If you’re looking for a reason to care about tree loss, this summer’s record-breaking heat waves might be it. Trees can lower summer daytime temperatures by as much as 10 degrees Fahrenheit, according to a recent study, US cities are losing 36 million trees a year.
But tree cover in US cities is shrinking. A study published last year by the US Forest Service found that we lost 36 million trees annually from urban and rural communities over a five-year period. That’s a 1% drop from 2009 to 2014. If we continue on this path, “cities will become warmer, more polluted and generally more unhealthy for inhabitants,” said David Nowak, a senior US Forest Service scientist and co-author of the study.
Nowak says there are many reasons our tree canopy is declining, including hurricanes, tornadoes, fires, insects, and disease. But the one reason for tree loss that humans can control is sensible development. “We see the tree cover being swapped out for an impervious cover, which means when we look at the photographs, what was there is now replaced with a parking lot or a building,” Nowak said. More than 80% of the US population lives in urban areas, and most Americans live in forested regions along the East and West coasts, Nowak says. “Every time we put a road down, we put a building and we cut a tree or add a tree, it not only affects that site, but it also affects the region.”
The study placed a value on tree loss based on trees’ role in air pollution removal and energy conservation. The lost value amounted to $96 million a year. Nowak lists 10 benefits trees provide to society: Heat reduction: Trees provide shade for homes, office buildings, parks, and roadways, cooling surface temperatures. They also take in and evaporate water, cooling the air around them. “Just walk in the shade of a tree on a hot day. You can’t get that from grass,” Nowak said.
To get the full temperature benefit, tree canopy cover should exceed 40% of the area to be cooled, according to a recent study in the Proceedings of the National Academy of Sciences. “A single city block would need to be nearly half-covered by a leafy green network of branches and leaves,” the authors wrote. Air pollution reduction: Trees absorb carbon and remove pollutants from the atmosphere.
