Palm trees: admittedly not the most beneficial urban tree, but still nice to look at. REUTERS/Carlos Barria

If the state has a better grasp on the dividends it reaps from trees, it might support investing in them.

It’s hard to motivate meaningful responses to abstract environmental problems. That’s why some scientists have thrown their weight behind putting dollar signs on nature. Their hope is that if people better grasp the dividends society reaps from rivers, forests, soil, and the atmosphere, they might support investing in protecting it.

Certainly, that’s the philosophy the U.S. Forest Service has adopted in the agency’s work with urban canopies. Scientists there have calculated the dollar value of urban trees in cities around the U.S., measuring and monetizing the benefits trees provide by sequestering carbon, improving air quality, providing shade, reducing noise, and absorbing rainfall. They’ve built specialized software, i-Tree, to inventory urban trees and help evaluate their worth.

Their latest coup: adding up the value of California’s 9.1 million street trees. A new report from the U.S. Forest Service's Pacific Southwest Research Station used i-Tree to find that the canopies lining the state’s streets, avenues and boulevards are worth about $1 billion in benefits to cities and residents. That number breaks down like this:

  • $10 million in carbon storage;
  • $18 million in removing air pollutants;
  • $41.5 million in catching rainfall;
  • $101 million in energy savings; and
  • $839 million in boosting property values.

Those sound like huge numbers and big boons to society, and they are. "We've calculated for every $1 spent on planting or maintaining a street tree, that tree returns, on average, $5.82 in benefits," the forester and lead author Greg McPherson said in a statement. "These trees are benefiting their communities 24 hours a day, 365 days a year."

Still, California’s urban forests could be in much better shape. The study found that, although the number of street trees has increased over the past three decades, their density has fallen. In the late 1980s, there were roughly 105 trees per mile on average; now there are only 75. And the majority of California communities surveyed were over-reliant on a single species of tree, making those ecosystems vulnerable targets of pests or pathogens. (And no, an overabundance of palm trees wasn’t a major concern; the London plane tree was.)

With the findings of this study, state foresters should have a better sense of what kind of attention is needed where—and a $1 billion argument for why. “Money drives decisions,” David Nowak, the U.S.F.S. scientist who developed i-Tree, told me in April. “It’s the necessary evil of the game.”

There are drawbacks to monetizing forests and other environmental features. A blanket number can never account for all the hidden, intertwining good that ecosystems do for humans. But at least it might prod us to do some good for them.

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