Mangroves: The Forgotten Forests
Alexandria Gabb
Uniquely adapted to a life of extremes, mangroves have evolved to not just survive, but *THRIVE* where most plants cannot. Their secret? An arsenal of awesome adaptations, each specialised to help mangroves cope with life in the harsh and unforgiving space between land and sea.
Mangrove tunnel in the Everglades. Photo: Pixabay.
Mangroves are a group of salt-tolerant trees and shrubs that grow along coastlines in the tropics and subtropics. Perfectly suited to life on the coast, these plants can survive soils up to 100 times saltier than what other plants can tolerate. The term “mangrove” itself can get a little confusing as it can be used to describe different mangrove plants AND the habitat that they make up, so let’s clear things up with some good old tree-minology...
True mangroves: While there are roughly 80 species of mangrove plants, only 55 of these are what scientists call “true mangroves''. They are not very closely related but share a handful of unique morphological and physiological adaptations that help them cope with life in the salty, waterlogged soils that they call home - so much so that you won’t find them anywhere else!
Mangrove associates: Though mangrove associates are capable of surviving in salty soils, they lack the specialised adaptations of the true mangrove group, so are often found bordering collections of true mangrove species and growing further inland.
Mangrove habitat: Together, true mangroves and their associates form wetland habitats called mangrove forests or mangrove swamps. Mangrove forests made up of similar species can look widely different depending on the water present and the rise and fall of surrounding land; forests are often grouped as either riverine, lagoon, estuarine, basin or dwarf mangroves.
Mangrove distribution map. Photo: Wiki Commons.
Mangrove forests occupy just 150,000km2 worldwide - a space just 20,000km2 larger than the area of England - but what they may lack in cover, they more than make up for in ecosystem services. Mangroves are becoming increasingly recognised as some of the most complex and productive systems in the world, offering food and livelihood support to upwards of 120 million people around the globe… and they don’t stop there. Mangroves provide shelter to over 3000 fish species (many of which are commercially important), they protect shorelines from storms and hurricanes and they are also 3-5 times more effective at storing carbon than parts of the rainforest!
What doesn’t kill you...
Between 50-70 million years ago, mangroves evolved to boldly go where no plant had gone before – the intertidal zone. Characterized by regular flooding, high levels of salinity, strong winds, AND acidic, low-oxygen soils, this space offers less than ideal real estate for plants looking to settle. Did this stop them? NO! Instead, mangroves developed several unique and quite frankly in-tree-guing solutions, allowing them to colonize land that others could not. Let’s take a look at some of the ways they cope with their harsh and ever-changing environment.
Rhizophora mangroves at sunset, propped up by tall stilt roots. Photo: Pixabay.
Rooting for you!
Like most living organisms, mangroves need oxygen to provide their cells with energy and help them grow. While most terrestrial plants source oxygen from the soils around them, soil in the intertidal zone is typically soft, muddy and water-logged due to tidal flooding - making them anaerobic or oxygen-poor. Getting straight to the root of the problem, Mangroves have evolved to produce “aerial roots” that grow above-ground and partially exposed to the air instead of deep into the soil. Members of the Rhizophora genus sprout “prop” or “stilt” roots that branch out side-ways from their trunks, often extending as much as 3 feet off the ground!. Members of the Avicennia genus take an alternative approach, first growing their roots (called pneumatophores), down and then back up out of the soil where they stand-upright. While different mangrove types grow different types of aerial root, they all feature specialised organs called lenticels. These pore-like openings take in air for transport down to the below-ground root tissues.
A mangrove tree surrounded by its breathing roots (pneumatophores). Photo: Pixabay.
Indus-tree-al strength
These aerial roots don’t just allow mangrove trees to breathe, they also help them sit strong and sturdy against tidal action, waves, storms and even hurricanes! By growing their roots out laterally instead of down into the mud, mangroves increase their stability and strengthen their grip. Providing yet another iconic example, the prop roots of Rhizophora members form tangled nests that, when knotted with those of their neighbours, form an incredibly effective natural barrier - capable of reducing wave action by up to 66%! Mangrove roots come in all shapes and sizes from the HUGE buttress roots of Heritiera littoralis to the knee-joint roots of Bruguiera species, each fine-tuned to help mangroves get a grip!
A wall of tangled mangrove roots bordering the water. Photo: Pixabay.
Unbe-leaf-able leaves
Perhaps the most interesting challenge faced by mangrove trees is the high levels of salinity they have to endure. Mangroves have evolved to cope with salt concentrations of up to 2 or 3 times saltier than the sea (35ppt)! They achieve this impressive feat using one of two approaches, either excreting salt after absorbing it or blocking its uptake entirely. Rhizophora members chose the latter, evolving special barriers at their roots to take up water while blocking salt all together. Salt-excreter species take this one step further. Often found slightly more inland than Rhizophora members, mangroves like Avicennia germinans and Laguncularia racemosa can be exposed to salt concentrations of up to 90ppt! They take up salty water and later excrete it from their leaves, often forming whole salt crystals on the surface. White mangroves also channel lots of water to their leaves to balance the salt concentrations inside them. This leads to the formation of thick, succulent leaves that eventually drop-off and take the salt with them. Not only do these adaptations allow mangrove trees to survive their salty situations, but they also make them very effective at filtering contaminants and pollutants from the water.
Salt crystals on a Black Mangrove leaf (Avicennia germinans). Photo: Wiki Commons.
Sea-worthy seeds
Mangroves don’t stop there! Even their approach to reproduction is unique! Mangroves are viviparous, meaning that just like us (and unlike most plants) they produce live young! Okay, well... not exactly like us. Mangroves produce seeds called propagules that begin to grow whilst still attached to the parent tree so right from day one, mangroves come into the world ready to go (or grow)! The propagules are buoyant and, after falling from the parent tree, float or drift before eventually sinking or washing up somewhere to take root. The propagules of each species differ greatly in size and shape and can float for varied lengths of time. Rhizophora propagules look a lot like green beans, long and pointed at the end to help them lodge themselves into the substrate. Black mangrove propagules are much smaller, round and quite flat, helping them get washed further inland where they are better suited to grow.
Floating Rhizophora propagules, long and pointed at the tip. Photo: Flickr.
Stuck between a rock and the ocean
Despite their importance, mangrove forests are disappearing at an alarming rate, facing threats from both land and sea. Overexploitation, pollution, erosion and clearance for alternative land uses (coastal developments and aquaculture ponds) are driving mangrove loss and degradation and leaving little room for the forests to fight against and recover from threats of rising sea-levels due to climate change.