IELTS Reading: Choose two

A mangrove is not a kind of tree. It is a way of life adopted independently by about seventy species drawn from a dozen unrelated plant families, which have arrived at the same set of solutions to the same set of problems: how to live with roots in salt water, in mud that contains no oxygen, on a shoreline that moves. The solutions recur across the families in a way that makes the convergence obvious. Roots that grow upward out of the mud to breathe. Membranes that exclude most of the salt at the root, or glands that excrete it through the leaves. And, most strikingly, seeds that germinate while still attached to the parent and drop as living seedlings, ready to root within hours if they land somewhere suitable.

What a mangrove forest does for the people living behind it has been studied in considerable detail. It is a nursery: a very large proportion of the fish caught on tropical coasts spend part of their early life among the roots, where a predator large enough to be dangerous cannot manoeuvre. It supplies timber, fuel, honey and thatch. And it dissipates wave energy, measurably, in a way that has been quantified by placing pressure sensors at intervals through a belt of forest. A hundred metres of dense mangrove reduces wave height substantially, and the effect is greatest for exactly the short, steep storm waves that do most damage to property.

There is a further service that was overlooked for a long time. Mangroves hold an exceptional quantity of carbon for their area, several times what a tropical rainforest holds per hectare, and the great majority of it is not in the trees at all. It is in the sediment beneath them, which accumulates slowly in anoxic conditions where decay is very slow, and which may be several metres deep and thousands of years old. This changes what clearance means. Felling the trees releases what is in the trees; disturbing the mud releases a store that took millennia to build, which is why converting a mangrove to something else is far worse in carbon terms than the standing biomass would suggest.

Somewhere between a third and a half of the world's mangroves were lost during the twentieth century. The causes vary by region, but in much of Asia the single largest was conversion to ponds for farming shrimp. The economics of that conversion are worth understanding, because they are not the economics of a permanent land use. A pond cut into a cleared mangrove is highly productive for perhaps five to ten years, after which the accumulated waste, the acidity released from the disturbed soil and the build-up of disease make it uneconomic, and it is abandoned. The operator moves along the coast and clears another. What is left behind is neither forest nor farm.

Restoration has accordingly become a large and well-funded activity, and most of it does not work. The standard project plants nursery-raised seedlings of a single species in rows, frequently on open mudflats that never supported mangroves in the first place because they sit at the wrong tidal elevation, and reports success by counting seedlings planted rather than trees alive some years later. Survival rates below twenty per cent are common and rates near zero are not unusual. The alternative approach starts from the observation that mangrove seeds arrive by water in enormous numbers without any help: if the site is at the right elevation and the water can move across it as it used to, the forest re-establishes itself. The work is therefore hydrological, removing the embankments and reopening the channels, and it costs a fraction of planting.

The obstacle is not knowledge but ownership. The benefits of a standing mangrove are diffuse and public: the fishery improves for everybody, the storm surge is reduced for the whole village, the carbon matters to nobody in particular. The land, on the other hand, is held by somebody, formally or informally, and a shrimp pond pays that person directly for a decade. Various mechanisms attempt to bridge this, including carbon credits and insurance policies written on the protective value of the forest, and several are promising. None has yet been scaled to the size of the problem, and all of them founder on the question of who exactly holds a title to a piece of tidal mud.

It is rare in environmental policy to find a case where the ecological argument and the financial argument point the same way, and this is one. A hectare of mangrove left standing is worth more than a hectare of shrimp pond on almost any reasonable accounting, including one that ignores carbon entirely and counts only fish and flood damage. That makes it all the more striking that the majority of the money spent on restoration over the past thirty years has gone into planting seedlings in rows on the wrong part of the shore, and that the method that works is the cheaper one.