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US urban areas are losing 36 million trees every year, study finds

Urban areas in the U.S. lost a collective total of 1 percent of their tree cover between 2009 and 2014.

Article by Morgan Erickson-Davis.

Urban and community areas in the U.S. are losing their trees, according to a new study conducted by the U.S. Forest Service. And this loss is happening at a fast clip – the study finds that, overall, these areas lost around 175,000 acres of tree cover annually between 2009 and 2014.

This loss, Forest Service researchers say, equates to the disappearance of some 36 million trees every year.

Urban forests aren’t just esthetically pleasing; they provide a variety of benefits to cities from shielding buildings from the sun and reducing cooling costs and energy consumption, filtering pollutants from water and air, mitigating flooding and erosion, and helping in the fight against global warming by storing carbon. In total, analysts estimate urban trees save the U.S. around $18.3 billion every year.

But new research published recently in Urban Forestry and Urban Greening finds urban tree cover – and the myriad benefits it provides – appears to be declining in the U.S. When researchers with the USDA Forest Service researchers David Nowak and Eric Greenfield analyzed tree cover extent in urban areas between 2009 and 2014, they found significant drop-offs in many areas.

Overall, the researchers estimate that urban areas lost a collective total of 1 percent of their tree cover in five years. When they looked at a broader land classification called “urban/community” that encompasses both urban areas and other areas of less-dense human habitation, they found that those regions lost a collective total of 0.7 percent.

Broken down by state or district, the study’s findings indicate tree cover losses were greatest in Oklahoma (0.92 percent lost per year), Washington DC (0.44 percent per year), Rhode Island (0.40 percent), Oregon (0.38 percent) and Georgia (0.37 percent).

Along with losses in tree cover, the researchers found “impervious surfaces” like cement and asphalt roads and buildings increased by 1 percent in urban areas and 0.6 percent in urban/community areas. These are called “impervious” because, unlike soil, water cannot penetrate it and soak into the ground below. This can lead to flooding in areas without adequate drainage. Pavement also has the opposite effect of trees in that it can increase air temperature, which can create “heat islands” and require buildings to spend more energy for cooling.

The study estimates around 40 percent of new impervious areas came from areas where trees used to grow.

“The pattern of decreasing tree cover and increasing impervious surfaces indicate a synergistic pattern of loss of environmental benefits (e.g., air temperature cooling by trees) and increased environmental issues (e.g., air temperature increases associated with impervious surfaces),” the authors write in their study.

The researchers caution that this trend is likely to continue unless management policy is changed to prioritize tree cover and create more programs focused on protecting urban forests.

“Urban forests are an important resource,” Nowak said. “Urban foresters, planners and decision-makers need to understand trends in urban forests so they can develop and maintain sufficient levels of tree cover – and the accompanying forest benefits – for current and future generations of citizens.”

Citation:

Nowak, D. J., & Greenfield, E. J. (2018). Declining urban and community tree cover in the United States. Urban Forestry & Urban Greening, 32, 32-55.

Banner image of New York City’s Central Park by user King of Hearts via Wikimedia Commons (CC BY-SA 4.0)

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