Trees are vital for solving the climate crisis. But there’s nothing simple about the forested world, as forest people know.

LONDON, 23 August, 2021 − Here’s something you perhaps didn’t know (but you can be sure forest people did). Rainforests make their own rain. Just how much rain they make is a revelation. The process starts with evaporated ocean, which condenses over coastal forest: thereafter, the trees get to work.

The initial deposit of rain will be transpired through the foliage, back into the air to be caught in a pattern of winds that might even be helped by the trees themselves: the same water will fall again across the forest five or six times before journey’s end.

The scale of this natural corporate utility service is colossal: one pilot followed the Amazon’s own flying river from Belém near the Atlantic coast across to the Andes, where the airstream and clouds of vapour turned south to reach the coast again at São Paulo, at the same time transporting 3,200 cubic metres of water a second.

There’s no case for doubt. One of the plane’s passengers collected air samples along the way: once inland, the water vapour had the molecular signature associated with vegetation rather than freshly evaporated seawater.

And somehow the forest actually adds to the delivery: at one place near the ocean, the fall is 215 cms a year; at the heart of Amazonas it is somehow 245 cms a year.

Trees as rainmakers

The phenomenon that is the flying river is not unique to the Amazon. Others cross North America, the Congo rainforest, the Sahel and Ethiopia. The world’s most mighty high-altitude aquifer runs for 6000 kms west-to-east across the Eurasian landmass, taking six months, at the end of which four-fifths of the rain in northern China has been generated by the great boreal forest that begins in Norway, Sweden and Finland.

Trees make the rain. Arid places may be treeless not because they are arid; they could be arid because someone cleared the foliage.

A Trillion Trees: How We Can Reforest Our World, by Fred Pearce, (Granta, £20) is a reporter’s book. Pearce has been reporting the science and impacts of the environment for the New Scientist and other journals for four decades or more.

He doesn’t just deliver the big picture: he illuminates the detail. He goes to forests and the desolate landscapes where forests had once flourished. He meets scientists, activists, campaigners, government officials, loggers, farmers, businessmen, politicians and where possible the indigenous peoples of the forest.

He isn’t just there for the rainforest: he knows the American landscape, the great forests of the north, the plantations of Israel, the woodlands of Europe and the mangroves of the African shore, and he introduces the people to whom these places matter.

“If natural regrowth has to be the basis for the renaissance of the world’s trees, then the custodians of that process must be the people who live in them”

This is the book’s strength, and occasionally its weakness: just as the dense understory slows the trek through the great forest, so the vigorous tangle of evidence and counter-argument sometimes leaves the reader a little confused.

That seeming weakness is best considered part of the book’s big message: forests and trees may be simply marvellous, but they are never simple. There is good evidence that trees cool the planet, and manage their own airflow, but not so good that it is not disputed.

There is convincing evidence that trees emit volatile organic compounds that help the rain-making process but also extend the life of that potent atmospheric greenhouse gas, methane: convincing enough to permit at least one scientist to argue, seriously, that forests might not cool the world after all, even as they absorb that other greenhouse gas, carbon dioxide.

And along the way Pearce and his articulate arboreal experts deliver other challenges to the orthodoxies of popular ecology. Big money and unthinking greed help in the destruction of forests everywhere, but the richer the nation, the more likely it is to be extending its own canopy. Between 1990 and 2015, high-income countries on average increased forest cover by 1.3%. Low to middle-income countries however lost 0.3%, while the poorest of all bade farewell to 0.7%.

It would be nice to think that “levelling-up” would play its role in slowing climate change. But, of course, the rich nations are exporting deforestation in the service of trade. The poor world’s forests are being felled and land cleared for our beef and cattle fodder, our coffee, our chocolate.

Second thoughts

In the course of this absorbing book, Pearce undertakes some enthusiastic root-and-branch re-examination of other arboreal orthodoxies. North America was not once covered by “endless pristine forest”. For millennia, forests have been managed by indigenous peoples; the same is true for African and South American jungles.

Plantation − commercial or otherwise − may not be a good way to restore global canopy. Systematic, government-endorsed “greening projects” may not be the best solution to either carbon absorption or biodiversity restoration. It might be better to leave nature to do what nature does best: the results of “wilding” what was once degraded or deserted land can be remarkable.

Agroforestry, − partnering of trees and crops − on the other hand, also has a lot going for it. Unexpectedly, the seeming connection between land degradation and over-population isn’t really there. In the words of one research paper, “population density is positively correlated with the volume of planted woody biomass.”

And on the evidence so far, centralised policy and government initiatives might be less effective than indigenous or local guardianship. Where communities do have genuine control of their own woodlands, community management of the world’s forests “works staggeringly well.”

There is a case for people power after all. Pearce writes: “If, as I believe, natural regrowth has to be the basis for the renaissance of the world’s trees, then the custodians of that process must be the people who live in, among, and from them … They know them best and need them most.” − Climate News Network

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A Trillion Trees: How We Can Reforest Our World, by Fred Pearce: Granta, £20, ISBN: 9781783786916