Wanna Save the World? Make Sure You Plant the Right Tree

Problems like global warming are so overwhelming that most citizens feel there's nothing they can do to help. That's wrong.

If you want to help, scientists across the country are saying these days, plant a tree. Better yet, work with your city to improve its urban forest.

But here's the hard part. We've all known for years that trees are good for the environment, soaking up air pollutants and greenhouse gases, and a little shade in the middle of what scientists are calling urban heat islands can't hurt. However, not all trees are the same. They all help to clear the air, in varying degrees, but many also contribute to air pollution by emitting volatile organic compounds, such as isoprene, which help form ozone, a health hazard.

That led researchers at the State University of New York in Syracuse to come up with a new angle. There's lots of trees in Syracuse, but many of them are the wrong kind.

Allan Drew, a forest ecologist in SUNY's College of Environmental Science and Forestry, teamed up with several colleagues to see if Syracuse could create a much more effective urban forest. They found that simply mixing the types of trees could make a huge difference.

"We've demonstrated that it's feasible," Drew says.

The researchers took a national program, sponsored by the Forest Service, a step further. For years now, the Forest Service has worked with cities to help local officials understand the complex problems associated with creating an effective urban forest. If you want to clean the air, don't plant willows, for example, because they don't absorb very much carbon as they grow, and they emit a lot of junk.

Syracuse researchers found that if they could replant their city with trees that are great at sequestering carbon compounds, especially carbon dioxide, they could increase the removal of carbon by more than 300 percent. But they also found that air quality would actually suffer from an increase in volatile compounds.

So they looked at mixing the forest, emphasizing trees that are good performers when it comes to carbon sequestration and don't emit a lot of junk. They came up with a list of 31 species, including American basswood, dogwood, Eastern white pine, Eastern red cedar, gray birch, red maple and river birch. That combination, they found, would increase carbon sequestration by 86 percent, and reduce the emission of volatile compounds by 88 percent.

They also eliminated trees that are so invasive they become pests, like the European buckthorn, and the American elm, which is vulnerable to Dutch elm disease.

Drew says the Syracuse plan would probably work in many other areas, but trees can be fickle. They don't perform the same way in different regions, and their ability to clean up the air varies with age, rate of growth, and many other factors.

"You could predict what the emissions might be for a red oak (fairly high) but if that tree is growing in a shaded area, its emissions are going to be less than if it was out in an open, sunny area," he notes. "There's a certain amount of variability."

Emissions increase with temperature.

"The warmer it is, the more of a potential problem you have," Drew says. "It's probably more of a factor in the sun belt, but the potential to vary the species mix and achieve good results are probably better."

Many cities across the country have completed studies of their urban forests, and some of the figures are impressive.

Atlanta has more than nine million trees which soak up 46,345 tons of carbon every year. By contrast, Calgary, Canada, has nearly 12 million trees, but they only sequester 21,422 tons per year. The difference is due largely to different species -- many conifers in Calgary, many mature, broad-leafed trees in Atlanta.

According to the Forest Service, large diameter, long-lived, leafy trees tend to be the most beneficial.

This is still a relatively new field, and scientists are finding that it's more complex than it seems on the surface.

Colorado State University researchers studied a eucalyptus plantation in Hawaii to see how good those popular trees are at sequestering carbon. Surprisingly, they found that the eucalyptus performed better if they were interspersed with another local tree, the mimosa. They think the mimosa enriched the soil with nitrogen, stimulating the growth of the eucalyptus. Carbon is absorbed in the new growth.

Not everybody, however, is on this band wagon. Some researchers point out that trees require maintenance, which frequently involves power tools, which gunk up the air.

And water is a big factor. Many plantations around the world now grow trees as part of the fight against global warming, but researchers at Duke University warned recently that there's no free ride here.

"We believe that decreased stream flow and changes in soil and water quality are likely as plantations are increasingly grown for biological carbon sequestration," the Duke scientists concluded.

And, by the way, trees are only a short-term fix. Eventually they die, which is why the Forest Service recommends trees that are expected to live at least 50 years. Dead trees rot, releasing all that sequestered carbon.

So it's not the only answer, and it won't end the threat of global climate change. But it could buy time.

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