The potential of the urban forest

In the city, trees can be very valuable, but it’s often underestimated how valuable. Chris Smith spoke with Mat Disney. He's from University College London where he has an interest in Urban Forests and he’s urging people to plan new developments more carefully with trees in mind…

Mat - Well, that's a, that's a good question. And, you know, depending on who you ask, there's several hundred definitions of what a forest is, nevermind an urban forest. So, you know, what's an urban forest, it's a forest that's got people in it basically, but more technically, if we look at the UN's food and agricultural office definition, which is one of the ones that many people use, it's a chunk of land with a tree canopy cover of more than about 10%, and an area of more than about half a hectare. So it's about, half a football field size. And, it has to have trees that can reach a minimum height of five metres. So you can see straight away that that's not a very big ask, you know, half a football field, 10% tree cover, five meters. That's not much of a forest in many cases, but the critical thing in their definition that is, it exists in the absence of other land uses. And of course that's the difference in cities. The land use is what we put it to.

Chris - And apart from looking nice and providing a place for park benches, where you can sit under normal circumstances and have pigeons poo on you, while you're having your packed lunch, what actually do they return to the city? Why is a tree so important? What's the sort of, raison d'être behind you being on this mission?

Mat - Well, trees, you know, as we've heard from the sort of earlier reports here, that they provide all sorts of different services, not least the kind of aesthetic benefits that we appreciate so highly. Everybody loves trees, but they act to absorb carbon. That's one aspect, but in cities, particularly they do things like keeping the temperature down, particularly when temperatures are reaching extreme levels in the summer. And as we are, you know, climate models and forecasts are predicting that we're going to see spikes in temperature, and longer, more protracted heat events in cities. Trees keep that temperature down through transpiring water, and through shading. They also act to mitigate stormwater runoff, as well as providing a wider ecosystem for biodiversity in cities, birds, pollinators, insects that keep the other aspects of the green city alive.

Chris - Some parts of some well-established cities do have lots of big green open spaces. New York has Central Park. London has a number of different parklands. I mentioned when I was introducing you, that you've been trying to encourage people to think about this at the planning stage, I guess where you're coming from is that to plant a decent tree, isn't something you just do. And it's done like building a building. It's something you've got to think about, what will be there in 30 or 40 years, which is the time it will take to establish.

Mat - Absolutely. And in fact, more than that, the values that we get from a large mature tree that's 50 plus years old, far outweigh the value that we get in essentially reaching that point. So the problem is if you lose a large mature tree, you can play the numbers game and replace it with 20 smaller trees. And you can even get an equivalent tree canopy cover, but the overall value of those smaller trees, even though the canopy cover may be the same is much less than the large mature trees that take a long time. So we have to think, well in advance of how we plan cities, to build this urban green infrastructure as we go, and, you know, the current way we do it is rather piecemeal. And if we don't put the right value on trees, what they're really worth to us, then it is easy to overlook them because ultimately everything has a bottom line.

Chris - It sounds like common sense though, doesn't it? Plant some trees now, think about the future. We're very good at being wise, in retrospect, as a human race though aren't we? Are there actually any decent rules that have some teeth in place that make sure that the sorts of common sense things that you're saying, which actually translate longterm into a really much better environment for everybody, and nature, actually gets put into play. Because there's a huge tension and pressure on developers, isn't there? They see a patch of land and they see pound signs there. We could put 15 office blocks on there and make loads of money instead of planting some tree.

Mat - Absolutely. But you know, we know those things are also somewhat temporal as well. People might be looking at office space at the moment and thinking that's perhaps not such a safe investment. What we do know is that if the value of trees is anywhere near what we think it is, then building that in to how we plan our cities, given that we're more and more of us are living in cities, 60% of the world's population by 2030, that's going to be critical to making cities livable. And of course we've seen how important that is. I think many people in Britain particularly, would look at trees and parks very differently in the last six months, then they perhaps have done before. And I think that's something we need to really emphasise, and that we can't make decisions in hindsight, as far as green infrastructure and trees are concerned, because they are long lived things. We need to think about them now. And we need to think about them for the long term.

Chris - Are you gathering actual objective evidence on this, or are you mainly sort of, gathering qualitative, people's reactions to what they think about trees, or are you actually out there, boots on the ground, making measurements of cities and what's going on in this space?

Mat - Yes. I mean, my perspective on this is I come at it from a kind of physical science point of view. So we're trying to use new observations from satellites, and aircraft, and even drones, and cameras on the ground, and new three-dimensional measurements, to make these kinds of quantitative estimates of what trees do. But at the same time, not overlooking those less quantitative, more qualitative aspects, because of course in the end, that's what many of us do value, even though we know that yes, trees are good for us, but how good they are, we might not put the right value on, as I say. So making those kinds of measurements across a range of scales, right from the ground up, through to using these kinds of new, high resolution, satellite data is giving this bigger picture look of things.