Mangroves grow like weeds, making them easy to restore. But conservationists still must plant carefully. Some mangroves, like this Avicennia, require a bit more care.
(Ahead of World Oceans Day this Saturday, June 8, Cool Green Science is running posts this week looking at the science and issues of marine restoration. Here’s more information on Restoration Week and what the Conservancy is doing to restore marine habitats such as coral reefs, eelgrass, shellfish, and mangroves.)
by Mark Spalding, senior scientist, The Nature Conservancy’s Global Marine Team
Here’s something you may not realize about mangroves: They are opportunistic weeds.
Given half the chance, mangroves settle quickly, grow fast, thrive and bear much fruit in the toughest of places.
They have to be this way – they live in muddy coastal waters which are among the most dynamic places on the planet. It’s the narrow line between land and sea: sometimes wet, sometimes dry, sometimes salty, sometimes fresh. These are places ripped by storms. Places where sediment can be dumped by the ton, or scoured away by waves and currents.
But mangroves have been hammered by human development: cleared and drained, the rich soil banked up to keep the sea out or to control its flow, creating rice paddies or shrimp ponds. Elsewhere, the level land has become roads, airports, industrial complexes, marinas or waterfront homes. And all too often, such schemes fail.
Disease affects the aquaculture, the drained soils dry out and turn acidic, the expensive infrastructure is lost to the relentless scour or pounding of the ocean. What then? No mangroves, no fish, no aquaculture, and a crippling cycle of ongoing, expensive coastal engineering. Consider Guyana, where drained mangroves provided critical, fertile agricultural plains — until the dikes were breached and the soil became saline. Now even the huge concrete bunds are collapsing into the eroding shore.
But this is not a gloomy blog post.
It turns out that mangroves are among the easiest of habitats to restore. In one unique adaptation to their watery world, many mangrove plants actually start to sprout while still attached to their parent plant. The seed starts to grow within the fruit, and in a host of species growing out into “propagules,” up to 80cm long — which are, in effect, young saplings. All you need to do is snap them off and stick them in the ground and presto, you’ve planted a tree. In a wonderful, crazy example 300 volunteers, with no mechanical aids, planted over 500,000 trees in a day. That same year 30 million trees were planted in Senegal.
Mangroves are among the easiest of habitats to restore, due to the plant’s unique adaptations.
But a depressing number of these projects fail. This is a total failure to understand very simple ecological science. In fact it’s often a classic case of imagining that science is somehow flexible, that its simple rules can be bent to accommodate political needs.
The basic story is this. Mangroves grow between the tides, but only really thrive above the mid-tide. Below that, there’s too much water and not enough air for their roots. And some mangrove species grow better at ever-so-slightly higher or lower elevations and salinities than other species. So different species prefer different zones — in fact, just like the different zones you see on rocky shores or salt marshes. Legalities and laziness turn out to be our undoing.
Legalities relate to “who owns the land.” In many countries, land is owned right down to the mid-tide, often by a complex mess of local owners and absent landlords. They don’t want to let go. So the authorities think they can just avoid the cost and the challenges and defy nature instead. Millions of mangroves have been planted in the wrong place – on the state-owned land just below the mid-tide mark. These may sprout. They may grow a few leaves, but then they falter and die.
Property boundaries or just plain laziness can lead to failed mangrove plantations.
The other challenge is laziness. The sheer, unadulterated ease of planting a long propagule of a Rhizophora tree is just so tempting. You can scoop up an armful and simply walk along planting as you go, barely having to change your stride. But actually they might not be the right species for that particular place. Another species, such as Avicennia, will grow better on the front line, where tidal inundation is longer, or salinities higher. Now Avicennia isn’t exactly hard to plant, it’s just not as easy as Rhizophora. So they opt for speed planting and the trees don’t take. So many projects have failed. And in some places this has even led to a sense that planting mangroves is difficult.
The key lesson from all this is that science needs to be communicated. If the restorers aren’t going to bother putting the right mangroves in the right places they might as well save their efforts and stop raising people’s hopes.
But there’s space for more science too. One of the most compelling stories is coming from the ideas of restoration without planting. This is Field of Dreams restoration and it makes complete sense. The reason mangroves aren’t in a place is usually because they can’t get there or gain a toe-hold. So, the argument goes, you just need to prepare the ground.
Restore the hydrology so water drains freely again through the old shrimp ponds and fields, with channels acting as arteries bringing vital water in and out with tides. This might need earth-moving gear, or just the good will and many hands of local villagers. “If you build it they will come.”
Mangrove restoration is one of the most optimistic conservation tools I can think of. Low cost, massive gain, local ownership, ecosystem services, community empowerment, climate change adaptation, carbon storage, fisheries enhancement, tourism, timber, fuelwood without fossil fuels. It won’t work everywhere, but the win-wins are wonderful.
A mangrove plantation in Mauritius.
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