Can’t See The River For The Trees

4:33 minutes

A waterfall in a wooded glen
Credit: Pixnio

This week business leaders, celebrities, and government officials from around the world met in Davos, Switzerland—and one of the topics was trees. The Trillion Tree campaign, a collaboration between several of the world’s largest environmental organizations, wants to combat global deforestation around the world  But at the same time, work published in the journal Global Change Biology indicates that tree planting can lead to unintended consequences. 

The researchers found that increased levels of forest can reduce the available water in nearby rivers dramatically, cutting river flow by as much as 23% after five years and 38% after 25 years. The effect of trees on river flow is smaller in drier years than wetter ones. The type of soil conditions also have an effecttrees planted on healthy grassland have a larger impact on river flow than forests on former degraded agricultural land. 

David Coomes, Director of the University of Cambridge Conservation Research Institute and one of the authors of the paper, joins Ira to talk about the pros and cons of reforestation.

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Segment Guests

David Coomes

David Coomes is a professor in the Department of Plant Sciences and Director of the University of Cambridge Conservation Research Institute at the University of Cambridge in Cambridge, UK.

Segment Transcript

IRA FLATOW: And now it’s time to play Good Thing, Bad Thing.


Because every story has a flip side, and we have one this week. This week, business leaders, celebrities, and government officials from around the world met in Davos. And one of the topics were trees. Yeah. Remember the Trillion Tree initiative? It got kicked off this week. And really, who could be against planting more trees.

But research published this week argues that sometimes a new forest may have unintended consequences. Joining me to talk about this is one of the authors of that study, published in the journal, Global Change Biology. David Coomes is director of the University of Cambridge Conservation Research Institute in Cambridge, UK. And he’s just back from Davos himself. Welcome to Science Friday.

DAVID COOMES: Thank you very much indeed.

IRA FLATOW: You’re welcome. How big an impact does planting a forest have?

DAVID COOMES: Well, I’m very much in favor of protecting the world’s intact forest and restoring natural forest, and also planting trees, which was the focus of attention in Davos this week. And we know that already forests are soaking up about 28% of the CO2 we put into the atmosphere each year. And so planting more trees is only going to soak up more and store more carbon. So it’s really important to plant and protect forests. So I’m all in favor of that.

IRA FLATOW: Well, that’s the good news, as we talk about in Good Thing, Bad Thing. Give us the possible downside to this.

DAVID COOMES: Well, the possible downside in some parts of the world– and it’s not everywhere– is that if you plant trees, they’re actually much better at taking rainfall, capturing it in the leaves, capturing it in the stems, and capturing it in the roots and sending it straight back into the atmosphere and turning it back into water vapor. And so what you see if you plant trees in grasslands or old crop lands is that the river flows go down.

And so that’s potentially bad news if you lived in an arid region and forests are replanted on hills. And then you just haven’t got as much water going down into the valleys where it could be used by agriculture or washing or whatever in the cities. So that’s the downside.

But here in the UK, we’re not short of a drop or two of rainfall. And planting trees on the hills is thought to be a good thing. It allows water to be stored on the hills after heavy rain and not go through and flood the lowlands. So depending on where you are in the world, this could be regarded as a good or a bad thing.

IRA FLATOW: So you have to be, as you say, you have to be conscious of what you’re doing and where you are when you do this.

DAVID COOMES: Exactly. So I just think as we’re– I mean, it’s a tremendous opportunity with everyone– all the politicians seem to be on the side. Donald Trump this week planting a trillion trees is something America’s into, as well as Europe. We’ve just got to think what trees and where. What’s the right thing to do here? And it does require a bit more thought than plonking trees everywhere.

IRA FLATOW: The climate change in this country has caused lots of flooding, overflowing of river banks and things. In those cases, could the trees actually soak up a significant amount of water?

DAVID COOMES: Well, that’s a more complicated one. And certainly, I mean I think up in the planting trees in the uplands has a very clear effect on river flows down in the lowland areas. To be honest, I’m not sure about the lowland vegetation, what the benefits or otherwise would be. I wouldn’t like to tell you something other than what I’m knowledgeable about.

IRA FLATOW: Well, we wouldn’t like you to do that either, so. But what I’d like to ask you is what kinds of trees did you look at? Are there certain trees that work different than ours?

DAVID COOMES: Well, like your last guest, this was a meta analysis, so it’s taking lots of previous studies. And actually, there’s hundreds of studies. And we’ve just focused on 43 really top quality ones, where you have what’s called a time series. People have followed the forest after it’s been planted and following it over a number of years. And so that’s what we’ve looked at. We’ve looked at all the studies which are high quality throughout the world. But most of them were in the Mediterranean sort of climate system, drier climates of the world that people were worried about this very issue.

IRA FLATOW: Yeah, and they should be, according to your study. Thank you, Dr. Coomes, for taking time to be with us today.

DAVID COOMES: Yeah, it’s a pleasure.

IRA FLATOW: David Coomes, professor in the Department of Plant Sciences, director of the University of Cambridge Conservation Research Institute.

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As Science Friday’s director and senior producer, Charles Bergquist channels the chaos of a live production studio into something sounding like a radio program. Favorite topics include planetary sciences, chemistry, materials, and shiny things with blinking lights.

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