/ 31 October 2019

The trouble with indiscriminate tree-planting in Africa

The trouble with indiscriminate tree-planting in Africa
Britain's Prince Harry, Duke of Sussex, helps a schoolboy to plant a tree at the Chobe National Park, on day four of the royal tour of Africa, in Botswana. (Dominic Lipinski/Pool via Reuters)



On July 25 in Addis Ababa, Ethiopian Prime Minister Abiy Ahmed picked up a spade and started to dig a hole. When he was finished, he gently lowered a sapling into the hole he had dug, and then covered its roots with the loose soil. The tree that he had just planted was one of 350 million planted in Ethiopia that day, part of an unprecedented push to reforest the country — and, in the process, save the world from the climate apocalypse.

Unfortunately, it’s a little more complex than that.

The idea that the world needs to plant more trees is not controversial. Trees suck carbon out of the atmosphere, storing it in the earth instead. So if we plant enough new forests — these would need to cover as much as one-third of all the land in the world, some estimates suggest — we should be able to prevent the planet from becoming any hotter.

This is the premise of a recent paper published in Science, an academic journal. In it, researchers led by Jean-François Bastin, a Swiss-based academic and self-described “tree doctor”, say that the Earth is capable of supporting an extra 900-million trees. These new forests would capture and store 205 gigatonnes of carbon dioxide in the next century (that’s equivalent to two-thirds of all the carbon dioxide generated by humans since the industrial revolution, according to Scientific American).

But where to find so much available land? Attention in recent years has focused on Africa, the grassiest continent, whose wide, empty savannahs can be transformed into enormous forests with relative ease. The African Forest Landscape Restoration Initiative, which styles itself as AFR100, aims to do exactly that: it has secured $1.4-billion in funding from the likes of Germany and the World Bank to pay African leaders to reforest 100-million hectares of African land.

AFR100 is a win-win: for foreign donors, who can bolster their green credentials; and for African leaders, who can claim to be fighting climate change while generating a sizeable financial windfall for their governments. That’s why it has been so enthusiastically embraced by the leaders of 28 countries, who together have committed to plant trees on 113-million hectares — comfortably exceeding AFR100’s original target.

But it might not be a win for the environment.

Arboreal imperialism

When Sally Archibald first read Bastin et al’s paper in Science, she was outraged. Not only by the numbers — which at first glance appeared totally wrong to her — but also because she noticed that much of Southern Africa’s grasslands had been included among the areas ripe for reforestation.

Archibald, an associate professor at Johannesburg’s University of the Witwatersrand, has spent much of her career studying these grasslands, and she believes that planting trees there would be a disaster. “Just because trees can grow on a patch of land, does not mean that trees should grow there,” she told the Mail & Guardian.

Archibald’s frustration has been growing in recent years, as the idea of “reforesting” Africa has become more popular with her Western colleagues — and with global policymakers.

“It is true that you could store lots of carbon if you planted more trees. But when I engaged with European scientists about why you can’t plant trees in Europe, they say, ‘Well, we are using that land.’ There is a sense that land in Africa is available and can be used to fix global issues.”

Besides, it is far from settled whether planting more trees in Southern Africa’s grasslands would actually have the intended effect. “Several researchers have argued that the grassy biomes targeted for afforestation are better than forests at conserving carbon,” noted a recent paper on this subject in the Trends in Ecology & Evolution journal.

“This is partly because forests, especially plantations of eucalypts and pines, are vulnerable to high-severity fires and will become more so as the world warms. Most of the carbon stored in grasslands is below ground, where it persists through fire.”

There are other problems with indiscriminate afforestation, this paper explains. Digging up the soil to plant the trees may inadvertently release carbon into the atmosphere, because grasslands themselves are hypothesised to be formidable carbon sinks; and trees, with their deep roots, use up lots of water — leaving less for other species, including humans.

This science is not new, nor is it controversial: in fact, South Africa heavily regulates the planting of new forests because of their negative impact on water supply. And this is what really frustrates Archibald. When it comes to the global discussions on tree-planting and climate change, the work of local scientists is being sidelined.

“There is an inequality of different people’s voices in these discussions,” she said.

Local knowledge

In October, Science published a technical comment from 46 academics from all over the world — including a number from Africa. The comment was a rebuttal of Bastin et al’s methodology, and claimed that the original paper had overestimated the global potential for tree restoration by a factor of five — as well as ignoring crucial local context that undermined their projections. Two other technical comments were published by Science, both of which raised serious additional flaws in the methodology of the original paper.

“This paper shows that the authors have little knowledge of savanna ecology,” said Alfan Rija, from the Sokoine University of Agriculture in Tanzania, who co-authored the rebuttal. “I am worried that these programs could be used by stronger countries — the G20 — to craft global policies to exploit land and forests of the tropical world.”

The problem is systemic, argues another co-author, Coert Geldenhuys from the University of Stellenbosch: “Most forest and woodland policies in Southern Africa are basically forced on to Africa from outside (the World Bank, the Food and Agriculture Organisation, the International Union for Conservation of Nature, et cetera), and ignore the diverse ecology of the African systems.”

The publication of this rebuttal was organised in part by the Miombo Network, a regional alliance of academics across Southern Africa who work together to figure out the best ways to manage land in the vast Miombo Woodlands biome, which stretches from Angola to Tanzania and from southern Zimbabwe all the way up to the Democratic Republic of the Congo. When it comes to this part of the world, they are the real experts — and, thanks to their coordinated efforts, we know that if a tree falls in a reforested biome, it will make a noise.

Crowther Labs, under whose auspices the Bastin study was conducted, declined the M&G’s request for an interview. However, Bastin and his co-authors responded to the criticisms of their work in their own technical comment in Science. They insist that their numbers are accurate, and that their work is not meant to be prescriptive: “Generally, we must highlight that our analysis does not ever address whether any actions ‘should’ or ‘should not’ take place. Our analysis simply estimated the biophysical limits of global forest growth by highlighting where trees “can” exist,” they said.

Archibald, however, is not buying it. “We can have a scientific debate about how many trees should be planted, but once you provide a map showing where they should be planted [such as the one on the Crowther Labs website] then governments and policymakers are going to use it to make decisions … We feel, rather engage with scientists in Africa about how storing carbon can align with other human objectives. If they would like to engage with us, we are very happy to engage.”