US Cities Lose Tree Cover Just When They Need It Most (scientificamerican.com)

An anonymous reader shares a report: Scientific evidence that trees and green spaces are crucial to the well-being of people in urban areas has multiplied in recent decades. Conveniently, these findings have emerged just as Americans, already among the most urbanized people in the world, are increasingly choosing to live in cities. The problem -- partly as a result of that choice -- is that urban tree cover is now steadily declining across the U.S.

A study in the May issue of Urban Forestry & Urban Greening reports metropolitan areas are experiencing a net loss of about 36 million trees nationwide every year. That amounts to about 175,000 acres of tree cover, most of it in central city and suburban areas but also on the exurban fringes. This reduction, says lead author David Nowak of the U.S. Forest Service (USFS), translates into an annual loss of about $96 million in benefits -- based, he says, on "only a few of the benefits that we know about." The economic calculation involves several such benefits that are relatively easy to express in dollar terms -- the capacity of trees to remove air pollution, sequester carbon, conserve energy by shading buildings and reduce power plant emissions.

Comments

Compensating

[zmaragdus]

With the emerging (but still very small) movement to add a lot of plants to the roofs / sides of buildings, I would like to see a study making a quantitative evaluation as to how much said plants can compensate for the loss of trees.

Re:Compensating

[drinkypoo]

With the emerging (but still very small) movement to add a lot of plants to the roofs / sides of buildings, I would like to see a study making a quantitative evaluation as to how much said plants can compensate for the loss of trees.

I don't know, but I can take a semi-educated guess. Old forests with mature trees typically (that is, for almost all species) sequester more carbon than young ones which are still growing. It seems counterintuitive, but the key is that old forests have substantially more biomass — the trees have to respirate in order to maintain it, and it's their respiration which is the basis of most of the benefits of trees. Therefore, the benefits will essentially scale with the biomass, which is to say that trees are more beneficial.

Some of the benefits of trees are irrespective of their biomass, for example their effect on albedo. This can as easily be produced by leafy salad greens as a redwood tree. But carbon sequestration, air filtration, and regulation of humidity (just off the top) are all linked to biomass.

Re:Compensating

[Tool Man]

You know those colored bands, known as "rings", in wood? The ones that are created anew year after year? What do you think they're made of?

Now, consider that they'll often be roughly the same thickness on average each year. Each circle is larger than the one before, so it's layering on more circumference all the time. As long as the tree is still alive, it's adding more to the trunk alone.

I'll allow that some trees die fairly soon in human terms, but many do not. Still though, even alders or poplars are bigger each year for their relatively short, fast-growing lifespan.

Re: I like trees, but

[Anonymous Coward]

Google "urban heat islands." As summers get longer and hotter, the problem is exacerbated in cities due to low albedo surfaces, heat retention by high thermal mass cement and asphalt, and runoff of surface water that in other areas would be absorbed by the soil. As a result, cities end up being several degrees hotter than their surrounding regions. Vegetative cover helps to offset these effects.

Not a Choice

[Tablizer]

Americans, already among the most urbanized people in the world, are increasingly choosing to live in cities. [Emphasis added]

I don't believe it's a choice in the direct sense, but rather an economic reality. To be competitive globally and against automation requires high collaboration among experts and specialists. You mostly find this in populated areas.

Farming automation has reduced the need for rural workers, and mining automation is also ramping up.

If your job allows you to do remote work, then it more likely can be outsourced to Timbuktu for 1/3 your wages.