It is estimated that between 1980 and 2000, 180,000 ha of mangroves were degraded or destroyed each year. While the rate of loss decelerated to around 100,000 ha/yr in the 21st century, both mangrove conservation as well as rehabilitation are clearly imperative.
Unfortunately, most mangrove rehabilitation efforts world-wide fail to re-establish mangrove forests. The majority of these efforts are over-simplified planting projects, largely attempts to force mangroves to grow in intertidal mud-flats, usually below Mean Sea Level – where mangroves simply do not grow. This takes place for a pair of reasons:
1. Land tenure and ownership issues make it difficult to put mangroves back where they belong, that typically being areas that were converted due to unclear policy and inadequate prior management.
2. Poor understanding of the ecological requirements of mangroves, and the processes which lead to their establishment and early growth.
Our work combines an awareness of the socio-political problem of land tenure with issues of properly understanding the ecological underpinnings of successful mangrove rehabilitation from point (2). By understanding both pitfalls and recommended practices around mangrove rehabilitation, it is our hope that the rather simple, but enigmatic practice of restoring mangrove forests is approached more scientifically and rationally and that practitioners become more reflective about their actions.
Notes on terminology:
Ecological mangrove restoration (EMR) is defined as:
an approach to coastal wetland rehabilitation or restoration that seeks to facilitate natural regeneration in order to produce self sustaining wetland ecosystems.
EMR is a general approach (not a mandated method or sequence of steps), that is designed to provide a logical sequence of tasks that are likely to succeed in restoring or creating mangrove habitat with a diverse plant cover similar to that of a natural reference mangrove forest, with functional tidal creeks connected to upland freshwater flows if available, and supporting a diverse faunal community. All of the above is designed to persist over time without significant further human intervention. Planting of mangroves may be needed in certain circumstances, but in most cases we have studied, volunteer mangroves (not planted) will provide the diverse forest cover over time. The contemporary practice of EMR includes local stakeholder engagement and negotiation in order to achieve agreement about shared objectives and restoration methodologies.
The initial five steps of EMR was first published as an abstract of a presentation by Lewis and Marshall at the 1998 World Aquaculture Society meeting in Las Vegas, Nevada, USA. Further revisions were made by Lewis and Brown and Lewis. With the publication of an updated approach by Lewis the number of steps was increased to six with the addition of a “site selection” process. With this current updated version we have increased the number of steps to eight,but it is important to remember that these are only “suggested steps” and each EMR project is unique, with individual problems and opportunities.
For example, in many cases there is little or no real site selection process, but instead a local community may have a specific site already selected and need guidance in how to manage the EMR process. In this case, our current step 4 (select a site) is moot. All the authors understand this but also firmly believe that some guidance on a logical approach to successful EMR is essential as a starting point, since the history of mangrove forest management and restoration is replete with failed efforts. In fact, we all believe the failures far exceed the successes, with as much as 80-90% of the projects not meeting their goals (if any exist) or simply failing to establish a biodiverse ecologically functioning mangrove forest.
EMR focuses on removing the barriers which may prevent nature taking its course by interventions that restore or create the topography and hydrology which wetland plants and animals require. EMR may also intervene by delivering propagules and in specific cases provide complementary planting of species known to thrive in the particular conditions which exist.
EMR recognizes that wetland species are found in identifiable habitats and does not promote rehabilitation or establishment in locations where these conditions do not exist or cannot be created. EMR exists because its practitioners recognize the value of wetland goods and services to people. They also recognize that people are in most cases a part of the coastal environment and that unless these users agree to the perpetuation of the wetland it will be degraded or converted to alternate purposes, oftentimes dictated by wealthier individuals at the detriment to the public-at-large .
The term “restoration” has a very broad meaning. It generally follows the definition of Lewis:
Returned from a disturbed or totally altered condition to a previously existing natural or altered condition by some action of humans.
“Rehabilitation” is similar to restoration in that the goal is not to return to someprevious condition as defined by measurable reference criteria, but conversion of an altered wetland to some beneficial use as defined by locally agreed upon goals andcriteria. An example might be the conversion of abandoned aquaculture ponds to tidally influenced open water but not the mangroves that originally existed.
Rehabilitate or restore does not always indicate a return to some historical pre-human impact condition, nor a
return to the exact conditions that existed before some change occurred. Of course any attempt at pure restoration is bound to fail as the exact pre-human impact conditions of a mangrove forest are rarely known, so a proper measurable success criterion to define success of restoration could not be accurately determined if the goal is to produce a forest like those that existed hundreds of years ago. For this reason we emphasize quantitative comparison of a restoration or rehabilitation site to an adjacent reference site supporting more less natural mangroves characteristic of the local area.
Both restoration and rehabilitation may also mean returning a site with existing but stressed mangrove forest cover to a more hydrologically connected system prior to a die-off of trees occurring due to extended flooding, or hypersaline conditions. This approach requires being able to measure the existing hydrology and determine if stress is present and intervene prior to a mangrove die-off.
Another term of the art is “mangrove forest creation.” Creation generally refers to the conversion of uplands to wetlands. Planting mangroves on a mudflat that historically did not support mangroves is a similar type of effort, which could properly be called “mangrove forest afforestation.” Most mangrove afforestation efforts are not successful as documented by Samson and Rollon (2010) in the Philippines (but has worked in the Sudarbans and Guyana).Mangrove creation through excavation of uplands to mangrove wetland elevations and connecting these to tidal flow can work but is very expensive due to the excavation costs.
Finally, the term “mangrove forest enhancement” or “replenishment” is often used to describe some sort of planting within existing mangroves for the purpose of improving existing ecological conditions. These are also called “enrichment plantings.”
There is no scientific evidence that any of these enhancement efforts really enhance the existing functions or benefits of mangroves. Often they may actually further degrade the system, if for example, planting of mangroves takes place if areas within mangroves that are devoid of mangroves. Often these are important habitat areas for wading birds to feed for example, or they may be tidal flushing channels, and their planting while successful, may ultimate doom the forest to reduced tidal flooding, encroachment into channels and the final death of the forest due to human-induced hydrologic restrictions.
Although the above foray into differing terminology may seem like an exercise in semantics, it is important that appropriate terminology be used around mangrove rehabilitation. Oftentimes, a single guiding document will determine the fate of an activity or series of activities. The differences between rehabilitation, restoration, creation, afforestation and enhancement could very well be the deciding factor in the viability and success of a multi-million dollar project.
This is an excerpt from Ecological Mangrove Rehabilitation: A Field Manual for Practicioners, produced by Mangrove Action Project in 2014. Download the Manual, or view it online, or view our many other research papers in our Downloads section.