Does planting trees actually help the climate?

The recurring underlying driver behind the threats that trees face is of course our warming climate. But, even though I’m sure you’re ‘sycamore’ climate doom, we know for a fact that trees can act as a clean and relatively quick way of storing carbon. Indeed, tree planting has, in recent years, become a very ‘poplar’ activity. China and India have planted over 4 billion trees combined in the past few years. The Great Green Wall is an international effort to plant a belt of trees across Africa to hold back the spread of the Sahara desert. The Trillion Tree Campaign is a global effort to… well I’m sure you can work that one out. But, whilst the task of planting a tree seems simple, the what, the where, and the how are often anything but. Mark Maslin is a professor of earth system science at UCL…

Mark - There were 6 trillion trees on the planet before agriculture, and we've cut down 3 trillion trees, so we've cut down half the trees on the planet. So we know Earth can actually support a lot more trees. And it's really interesting that it's sort of like people are saying, 'oh, but there's so many people on the Earth.' What is interesting is that actually we are becoming more urbanised. So even though the population will go up to about 10 billion by the middle of the century, we are living in more urban areas. So those places that were deforested, that were populated, and now becoming unpopulated, giving us lots of opportunities to replant forests everywhere.

Will - Are there cases of trees not acting as carbon sinks? Because there's quite troubling studies coming out from Southeast Asia and from the Amazon saying that the Amazon may no longer be a sink for that much longer. So are there instances in which a tree can take out more than it takes in?

Mark - So you have to think of trees not as a single static source of carbon storage. Remember, when you plant a sapling, it's really small, and as it grows, it's going to take on more and more carbon. And then when it becomes a mature tree, then that carbon absorption will slow down. And so the key thing here is which part of the life cycle are you at? How big is the forest? And therefore how mature is the forest? But the interesting thing is just because some of the mature tropical rainforests are no longer absorbing carbon, that doesn't mean we don't protect them, because if you lose them due to deforestation or increased drought length due to sort of like severe climate change and elongated El Niño, then you suddenly lose all that carbon. They may not be storing extra carbon, but they're storing a huge amount of carbon now, which you don't want to lose.

Will - I'd like to play you a clip from Tom Crowther, who we heard earlier in the program. He was part of the research that went into the 1 Trillion Trees Project, and he had this to say about reforestation

Tom - Planting trees should be done by local people to bring back the local biodiversity they depend on. It should not be this idea of mass plantations, plant rows and rows, carpets of monocultures, of species, of an individual tree species because that can be devastating to biodiversity and even worse for the people who depend on it. So it's really critical that we get the right type of restoration in the right way.

Will - What do you make of that?

Mark - Well, I think Tom has really hammered home the most important key message, which is the right trees in the right place. In the 1990s, Western China was becoming a desert, and the politicians were obviously very worried about this, and what they did was ask their scientists, 'well, look what's gone wrong.' And the scientists said they've cut all the trees down. So they went through a massive classic Chinese massive reforestation project. They reforest about a hundred million hectares of land. What happened is the trees stabilised, the soil stopped flash flooding, but because they planted so many, it stabilised the local rainfall and the consequences of that was agricultural production increased massively. So there is no necessary conflict between actually trees and agriculture if you get the balance right, because of course the trees provide the environmental requirements like stable rainfall soils and things like that, which are essential for agriculture. So it's getting that balance right and making sure that the local people are fully involved. In the case of western China, it was extremely poor. So the Chinese used it in two ways. One, they wanted to restore the environment, but they also then turned around to these incredibly poor farmers and go, you will take this money to plant these trees. So it was a social manipulation as well as 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 sort of agriculture and local culture.

Will - With all that being said then, what do you think is the most effective way of utilising trees in order to combat the climate crisis?

Mark - I'm going to qualify that the best way of fighting climate change is to stop burning fossil fuels. Now, even if we planted a trillion trees, that would only take 3 to 4 years of global emissions out of the system, and it would take 50 to 60 years to do that. So if we go back, what's really important is to stop burning fossil fuels. Second thing is, yes, of course we should plant trees because they are our carbon sinks, so that will help us on getting to net zero. But they're also incredibly important culturally, but also for biodiversity. Remember, we've cut half the trees down on the planet, so we've got a long way to go to actually restore all of that incredible nature, all that incredible biodiversity. And of course, we all love forests.