US cities are losing 36 million trees a year. Here’s why it matters

The Great American Forest’s crucial role as a carbon sink is under threat from urban sprawl, according to a new study

US cities are losing 36 million trees a year. Here’s why it matters

Woody limbs covering power lines. Housing buildings full of brick. Stagnant lawns topped with grass that never gets wet. America’s urban population is growing faster than forests can. In many places, forests are disappearing.

The results are stark. Nearly a third of the Great American Forest is degraded by urban sprawl, according to a new report from the US Forest Service (USFS).

“The United States is in the midst of a global challenge, and in our own backyard, those left behind are beginning to experience the impacts,” said Ted Diadiun, the agency’s acting director. “The interconnected nature of our urban forest system requires that we redouble our efforts to stop tree loss and to leave behind healthy forests for future generations.”

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Trees are perfect carbon sinks: they store carbon in their leaves and stems, then decay. When we see forests with a healthy canopy, it’s because plants like those on New York rooftops are going through a period of “de-forestation”. This is the depletion of woodlands that occurs when large expanses of trees are cut down and left to rot.

The US is engaged in such a process of “imperceptibly ground-up” forests. Since the 19th century, the US has lost an estimated 9.2m acres of forestland, according to the USFS.

“Seventy-five percent of humans live within 100 miles of a major city. This growing urbanization is driving the expansion of urban sprawl,” said Richard Lukso, co-author of the study and a forest ecologist at Montana State University.

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Urban sprawl, which is like townland laid out on flat sand, uses woodlands to build roads, take land away from prairie and forest habitats and lay out homes on prime lands. The tall, low, isolated landscapes of old forests have gone – more than 40% of all New York City’s “hardwood forest” has been cut down since 1872, according to the report.

Green areas of the Great American Forest. Photograph: Jeff Schaeffer

This is causing a dramatic decline in a crucial natural resource: as forests shrink, these areas are left polluted with more carbon than they had been producing before.

That means less wind which blows away pollution, less sun that reflects radiation back into space, and less carbon dioxide in the atmosphere. On top of that, global warming will cause additional effects such as droughts, longer fire seasons and global sea level rise, as forests burn. The research team calculated that forests in the Midwest alone are releasing 44m tonnes of carbon a year into the atmosphere.

This is not to say that urban sprawl is good for the forest’s health.

“Urbanization does not equal forest destruction,” said Lukso. “In fact, the success of urban development and the success of forest restoration is two sides of the same coin. Many communities are moving forward with environmentally friendly and ecologically sound urban development. The projects we observe would not be possible without thoughtful forest management.”

But the forests used to cover half of America, but are now 13% less than they were in 1890. They are becoming increasingly dry. These changes are often under-appreciated by policymakers.

“Some of the efforts to protect forests in America are either happening too late or are focused on the wrong type of forest,” said Lukso. “We want to focus on the trees left behind. We want to preserve forestlands where there is wildfire resilience, high carbon storage and carbon management.”

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