Trees and forests are often seen as symbolic of nature, and for good reason: They are vitally important to both the planet and to people. More than three-quarters of the world’s terrestrial biodiversity occurs in forests. Wooded areas filter and manage rainwater, provide shade, soak up pollution, regulate climate, provide oxygen, and salve our spirits.
In the face of threats from deforestation, ecosystem degradation, pests, and more, about 38 percent of the planet’s 166,000 tree species are now at risk of extinction, according to an October assessment from the IUCN. Increasing the planet’s tree coverage would have positive benefits for the climate and our ecosystems, scientists agree, but how we go about that is complicated.
In an interview with Yale Environment 360, Jake M. Robinson, a researcher in restoration genomics at Flinders University, in Adelaide, South Australia, talked about how reforestation efforts can go awry if done poorly, reducing biodiversity and harming local populations.
“If you plant trees in ecosystems where trees simply shouldn’t be growing, you can remove the biodiversity that evolved with those niches,” said Robinson, author of the recently published Treewilding: Our Past, Present and Future Relationship with Forests. “Tree planting campaigns are important, but it’s more important that we do it right.”
Jake M. Robinson
Courtesy of Flinders University
Yale Environment 360: How has the planet’s tree cover changed since humanity started to transition to agricultural societies some 10,000 years ago?
Jake M. Robinson: Back then, something like 45 to 60 percent of the terrestrial surface of Earth was covered in forests. It has rapidly changed. We have gone from about 6 billion hectares of forest to 4 billion. We have lost a third of the planet’s forests in the last 10,000 years. About 50 percent of that loss has been just in the last 100 years or so. That’s how much it’s sped up since the Industrial Revolution. But the nature of forests is changing too. There are pockets of natural virgin forests left, but much of it is monoculture plantation.
e360: Is there a consensus about how much of the planet should be forested for a healthy planet?
Robinson: No, there isn’t a consensus. Some people have the idea that we should be restoring ecosystems back to what they were like before humans started intervening in them. Another view is we need to restore the functions and the processes of ecosystems and the biodiversity they support, not necessarily this ideal picture of what it was a few thousand years ago. I would like to see us recognize that humans are part of the ecosystem and that it’s okay that we’re altering the land: Indigenous peoples have been changing ecosystems for tens of thousands of years.
e360: Tree-planting initiatives have really taken off. You write that, globally, the area of planted forests rose from 170 million hectares in 1990 to 293 million hectares in 2020. How did that happen?
Robinson: This was partly catalyzed by a 2019 Science study, which is quite controversial in the restoration ecology sector. They basically ran some models and showed that 900 million hectares is available to support forests, which could host about another a trillion trees. That simple idea took off, stratospherically. The authors weren’t literally advocating planting a trillion trees, nor did they mean to imply that there aren’t other essential ecosystems, like grasslands. It was all in good faith, and mostly a case of poor communication.
“Restoration seems like a fairly straightforward idea… but probably the majority of ecosystem restoration projects fail.”
e360: What’s wrong with the “plant a trillion trees” mindset?
Robinson: Restoration seems like a fairly straightforward idea, but it often goes wrong. In urban areas, around 30 percent of trees that are planted die within the first five years of life. If you plant trees in ecosystems where trees simply shouldn’t be growing, you can remove the biodiversity that evolved with those niches. Probably the majority of ecosystem restoration projects fail or don’t meet their stated goals. There’s a big social element as well, because lots of people are already on the land that is put aside for tree planting; you can end up displacing them. Tree planting campaigns are important, but it’s more important that we do it right, planting the right trees in the right places.
e360: Is there a key example of a tree planting project that has been thoughtful and successful?
Robinson: There’s one that I like to draw attention to in Oaxaca, Mexico. Several hundred years ago there were 100,000 Indigenous people supported by the forests in the Coixtlahuaca district, and now there’s fewer than 3,000 inhabitants. The land can’t support them, it’s so degraded from agriculture and poor land management. Twenty years ago the soil was essentially rock. The local communities had to crush the rock so it could retain water, and then they planted these little nurse species — a non-native tree called Gregg’s pines that’s well adapted to poor conditions and won’t reproduce — just to kickstart the ecosystem. These trees died and their carbon made it into the soil. It’s an ongoing project — it’s not yet a flourishing ecosystem — but it just shows that it can start from zero.
Pine trees planted as part of a reforestation program in Oaxaca, Mexico.
Wolfgang Kaehler / LightRocket via Getty Images
e360: In other projects, people are very firmly in favor of using only native species.
