Tidal systems are highly dynamic and harsh environments seemingly engineered to prevent plant life from flourishing. They have loose, shifting sediments that have no air for plant roots to breathe. Tidal zones are frequently inundated with water capable of drowning most plants, and have salt concentrations that are much higher than most plants can tolerate. Yet, even in these inhospitable places, floral diversity has managed to adapt.
Mangrove forests, also known as mangals, are diverse ecosystems that populate every coastal state and union territory in India. India hosts nearly half of the world’s total mangrove diversity; 37 of the world’s 80 mangrove species are found here. The myriad mangrove species in India have uniquely adapted to the challenges in these ecosystems — be it the pencil roots of Sonneratia, the salt glands on the leaves of Avicennia, or the ribbon-like buttress roots of Hitteretia littoralis. But one genus stands above them all — quite literally on stilts — Rhizophora.
Rhizophora means root-bearing, which becomes evident after just one glance at them. They host an extraordinary system of specialised aerial stilt roots called rhizophores that descend from high up in the stem and branches of the plant, creating a complex tangle of roots at the base. These strong roots allow Rhizophora to be often the first species to colonise a tidal marsh, forming an outer seafront fringe of trees. Their establishment shelters the coastline, allowing other species of mangroves to grow further inshore.
It is mindboggling to see how perfectly evolution has shaped these trees. They tackle daily tidal changes, frequent or permanent inundation, fluctuating and often very high salinity in the water, and anaerobic and shifting soils. And as if surviving such harsh environments wasn’t enough, they help create a vibrant and thriving mangrove ecosystem, performing several ecosystem-level functions that benefit us all. Let me show you how.
The soils of tidal marshes pose two major problems for plants. The first hurdle these trees face is the loose silt sediments that make it difficult for plants to stay upright. The second is a trickier one. The soils are anaerobic, i.e., they have little to no oxygen.
Rhizophora get directly to the root of both challenges. Their rhizophores descend from branches and stems, forming a dense intertwined mesh at the tree’s base. Their broad outward expansion works perfectly to anchor them in the moving sediments of estuaries, while their aerial nature allows the tree to breathe despite the choking soils beneath them.
Even small quantities of salt can be fatal to trees. The high amounts of salt in seawater cause water to rush out of cells, dehydrating the plant from the inside. But mangrove species have developed unique strategies to combat the stresses of life in seawater. Avicennia, for example, have evolved special glands on the surface of their leaves through which they excrete the excess salts they absorb.
Rhizophora use a different strategy. Although their roots are specially designed to limit the amount of salt absorbed, they still absorb more salt than they need. (2) These excess salts are then pushed outward towards the terminal leaves of the tree, which when saturated with salt, turn yellow and fall off. The sacrifice of a few leaves allows the plant to survive.
In such harsh and unpredictable environments, delicate plant seeds aren’t very likely to survive. To counter this, Rhizophora trees do not have a dormant seed stage at all. Once fertilised, the flower immediately begins germination into a seedling — called a propagule — while still attached to the parent plant. The trees are thus often touted as being viviparous — i.e., capable of giving live birth. If seed-bearing plants are the floral equivalents of egg-laying vertebrates, then the Rhizophora propagules are the placental mammals of the plant world.
Rhizophora propagules are hydrochorous, i.e., they are water dispersed. (1) Once developed, they detach from the parent tree and fall straight into the water below them. Being highly buoyant, the propagules float on the water’s surface as they are carried out to sea.
To reach their ideal habitat, Rhizphora use the principles of buoyancy to their advantage. As the days pass, chemical changes within the propagule cause its density to gradually increase, making it less buoyant. (2) In combination, the low salinity environments at the boundary of a river-sea interface cause these propagules to become even less buoyant and float vertically just below the water’s surface or even sink to the bottom. (3) On coming in contact with the silty substrate, they quickly take root, firmly anchoring themselves in place.
Rhizophora, with its dense canopy, winding branches, and interlocking aerial roots, provide a complex habitat that spans both land and sea. This diversity in microhabitats provides a home to an equally diverse variety of fauna, birds, crustaceans, reptiles, and fish.
Their dense thickets provide ideal cover for roosting birds, such as the endemic Andaman nightjar (1). Being at the edge of the waterfront, they provide birds like the ruddy kingfisher (2) an ideal vantage point to scan the landscape and make easy pickings of crabs and fish in the marsh below. (3) Snakes (like this red-tailed pit viper) coil around branches and roots, waiting to ambush their next meal.
At the land-sea interface, we see a more amphibious species. Having evolved the ability to breathe outside water for short spans, mudskippers (1) are commonly found skittering across the water between rhizophores. (2) Graspid crabs are uniquely adapted to feeding on mangrove leaf litter, scurrying up and down roots searching for food. These crabs digest and assimilate leaf nutrients and aid the cycle of decomposition. (3) Hermit crabs feed on leaf and seedling detritus. In addition to their burrowing and feeding habits, the crab community also plays a critical role in the nitrogen cycling of mangrove ecosystems by hosting unique nitrogen-fixing bacteria on their shell.
Beneath the water’s surface, the interwoven network of Rhizophora roots provides the ideal habitat complexity for fish. Herbivorous fish feed on the algae that grow on the submerged Rhizophora roots. The nooks and crannies between roots provide juveniles of reef and pelagic fish with shelter from predators, making them ideal nursery sites. Mangrove systems are thus integral from the perspective of recreational and commercial fisheries across the globe.
Mangals are some of the most productive and vital marine environments. (1) They trap and bind sediment together, enabling land expansion in tidal floodplains. They buffer coastlines from the harsh wind and storm surges. (2) These biomes are also great at capturing and storing carbon within their sediments, sequestering up to five times as much carbon as tropical rainforests do!
Nature’s brilliance is on full display in a mangrove forest, and its benefits to us are clear. Despite this, the mangals of India are threatened by urban development. The latest is the concession to fell vast tracts of Mumbai’s mangroves for the upcoming bullet train. Given the burgeoning threat of climate change and the importance of these ecosystems, it is our responsibility to safeguard them for future generations.
About the contributors
Wenzel Pinto
Dhritiman Mukherjee
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