Trees in urban areas provide tremendous benefits to society. They remove air pollution, reduce energy use, filter storm-water runoff, and reduce ultraviolet radiation. They also improve the physical and mental health of those who live in cities. But a new analysis by researchers at the US Forest Service has found that cities across the United States are losing the equivalent of 140,000 acres of urban forest each year. That’s a decline of one percent of tree cover annually, while pavement – or what the researchers call “impervious cover” – is increasing by one percent.
“The urban forest in the US produces $18 billion in annual benefits. If we lose trees, we lose the benefits associated with those trees,” said David Nowak, a senior scientist with the Forest Service’s Northern Research Station in Syracuse, New York.
Using high-resolution images from Google Earth, Nowak and his colleagues analyzed more than 50,000 pairs of photos of urban areas nationwide taken about five years apart – a pair for each of the 1,000 sites in every state. They then determined the land use category depicted in each photo and calculated how the land use changed over time.
Twenty-three states have had significant declines in urban forest cover, which includes street trees, yard and park trees, and densely forested areas; 25 states showed modest declines; and the rest had no change or a statistically insignificant increase in tree cover. In New England, which Nowak said leads the nation in urban tree cover, Rhode Island had the greatest annual percentage loss at two percent, while Connecticut had the least loss (0.3 percent).
“A one-percent decline nationwide is not devastating, but it’s a trend downward. The question is, ‘will it continue to do this?’” Nowak said. “Whether it continues downward is a choice we have to make as a society. What do we want the future to be? If we continue the same patterns, we’ll have fewer trees and more impervious surfaces, which create more problems with runoff.”
Nowak noted that urban trees are going to die naturally from old age, storms, insects, and diseases, but some will regenerate. He advocates protecting existing trees and planting new ones to restore the benefits the trees provide.
“Management efforts to minimize the loss of existing trees would likely be the most beneficial efforts to sustain or enhance tree cover,” Nowak wrote in the journal Urban Forestry & Urban Greening. “Enhancing planting or regeneration rates would also be beneficial in stabilizing or reversing this trend, but newly established trees will take time to reach their potential and often have relatively high mortality rates.”
The key, Nowak said, is for city leaders to imagine what they want their urban forests to look like in 50 years. “Instead of having forests occur through happenstance, have a plan of what you want. We need an intelligent discussion about what and when and where you want forests to be. Every region is going to be different; there is no one answer, but those decisions should be made locally.”