Amidst the myriad of unique plant assemblages found around the world, perhaps none are as tenacious and resilient as mangroves. Mangrove is an umbrella term that groups all tree and shrub species depicting physiological adaptations to high salinity conditions and inhabiting tropical and sub-tropical inter-tidal zones. Mangrove restoration is currently gaining a lot of interest globally.
Wherever fresh water meets the sea, estuaries are formed receiving an influx of muddy sediments which together with the brackish water form a unique biome. These areas are intermittently inundated due to the tidal actions of waves, undergoing periodic cycles of submerging and emergence from seawater. Furthermore, the high salt and mineral content in the water make the nearby soils anaerobic. All these peculiarities create a harsh intertidal habitat where only a few specialist life forms survive.
‘Adapt to salt water my friend’
Mangrove species, however, have developed several morphological adaptations to not only survive but thrive under these conditions1. They have stilt roots akin to a banyan tree that not only extend support against the tidal forces but also act as extended breathing appendages during high and low tides. These roots have specifically adapted cellular pores called lenticels that allow for the ready diffusion of oxygen into the plant while also being hydrophobic, preventing salt water from entering the cells and dehydrating the tissues.
Apart from stilting, some roots also prop up from the ground like arrows and perform both photosynthesis as well as respiration as required by the plant and are called pneumatophores. Astonishingly, several mangrove species depict the animal-like trait called vivipary! This is the phenomenon in which the offspring begins development while still attached to the parent plant (as in humans). Besides this, they have several other adaptations including leaf succulence and ultrafiltration mechanisms to ensure the salt is selectively filtered out.
Genetics reveal that these fitness traits have independently arisen across fern, monocotyledons, and dicotyledons from initially non-mangrove species (convergent evolution). Only around 70 species of true mangroves, i.e. species that grow only in mangrove environments and do not extend into terrestrial plant communities, are found across the world.
But why do we need mangrove restoration?
They are your coast guards
As remarkable as these adaptations are, they aren’t the only characteristics that make mangrove forests special. Mangroves act as sentinels of our coastline by providing a buffer from tropical storms, cyclones and tsunamis. The roots slow the flow of water helping reduce soil erosion while their prop roots (stilts) act as filtration meshes for inorganic pollutants and harmful metals brought by rivers and streams and therefore let out clean water into the oceans.
They are your climate warriors
Mangroves are not only buffers against natural calamities but also of tremendous importance for mitigation and adaptation strategies in response to climate change. Globally, they have been modelled to sequester 21 gigatonnes of carbon which is remarkable considering they only comprise 0.1% of the terrestrial land cover2! This is because of their numerous niche physiological traits such as the proliferation of succulent foliage and root systems mentioned above. This means that on average, mangroves are ten times more productive as carbon sinks than terrestrial forests.
What corals are to marine ecosystems, mangroves are to terrestrial ecosystems ?
The nutrient-rich sedimentary beds near the mangrove roots act as a micro-habitat for a blistering number of micro-invertebrates and aquatic flora such as fishes, crabs, sea snails, shellfish, and algae. These in turn through the food web support a vast array of wildlife and other associated plant species. Leaf falls from mangrove species help in nutrient provisioning for other mangroves as well as coral reefs.
Several molluscs and crustaceans including sessile barnacles and mussels find refuge amongst the mangrove prop roots from rapid tidal waves. The soft mud banks in mangrove ecosystems also provided burrowing habitats for mudskippers and crabs and nesting habitats for sea turtles. Numerous lizard species and saltwater crocodiles also prefer the mangrove habitat as it offers them plenty of predation opportunities. Mangroves also provided nesting grounds for thousands of migratory birds from across the globe.
Estimates suggest that mangroves are refuges of 341 threatened species across the world! The Sundarbans, the largest contiguous stretch of mangroves in the world spread between India and Bangladesh, alone are home to over 42 mammal species, 35 species of reptiles, 300 species of birds, 250 species of fish, and over 350 species of vascular plants besides innumerous phytoplankton, fungi, bacteria, zooplankton, benthic invertebrates, molluscs, and amphibians.
Human livelihoods are inextricably linked to mangroves
Mangroves also support human societies, for example, Sunderbans is one the most population-dense regions in the world. These forests are named so because of the dominance of a mangrove species called the Sundari tree (Hereteria formes). Communities here are dependent on several species for charcoal, fuelwood, fodder, thatch for home-building, wildlife, fish, medicines, tannins and many more products. For example, Sonneratia alba also called mangrove apple and Nypa fruticans are widely harvested mangrove species.
Mangrove honey collected by the Moulis in the Sunderbans, also called blood honey because of the number of lives lost to man-eating tigers during collection, holds a plethora of medicinal advantages over regular honey3. Several forest-dwelling communities such as the Australian Aborigines eat the fruit of mangrove trees after removing harmful tannins from the fruits by soaking them in mangrove mud for three to seven days. In Tamil Nadu in India, the chemical extracts of a mangrove species called the Thillai tree (Excoecaria agallocha) are believed to treat leprosy!
