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Tree coverage is steadily shrinking in urban America, according to a new study of aerial photos.

Trees can do a lot for a city. From an environmental perspective, they improve air quality and also reduce temperatures, leading to lower energy consumption. From an economic one, they've been tied to increased property value, particularly in highly walkable neighborhoods. From a psychological view, urban trees even have an incredible restorative effect on our attention.

But cities aren't doing much for their trees in return. In fact, urban America is losing tree cover at a steady rate, according to a study published in the latest issue of the journal 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.

Atlanta has a great deal of urban tree cover, but that canopy is dwindling by the day. Across the study the city lost 1.8 percent of its coverage. Here's a look at Atlanta in April of 2005 (make a special note of that center square):

And that same spot in April of 2010: 

The average tree coverage for all 20 cities fell 1.5 percent, with an average annual loss of .37 percent a year. Those figures include the outliers of New Orleans and Detroit, which endured Hurricane Katrina and an emerald ash borer infestation, respectively. (New Orleans suffered the biggest tree coverage decrease, losing more than 9 percent of its canopy.) Even removing those outliers, the average tree coverage fell about 1 percent in the remaining 18 cities, at an average decline of .27 percent a year.

Here's New Orleans in 2005:

 Then again in 2009:

Meanwhile impervious coverage rose in 16 of the 20 cities over that same period, with four others (Syracuse, Chicago, Pittsburgh, and New Orleans) showing no change. Tacoma, Washington, had the greatest impervious increase, at 3.6 percent, following by Baltimore, Kansas City, and Spokane (roughly 2 percent each). The average rise for the 18 non-outlier cities was 1.4 percent, for an annual average increase of .31 percent.

The 20 major cities chosen by Nowak and Greenfield didn't technically constitute a random sample of urban America. So the researchers also took a true random sample of 1,000 urban geographical points in the United States and compared aerial photos of coverage there as well. They found a difference in degree, but not kind: on average, cities lost .2 percent of tree cover and gained 2.8 percent of impervious cover. Using those numbers, the researchers estimate an average net loss in U.S. cities of about 4 million trees a year.

Nowak and Greenfield recognize that many U.S. cities have implemented tree-planting campaigns that may slow, if not arrest the canopy decline. Those programs have almost certainly kept the situation from being much worse. Still they recommend that city residents interested in maintaining tree coverage take advantage of digital tools like i-Tree Canopy, which allows anyone to interpret canopy change using Google images. Without more regeneration projects and greater civic vigilance, urban tree loss stands to continue, the authors conclude:

Despite various and likely limited tree planting and protection campaigns, tree cover tends to be on the decline in U.S. cities while impervious cover is on the increase. While these individual campaigns are helping to increase or reduce the loss of urban tree cover, more widespread, comprehensive and integrated programs that focus on sustaining overall tree canopy may be needed to help reverse the trend of declining tree cover in cities. … Developing coordinated healthy tree canopy programs across various land ownerships can help sustain desired tree cover levels and better manage cover change.

Many thanks to the authors Nowak and Greenfield for providing the above images: Atlanta courtesy U.S.D.A. Farm Service Agency Aerial Photography Field Office; New Orleans via Google Earth.

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