SCOTT SIMON, HOST:
Trees are struggling to survive in many cities, right at a time when the benefits of trees are needed most. Research published recently in the Journal of Forestry shows the number of urban trees has dropped in the U.S. David Nowak is one of the authors of that study. He's an emeritus senior scientist at the United States Forest Service, joins us from Albany, N.Y.
Thanks so much for being with us, sir.
DAVID NOWAK: Hi, Scott. Thanks for having me.
SIMON: What did you discover in urban tree populations?
NOWAK: Well, what we did is we looked at photographic evidence from across the United States in urban areas. And what we found is that between 2009 and 2014, most of the states - 46 out of 50 states, or 51, if you include D.C. as a district - lost tree cover during that five-year period. And we estimated we're losing about 36 million urban trees in our country per year.
SIMON: What's happening to trees? Do we know?
NOWAK: Many things - one of the obvious ones that we saw was development. That tree was there. And all of a sudden there's a parking lot or a building. So we're taking out natural landscapes, if you will, to put in some new developments.
SIMON: Yeah.
NOWAK: Other areas, we're just losing trees, and there's multiple reasons. It could be choices of homeowners taking trees out. We have insects and diseases coming through. They're taking trees out. Some trees die just from old age. I mean, trees don't last forever. But right now we're in a phase that we're not getting enough tree cover coming in to offset the losses that we're seeing in the canopy.
SIMON: And remind us, why is it important to have trees in urban areas?
NOWAK: Wow. they have huge impacts on human health and well-being. I mean, we have a whole host of things that trees do that people may not realize. We stand in the shade of a tree; we feel cooler. Temperature leads to changes in energy production. So if we cool the environment and shade our buildings, we don't produce as much energy to cool, which then affects carbon emissions.
Trees remove pollution from the air. It affects water quality by changing the water flows by the trees evaporating water and slowing the flow down. Trees sequester carbon. This is one of the few benefits you can actually see a tree accruing 'cause as the tree gets bigger through time, half of the dry weight of that tree comes from carbon from the atmosphere.
SIMON: Yeah.
NOWAK: On the other side, when the tree decays and dies, that carbon goes back to the atmosphere. So trees are these vessels that can store carbon. And lastly, trees absorb UV radiation.
SIMON: Well, you've convinced me. The Inflation Reduction Act, I gather, has allocated $1.5 billion for the U.S. Forest Service to plant trees in cities. Is the solution to plant more trees or different kinds?
NOWAK: Probably yes to both - to plant more trees and different kinds. The other thing that you want to think about this, too, is also, we might want to change how we're managing our city systems because 2 out of 3 trees in urban areas in the United States come from natural regeneration, not from planting. And the reason we don't have more natural regeneration occurring in cities is because we mow back that natural regeneration to create lawn landscapes.
So one option is to change how we - our mowing patterns and allow natural regeneration occur - will not work everywhere. So on the other side, we need to plant more trees to help facilitate for the loss of trees from all those things we talked about in the past. And the reason we might want to consider - and this is a tricky question - new species is the issue of climate change.
So as climates tend to change, the natural pattern of species composition will change also. And cities tend to have more exacerbated impacts of climate change. They tend to be warmer already. So we need to look at what is our climate going to be probably 20 to 30 years in the future and pick the species that will survive under the conditions going forward. But you don't want to do it too soon. But you don't want to do it too late. So it's trying to find that window of, what are the best species that'll likely survive in the coming years?
SIMON: David Nowak is emeritus senior scientist at the U.S. Forest Service.
Thanks so much for being with us, sir.
NOWAK: Thanks for having me, Scott. Appreciate it.
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