As with many other ecosystems, mangroves are under severe pressures
Despite such a vast array of ecosystem services provisioned by mangroves, they are amongst the most threatened ecosystems. At the turn of the millennium it was discerned that global mangrove cover had shrunk by half compared to its pre-industrial range and the remaining showed alarming signs of degradation.
The reasons for mangrove loss are multifarious but can be broadly attributed to shrimp farming which requires the conversion of mangrove swamps to embanked ponds; sea level and temperature rise driven by climate change resulting in their habitat shrinkage as year-by-year lowland coastal areas are submerged; conversion of forests to agricultural land especially for paddy cultivation; coastal development for ports and tourism; and over-harvesting of forest produce such as fuelwood, fodder and fruiting species.
But what is the process of Mangrove Restoration?
In lieu of severe threats to mangroves amidst the pivotal role they play in sustaining biodiversity, natural resources and human livelihoods, great emphasis has been put on mangrove reforestation. Large tracts of land are being managed for mangrove plantation and assisted natural regeneration. These include the direct plantation of mangrove species and/or improving eco-hydrological conditions in estuarine zones that supplement mangrove regrowth.
Plantation drives consist of raising suitable species in nurseries from seedling to sapling stage followed by replantation on suitable mud banks. Seedlings are collected naturally i.e. seedlings dispersed by sea water instead of directly being harvested from trees using the mangroves’ natural seedling germination characteristics (vivipary). Species selection is of particular significance as successful survival rates are determined by choosing species keeping in mind their salinity requirements, days of inundation, and natural habitat zonation.
Species matter a lot in Mangrove Restoration
Mangrove species depict characteristic zonation (species adapted to lowland, midland or upland areas) and have differential thresholds for salinity. Plantation drives must only use the species native to nearby ecosystems and references are usually drawn from nearby tracts of natural undisturbed mangrove forms. Plantations must also consider the days in a month the area experiences inundation by seawater.
These are determined by the level of seawater during spring and neap tides, with usually lowland areas during neap tides considered as the lowest topographic threshold for plantation. The most successful restoration case studies attempt to emulate natural ecosystems. Furthermore, the restoration area must be surveyed beforehand for disturbances both natural (invasive species, erosion, embankment damage by cyclones) and anthropogenic (vicinity to grazing, agriculture and shrimp farms) and the local communities must be involved in their protection and management activities through educational outreach ensuring benefits are shared with the concerned stakeholders.
Besides nursery plantations, sometimes mangrove reforestation also includes direct seedling plantation when adequate skilled labour and/or infrastructure restraints don’t permit nurseries. In the state of Gujarat on the Western coastline of India, the topography along the coasts doesn’t depict a gradual slope but rather abrupt rocky undulations that give way to strong intertidal currents which damage seedlings and saplings. In such cases a local technique called the Otla raised bed method is used in which saplings are planted on earthen mounts raised to 10-15 cm in an alternating triangular pattern to reduce to ebb and flow of currents4.
The Role of Hydrological Conditions in Mangrove Restoration
Hydrological conditions play a tremendous role in mangrove growth and coupled with the incredible resilience displayed by mangrove species, more attention is being drawn to reestablishing the eco-hydrological conditions supporting mangrove species rather than direct plantations. This is called assisted natural regeneration wherein the focus of restoration is on establishing environmental conditions conducive to natural regrowth rather than plantations.
It has been widely observed that provided the hydrological conditions are appropriate (frequency, duration and level of inundation by sea water) and disturbances are kept to a minimum, mangroves show a remarkable ability to regenerate by themselves without requiring plantation! The most successful mangrove reforestation efforts put a greater emphasis on the latter, improving conditions of sustained inundation on gradually sloping mud banks in coastal backwaters to support natural regeneration rather than direct interventions through plantations. This allows mangroves to develop their natural species composition which is often more diverse than plantations of 2-3 successful species, resulting in a wider array of ecosystem services and species richness.
What about the fishbone channel?
The fishbone channel intervention utilised by the forest departments along the Kaveri and Godavari estuaries in southern India is a prime example of assisting natural regeneration5. The mangroves here were hampered in their regeneration because of poor inundation (some areas were over-inundated during high tide and others were cordoned off from inundation due to local geology). Channels were dug akin to a boney fish’s skeleton to ensure that seawater gradually inundated these mangrove sites leading to vastly improved restoration results.
Such instances of naturally assisted regeneration are favoured as they offer sustainability in the long run. These mangrove congregations upon establishment tend to be far more resilient to disturbances than artificially planted forests. Many reports cite a complete recovery in 3-5 years if restoration sites are monitored and protected well. These insights shed light on how nature can recover quickly on its own without too much human intervention if sound ecological principles are applied rather than forcing human-engineered mono-plantations in desperation for quick results.
Conclusion
Humans can take great inspiration from mangroves as they bridge the natural world between landscape and seascape, improving both in terms of the ecosystem services they provide and the biodiversity they harbour all while occupying some of the most adverse environments. Their sustenance is key for coastal life and with climate change looming over the horizon, our efforts only need to be doubled. Making use of nature’s resilience will be crucial in determining our success towards their restoration.
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Written by: Rishabh Srikar