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Public Radio's Environmental News Magazine (follow us on Google News)

Urban Forests

Air Date: Week of

Only one city, Syracuse, NY, exhibited an overall increase in tree cover. However, the increase was dominated by European buckthorn, an invasive shrub. (Photo: David Nowak, US Forest Service Northern Research Station)

Forests in U.S. cities are on the decline. Pests, drought, and urban development are taking their toll on the trees. David Nowak tracks the health of the nation’s trees for the U.S. Forest Service. He tells host Bruce Gellerman about the plight of the forests.


GELLERMAN: U.S Forest Service researcher David Nowak says, he thinks he’ll never see a harder worker than an urban tree. As Nowak tells it, not only do tree roots and leaves clean our air and water, the canopies cut energy costs, lower our fast paced, urban stress levels, and beautify our cities.

Problem is: America is losing its urban forests. According to Nowak, last year alone, our cities lost about four million trees. David Nowak joins me from his office at the US Forest Service’s Northern Research Station in Syracuse, New York. Hey David, welcome to Living on Earth!

NOWAK: Thanks Bruce, it's great to be here.

GELLERMAN: So, how do you know that we’re losing so many trees?

NOWAK: Well, we looked at historical imagery from the last five years or so over various cities and across the United States and looked at what we called paired image analysis to compare what cover was say five years ago and what cover is today. And we’re showing that most of the cities in the country are showing a decline in urban tree cover.

GELLERMAN: What’s happening to the trees?

NOWAK: It could be a whole bunch of things. We really don’t know exactly what it is. This trend of loss is both trees coming in and trees going out, so people are planting trees, trees are naturally regenerating, but we’re also losing trees to old age, to insects and disease and to development and the net change of what we’re looking at, it’s a trend of going downward.

GELLERMAN: So you looked at 20 U.S. cities.


GELLERMAN: And what’s the trend in terms of numbers?

NOWAK: It’s dropping at about, say, approximately point three percent of the city area per year on average. The big loser, obviously, which we expected, was New Orleans - because we targeted New Orleans specifically to pick imagery before the hurricane and then about five years after the hurricane. The other big losers were Huston, Albuquerque, Baltimore and Atlanta.

GELLERMAN: So, do we know why Huston, Albuquerque, Baltimore and Atlanta - why they might have lost trees?

David Nowak, Project Leader and Research Forester at the US Forest Service Northern Research Station (US Forest Service Northern Research Station)

NOWAK: It’s a combination of those natural factors, of those insects and diseases, and human factors, of development or people’s choices.

GELLERMAN: So, none of these 20 cities that you studied, none of them had an increase in the number of trees?

NOWAK: No, one city did.


NOWAK: The city of Syracuse was one city that had a one percent increase in canopy cover, and the reason that Syracuse has gone up is because of an invasive shrub, European buckthorn, has almost tripled in population in the last five to ten years. Syracuse had a Labor Day storm in 1998 that took out a lot of trees so it created more space. So, our theory partly is that some of this open space, that was created after the storm, we have this buckthorn in there that tends to be invading some of these sites.

GELERMAN: But a shrub is not a tree.

NOWAK: Well, it depends on how you classify a tree or a shrub. If you look in the literature, a lot of these plants are classified as large shrub or small tree, so by our definition is that if it has a diameter of about four and a half feet and is at least one inch across, we classify it as a tree in the woody class, which would be a large shrub or a small tree.

GELLERMAN: Well, as I said in my introduction, it seems that urban forests are the hardest working trees in the country, at least according to the chief of the U.S. Forest Service. How hard do they work? What do they do?

NOWAK: Well, they clean the air, they help clean the water by intercepting water and reducing runoff, they take in carbon dioxide, they shade buildings and reduce air temperatures in cities, it has huge impacts on energy use within urban areas. They produce wildlife habitat, they have effects on human health in terms of both by changing air quality and in terms of people just viewing vegetation and how our body responds to seeing the vegetation, we become more relaxed…

GELLERMAN: So, David Nowak, I was looking at a study about New York City going back to 2006, and it found that the value to the city was about 122 million dollars, that’s 209 dollars a tree of carbon sequestration, and flood control, that kind of thing - sound about right?

NOWAK: Sounds reasonable because there are many services that trees provide, and many we can’t even quantify. Some of the more direct ones - energy is fairly direct, pollution is more difficult, carbon and its effects on water. So there’s all these services that come from one plant or one plant system that have a whole multitude of benefits, so it sounds like a reasonable number.

GELLERMAN: Now, if you don’t have a tree, what you have then is bare ground and you can have grass, and you can have what you call ‘impervious cover,’ that would be what, like sidewalks, roads, that kind of thing?

NOWAK: Yes. It has to do with population densities and how the cities are structured in terms of where the people reside. If pack a lot of people in, you need a lot of infrastructure and you tend to increase the impervious surface.

GELLERMAN: What’s the most tree-covered city in the United States?

Aerial view of downtown Los Angeles. There's an average increase of about 550 hectares in impervious cover (such as pavement and rooftops) each year. (Google Earth)

NOWAK: Ah, that’s a good question. The…I don’t know offhand because we haven’t analyzed every city. On average the states that have the greatest amount of tree cover in urban areas tend to be Massachusetts and Connecticut, they tend to have on average over 60 percent canopy cover within their urban areas, which is fairly high. But for an individual city some of the cities down south had significant amounts of tree cover - Atlanta had 52 percent, Nashville had near 50 percent also, so those have lots of cover.

GELLERMAN: What can the average person do about the loss of trees, I mean, ok - I can plant a tree, I suppose, right?

NOWAK: You can plant a tree, you can allow natural regeneration to occur, there’s often reasons new trees don’t come in. Particularly in the east coast of the United States, we prevent trees from coming in, we mow our lawn, we put impervious surfaces down.

More importantly we can understand what our forest is and what it's doing so we can get better information about this system that we live with and this nature in our backyard. What type of species do we have in the cities, what are these services they are providing and how many trees do we actually need to plant to sustain cover into the future. We built a tool called iTree that’s out there for free to help people measure their forests and understand some of the services that it provides.


NOWAK: iTree. It helps you calculate, you can do, you can sketch an iTree design of your house and your backyard online and put a tree around there and get a quick estimate of benefit in terms of energy conservation, other benefits that it provides. With the new app that will be coming out this spring, you can actually use your smart phone and collect data in the field and then download it into the computer program.

And it takes in local weather data, local pollution data, and simulates some of these ecosystem services that we talked about. So, we believe that having data on your local area is very important to develop management plans and structures to help improve the forests through time. We’re hoping to get people to measure and engage school kids and homeowners to understand their landscape and help make decisions for a better future.

GELLERMAN: Well, David, thank you so very much.

NOWAK: Thanks Bruce, it's been great.

GELLERMAN: David Nowak is with the U.S. Forest Service in Syracuse, New York. Well, about two hundred years ago, John Chapman made a name for himself by walking the Ohio River valley preaching the gospel and planting orchards along the way. In doing so, Johnny Appleseed passed along food for thought, and created forests that frontier folk could feast on.



‘Tree and Impervious Cover Change in U.S. Cities’

David Nowak

i-Tree, Tools for Assessing and Managing Community Forests


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