Just a couple of months after the U.S. Forest Service alerted us to the staggering economic benefits provided by pollutant-scrubbing, emission-reducing, carbon-sequestering, efficiency improving urban trees, the USFS is back with some not-so-great news: the leafy multitaskers that make American cities livable are in decline.

Or, more accurately, America's urban tree cover was in decline from 2009 to 2014, when it dropped from 40.4 percent to 39.4 percent. And while a new tree canopy study headed by USFS scientists David Nowak and Eric Greenfield doesn't go as far as concluding that the urban tree cover is currently shrinking, there's also no reason to be believe that isn't the case based on past trends.

That being said, a 1 percent drop over a five-year period may not seem like a figure worth panicking about, especially when you don rose-colored glasses and assume that these lost trees have since been replaced. And in some cases they have.

But as Nowak and Greenfield's findings detail, a 1 percent drop when it comes to urban tree cover is a lot: roughly 175,000 acres decimated annually or a total of 36 million urban trees lost to disease, insect damage, development, storms and old age every year. What's more, the percentage of impervious cover in urban areas — rooftops, sidewalks, roads, parking lots and the like — increased from 25.6 percent to 26.6 percent during the same five-year period.

And just as previous studies have placed a price tag on the vast economic benefits that fast-growing cities can reap from urban trees, Nowak and Greenfield have given a conservative ballpark figure — a whopping $96 million — to the economic losses associated with five years of steady urban tree decline.

Writing for Scientific American, Richard Conniff points out that this $96 million loss only takes into consideration the aforementioned environmental benefits directly provided by trees: the removal or air pollution, increased energy efficiency due to increased shade, carbon sequestration and so on and so forth. Not taken into consideration are other significant, tree-related benefits including increased home values, reduced crime rates and happier, less stressed-out urbanites.

While the trees of Atlanta's Piedmont Park aren't going anywhere, nearly 19,000 acres of urban forest elsewhere in Georgia vanished each year between 2009 and 2014. (Photo: Tim Dorr/Flickr)

Thinning urban canopies in states big and small

Naturally, urban tree decline varied from state to state during the duration of Nowak and Greenfield's Google Earth-aided study, which was recently published in the journal Urban Forestry and Urban Greening.

Twenty-two states experienced relatively small declines in tree cover while Alaska, Minnesota and Wyoming experienced no change in tree cover at all. Three states — New Mexico, Montana and Mississippi — experienced modest but encouraging increases in coverage. Still, 22 states along with the District of Columbia experienced what Nowak and Greenfield considered to be "statistically significant" declines in tree cover in both urban cores (1 percent) and the outlying suburbs (0.7 percent) of metro areas.

Per Nowak and Greenfield, the states with the greatest annual statistical decline of tree cover were Alabama (-0.32 percent), Oklahoma (-0.30 percent), Rhode Island (-0.44 percent), Oregon (-0.30 percent), Florida (-0.26 percent), Tennessee (-0.27 percent) and Georgia (-0.40 percent). Washington, D.C., also topped the list with a -0.44 percent decline.

In terms of the overall acreage of urban forest lost, three southeastern states — Georgia, Alabama and Florida — along with Texas each exceeded 10,000 acres annually.

Not counting gains or losses, Maine had the greatest percent of urban tree cover with 68.4 percent while North Dakota had the least with just 10.7 percent.

But as Nowak explains to Popular Science, location always trumps size: "The trees in Montana might remove more air pollution than the trees in New York City, but the trees in New York City are more valuable because they are cleaning the air where people breathe, and reducing energy and air temperatures where people live and work. More than 80 percent of the U.S. population lives in urban areas. As a result, those trees are critical in terms of human health and well-being."

Providence, Rhode Island, is no doubt leafy in spots. However, the teeny-tiny Ocean State experienced the highest percent of urban tree cover decline during a five-year span. (Photo: Jeff Nickerson/Flickr)

Tree-planting and America's 'quick fix' mentality

So what can be done in states with urban areas that are shedding vital trees at an alarming rate?

Scientific American notes that some cities, in concerted efforts to counter the urban heat island effect, limit air pollution and manage stormwater, have gone out of their way to increase their urban canopies.

But seemingly more often than not, these tree-planting campaigns don't go far enough. In some cities — including those that have launched popular "1 million trees" initiatives — the target number is never reached due to funding issues and/or waning enthusiasm. As a result, newly planted trees are simply outnumbered by trees that lost to disease, age and rampant development. In cities that do reach the million-tree mark, the trees in questions are saplings that often don't get picked up by Google Earth imagery. Nowak suggests that with time, these young trees will make a difference.

Noting that American culture is "all about the quick fix," Deborah Marton of New York Restoration Project explains to Scientific American why urban tree-planting campaigns, however crucial and great for morale they may be, sometimes falter: "It's slow. It's not sexy. If you plant a new tree, that's exciting. If you water it for five years ... maybe it will grow a few inches."

"There's almost no public health, crime or environmental quality metric that you can look at that isn't made better by the presence of trees," Marton goes on to note.

William Sullivan, head of the Landscape Architecture department at the University of Illinois at Urbana–Champaign, suggests it would be helpful if cities with thinning canopies simply sat down and took time to consider the wide-ranging benefits of urban trees beyond just their aesthetic appeal. Sullivan believes that to be truly effective in an era of rising temperatures, wild weather and rampant urbanization, trees need to dominate the cityscape, not just be politely limited to parks and greenways. Cities need to be aggressive.

"Too many people think that living in closer contact with nature is nice, it's an amenity, it's good to have if you can afford it," he says. "They haven't got the message that it's a necessity. It's a critical component of a healthy human habitat."