove forests have a potential to mitigate climate change, such as through the sequestration of carbon from the atmosphere directly, and by providing protection from storms, which are expected to become more intense and frequent into the 21st century. A summary of coastal wetland carbon, including mangroves, is seen in the accompanying image. Wetland plants, like mangroves, take in carbon dioxide when they perform photosynthesis. They then convert this into biomass made of complex carbon compounds. Being the most carbon-rich tropical forest, mangroves are highly productive and are found to store 3 to 4 times more carbon than other tropical forests. This is known as Blue carbon. Mangroves make up only 0.7% of tropical forest area worldwide, yet studies calculate the effect of mangrove deforestation to contribute 10% of global CO2 emissions from deforestation. The image to the right shows the global distribution of above ground carbon from mangroves. As can be seen, most of this carbon is located in Indonesia, followed by Brazil, Malaysia and Nigeria. Indonesia has one of the highest rates of mangrove loss, yet the most carbon stored from mangroves. Therefore, it is suggested that if the correct policy is implemented, countries like Indonesia can make considerable contributions to global carbon fluxes. The UN estimate deforestation and forest degradation to make up 17% of global carbon emissions, which makes it the second most polluting sector, following the energy industry. The cost of this globally is estimated to total $42 billion. Therefore, in recent years, there has been more focus on the importance of mangroves, with initiatives being developed to use reforestation as a mitigation tool for climate change.
Reducing Emissions from Deforestation and Forest Degradation
In 2008, the United Nations launched the "Reducing Emissions from Deforestation and forest Degradation " program to combat climate change through the reduction of carbon emissions and enhancement of carbon sinks from forests. It is the opinion of literary scholars that the REDD program can increase carbon sequestration from mangroves and therefore reduce carbon in the atmosphere. The REDD+ mechanism, as part of the REDD program, provides financial support to stakeholders in developing countries to avoid deforestation and forest degradation. The estimated impacts of REDD+ globally, could reach up to 2.5 billion tons of CO2 each year. An examples of REDD+ implementation can be seen in Thailand, where carbon markets give farmers incentive to conserve mangrove forests, by compensating for the opportunity cost of shrimp farming.
Moreover, the Mangroves for the Future initiative, led by IUCN and UNDP, encourages the rehabilitation of mangroves by engaging with local stakeholders and creating a platform for change. In Indonesia, one project planted 40,000 mangroves, which then encouraged local government to take up similar initiatives on a larger scale. Mangrove restoration and protection is also seen as a climate change mitigation strategy under COP21, the international agreement to target climate change, with countries being able to submit the act in their Nationally Appropriate Mitigation Approaches. Ten of the world's least developed countries are now prioritizing mangrove restoration in their NAMAs.
Climate Change Adaptation
As well as providing the benefit of a carbon sinks, mangroves also host potential for climate change adaptation. They provide protection to local communities from sea level rise, coastal erosion and storms. These are all issues that are related to climate change and are expected to increase in severity in the future. Therefore, mangroves can help support the livelihood of those living in areas already vulnerable to climate change threats. In the IPCC AR5 report, the potential of ecosystem-based adaptation to climate change is discussed, which includes the restoration of mangroves. An example of this can be seen in Bangladesh, where the government initiated the plantation of 50,000 hectares of mangrove forest to stabilize coastal areas, in an attempt to tackle increasing erosion. Evidence suggests that this initiative was successful in increasing accretion of coastal sediment, which reduced coastal erosion in this area and protects coastal communities from flooding and storm events. It has also been found that areas surrounding mangrove forests are subject to less damage from cyclones than non forested areas.
Additional considerations
An important but often overlooked aspect of mangrove restoration efforts is the role that the local communities play as stakeholders in the process and the outcome. Since they may directly feel the effects of restoration projects, they should be involved in the process as much as feasibly possible, from decision-making to maintenance over the long term. Their involvement and local knowledge, as well as collaboration with other stakeholders such as sponsors and governing agencies, is crucial to the success of restoration projects.
Limitations
In some areas, restoration may be prohibitively difficult due to the degradation of the soil that regularly follows the clear-cutting of mangrove forests. Common effects include advanced erosion of the soil, loss of nutrients, high levels of salinity, and/or buildup of toxins. However, even without this extent of degradation, the soil may become unable to host plant life at all due to the loss of the live mangrove roots, which exuded oxygen and carbohydrate into the soil and maintained its quality. Using foresight early in the restoration process to carefully select sites that are likely to succeed as self-maintaining ecosystems, as well as ensuring that proper management is built into the conservation effort, can prevent the waste of time and energy that often accompanies restoration projects. The long-lasting aftereffects of mangrove degradation underscore the importance of eliminating its causes, since once sites are cleared, it is difficult for them to recover without a scientific intervention.
