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Does planting trees actually help the climate?

The recurring reason for the threats facing trees is, of course, our warming climate. But while I’m sure you’re a “plane tree” climate doomsayer, we know for sure that trees can be a clean and relatively quick way to store carbon. In fact, tree planting has become a very popular activity in recent years. China and India have planted over 4 billion trees together in recent years. The Great Green Wall is an international initiative to plant a belt of trees across Africa to halt the spread of the Sahara desert. The Trillion Tree Campaign is a global initiative to… well, I’m sure you can work that out. But while the task of planting a tree seems simple, the what, the where and the how are often anything but. Mark Maslin is Professor of Earth System Science at UCL…

Mark – Before agriculture, there were 6 trillion trees on the planet, and we’ve cut down 3 trillion trees, which is half of all the trees on the planet. So we know that the Earth can actually support a lot more trees. And it’s really interesting that people say, “Oh, but there are so many people on the Earth.” What’s interesting is that we’re actually becoming more urban. Even though the population will be about 10 billion by mid-century, we’re living in more urban areas. These places that were deforested, that were populated and are now becoming unpopulated, give us a lot of opportunities to reforest forests everywhere.

Will – Are there cases where trees don’t act as carbon sinks? Because there are quite worrying studies from Southeast Asia and the Amazon that say the Amazon may not be a carbon sink for much longer. So are there cases where a tree removes more carbon than it takes in?

Mark – So you can’t look at trees as a single static source of carbon storage. Remember, when you plant a sapling, it’s still very small, and as it grows, it takes up more and more carbon. And then as it grows into a mature tree, the carbon uptake slows down. So the key here is what stage of the life cycle are you at. How big is the forest? And with that, how mature is the forest? But what’s interesting is that we don’t protect the mature tropical rainforests just because they’re no longer taking up carbon. Because if they’re lost to deforestation or prolonged droughts due to severe climate change and prolonged El Niño, suddenly you’re losing all that carbon. They might not be storing any extra carbon, but they’re now storing a huge amount of carbon that you don’t want to lose.

Will – I want to play you a clip from Tom Crowther, who we heard earlier on the show. He was involved in research for the 1 Billion Trees Project and he said this about reforestation

Tom – Trees should be planted by local people to restore the local biodiversity that they depend on. There shouldn’t be this idea of ​​mass plantings, rows and carpets of monocultures, species, a single tree species, because that can be devastating for biodiversity and even worse for the people who depend on it. So it’s really critical that we get the right kind of restoration done in the right way.

Will – what do you think about it?

Mark – I think Tom has really clearly expressed the key core message, which is the right trees in the right place. In the 1990s, western China became a desert and the politicians were obviously very concerned about that. They asked their scientists, “Look what went wrong.” And the scientists said they had cut down all the trees. So they did a massive, classic Chinese reforestation project. They reforested about a hundred million hectares of land. What happened was that the trees stabilized, the ground was no longer subject to flash floods, but because they planted so many trees, the local rainfall stabilized and the consequence of that was that agricultural production increased massively. So there is no necessary conflict between trees and agriculture if you get the right balance, because of course the trees meet the environmental requirements like stable rainfall, soils and the like that are essential for agriculture. So it’s about finding that balance and making sure that the local people are fully involved. In the case of western China, it was extremely bad. So the Chinese used it in two ways. First, they wanted to restore the environment, but then they also turned to these incredibly poor farmers and said, “Take this money to plant these trees.” So it was both a social and an environmental manipulation. So I think Tom is on the right track, which is to support local people to actually plant the right trees to support biodiversity, but also to support their own kind of agriculture and local culture.

Will – With this in mind, what do you think is the most effective way to use trees to combat the climate crisis?

Mark – I want to be clear that the best way to fight climate change is to stop burning fossil fuels. Even if we planted a trillion trees, that would only remove 3 to 4 years of global emissions from the system, and that would take 50 to 60 years. So as we go back, it’s really important to stop burning fossil fuels. Secondly, of course we should plant trees because they are our carbon sinks and that will help us get to a net zero target. But they are also incredibly important culturally, but also for biodiversity. Remember, we have cut down half the trees on the planet, so we have a long way to go to actually restore this incredible nature, this incredible biodiversity. And of course, we all love forests.

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