New study: U.S. cities losing 4 million trees a year
Eric Jaffe, writing in the online news magazine The Atlantic Cities (a spin-off of what used to be called The Atlantic Monthly), reports on the findings of a new study in Urban Forestry & Urban Greening.
Using aerial photographs to compare changes over time in 20 major U.S. cities, researchers David Nowak and Eric Greenfield of the U.S.D.A. Forest Service found that tree coverage is on the decline, while impervious cover — roads, buildings, sidewalks, and the like — is on the rise:
Tree cover in 17 of the 20 analyzed cities had statistically significant declines in tree cover, while 16 cities had statistically significant increases in impervious cover. … City tree cover was reduced, on average, by about 0.27 percent/yr, while impervious surfaces increased at an average rate of about 0.31 percent/yr.
Nowak and Greenfield collected recent digital aerial images for at least 1,000 random points in 20 large American cities, and coupled them with images at the same points from roughly 5 years earlier. Trained photo interpreters then classified the various types of coverage at each point: tree coverage, grass coverage, building coverage, and so on.
Their subsequent analysis showed clear trends away from tree coverage and toward impervious coverage. All but three of the cities had a statistically significant loss in tree coverage, with two others showing a non-significant loss (essentially no change). Houston (3 percent) and Albuquerque (2.7 percent) suffered some of the biggest loses. Only Syracuse showed a gain in tree coverage — and that of 1 percent.