Robinson: Yes. In Japan there’s a technique called the Miyawaki Method. The idea is to plant a carefully selected mix of native species in very high densities, which stimulates them to grow faster. It’s essentially a rapid way of creating a functional ecosystem. But because this high density is artificial, it’s controversial. Many ecologists aren’t fans. But I’d say that most of the time, they’re in urban areas where the ecosystem’s just so different and novel anyway, and they have social benefits as well.
e360: The Great Green Wall initiative aims to plant a band of trees across the southern edge of the Sahara Desert, over a massive area of tens of millions of hectares. How is that going?
Robinson: That was [English forester] Richard St. Barbe Baker’s idea, to help stop desertification. It could be a good idea, but you’ve got to do it right. I think they’re planting some trees where they really shouldn’t be, and there are some human rights issues, potentially, as well.
e360: You talk about the concept of “growing” as opposed to “planting” trees. What’s the difference, and why is it meaningful?
Robinson: Sometimes you can leave a spot and let nature do its thing; nature will find a way and grow. But in certain situations, because we have degraded the planet so much, you have to create the right conditions for these trees and nourish them. That’s one of the challenges we’re trying to work with now: how to nurture at scale.
“We can tell the difference between a degraded ecosystem and a restored ecosystem simply by listening to it.”
e360: Of course, if you restore or plant a lot of forest there is less land for growing food.
Robinson: This is a huge issue. I don’t use the term revolution lightly, but I think we need a revolution in our food systems, because agriculture is one of the leading causes of ecosystem degradation on the planet, if not the leading cause.
One solution I speak about in the book is “syntropic agroforestry.” This is just the idea that you can still have a flourishing forest ecosystem while producing food within it. You have to really understand the lifecycle of the trees and know how to layer the plants and how communities turn over with time. Klaus Lotz, a farmer in New Zealand, is doing this. It looks quite like a rainforest. He mostly focuses on non-annual plants — fruits and vegetables, nuts and mushrooms. He says his yield per species is 10 percent lower per area of land than in a monocultural system, but his overall yield, of all produce, is actually higher.
e360: You are a microbiologist by training. How important are microbes within a forest?
Robinson: Incredibly important. And it’s not thought about enough. In a five-ton tree trunk, there’s around a trillion bacteria; and more in the leaves, the seeds, the roots, flowers, et cetera. A tree is between 68.75 and 99.9 percent microbial, according to my calculations. That’s kind of mind-blowing, isn’t it? Trees are holobionts [assemblages of a host and the many microbial species living in or around them]. Much like we now recognize the importance of the gut microbiome in human health, we need to recognize the importance of microbes for forest health.
A “syntropic” farm run by Klaus Lotz in Northland, New Zealand, produces food within a flourishing forest.
PermaDynamics
e360: Are forest stewards harnessing the power of microbes?
Robinson: Sometimes. Microbial inoculants are becoming more popular. The idea is to create your own liquid culture of microbes that you spray on leaves or put in the soil [to enhance soil fertility and plant growth], or you can include it with the little plugs of saplings if you are tree planting.
e360: You also study soil acoustics; how can that help to support forest soil health?
Robinson: We created these little modified microphones that we place in the soil, that detect the critters moving around: the earth worms, the spiders, the ants, millipedes, et cetera. We can tell the difference between a degraded ecosystem and a restored ecosystem simply by listening to it, like a doctor using a stethoscope to listen to your heartbeat. It’s been quite eye-opening, and I think it’ll be a useful monitoring tool in the future. We’ve also recently found that white noise can stimulate fungi, promoting growth. We need to figure out the evolutionary drivers of this — why are they responding? It could be they ‘mistake’ it for the sound of rain falling, which could trigger movement.
Climate change is “having different effects on different forests. While lots are reducing their range, some are actually expanding.”
e360: How is climate change affecting forests?
Robinson: It’s having different effects on different forests across the world. While lots of them are reducing their range, some forests are actually expanding. Along the 12,000-kilometer-long northern tree line [where forests give way to tundra], most of it’s expanding and only a tiny bit is receding.
e360: What do you think of helping trees migrate into new climate-friendly zones?
Robinson: I’m working on “climate-adjusted provenancing” in South Australia. We’re essentially collecting seeds from a climate gradient going north, where the conditions are warmer already, and then seeing if seeds from further afield fare better in changing conditions than the local tree seeds. I don’t have a strong opinion on it yet because it’s at an experimental phase in our lab.
e360: Overall, do you think humanity is moving in the right direction to restore our planet’s forested lands?
Robinson: I’m cautiously optimistic. There are pockets of people doing really good things. You’ve got to have hope.
e360: What are the biggest and boldest moves humanity can make to repair Earth’s forests?
Robinson: Protecting as well as restoring, that’s part of the big picture. And protecting Indigenous peoples and cultures that are doing less harm.
This interview has been edited for length and clarity.
