Debating the merits of planting forests on the sides of tall buildings.

The Bosco Verticale is a pair of residential towers being constructed in central Milan whose name translates into "vertical forest." Hundreds of full-size trees and thousands of shrubs and plants will scale the sides of the buildings — filtering dust, absorbing carbon dioxide, reducing urban heat, conserving energy, and generally making the city more environmentally friendly. In renderings, the 250- and 360-foot towers look a little like what you'd get if the game Jenga had a child with a Chia Pet.

Bosco Verticale certainly isn't the only verdant skyscraper out there. A series of six "farmscrapers," each designed as a self-contained ecosystem, is being planned for a province in China. A 70-floor commercial high-rise called the Urban Forest, also planned for China, is meant to reflect the country's mountainous landscape. About half of the surface of the 26-story EDITT Tower in Singapore will be covered in local vegetation.

On paper these plans seem like a winning compromise between the push for urban density and the need for urban sustainability. Their obvious environmental value aside, city trees have a refreshing effect on human attention — to say nothing of their economic benefits and potential impact on crime. They're also on the decline: one recent study found that American cities are losing four million trees a year, a trend that's poised to continue unless officials make tree-planting initiatives a priority.

Writing at his NPR blog, Robert Krulwich is certainly on board with the vertical forests of the world. Krulwich is "enthralled" by the idea that our concrete jungles may one day soon look more like — jungles. While recognizing that the innovation won't be perfect out of the gate, Krulwich nonetheless applauds the architectural designs chasing the green urban dream:

They are pioneers, new neighbors being asked to live with us in the sky. They'll take in the CO2 and breathe out oxygen. We'll take in the oxygen and breathe out CO2. We'll water them. They'll aerate us. It's a whole new neighborhood. Yes, we may stumble as we rise, but rise we shall. These towers in Milan will lead the way.

In two posts over at his blog Per Square Mile, however, Tim De Chant makes a pretty strong case that we don't quite want to go where the towers in Milan are leading us.

De Chant's first post gives some practical reasons it won't work to have trees growing on the top floors of tall buildings. (None of these structures has been finished yet, according to De Chant, with Bosco Verticale the closest to completion.) If the wind doesn't get them, De Chant fears the extreme temperatures will. Beyond that, he wonders about the logistical concerns of maintaining a small forest outside its natural habitat. The urban tree advocate in him likes the idea, the student of plant physiology in him has many doubts.

(The American in me, meanwhile, wonders who will be liable the first time a tree falls from the side of a building and lands on, let's say, a lawyer's car.)

But let's suppose the designers of Bosco Verticale (and other vertical forests) have accounted for these concerns from the outset. In his second post, De Chant writes that he still finds the idea a bit gimmicky — "a way to make your building feel sustainable without necessarily being so." He calculates that if you took the estimated $4.25 million that it cost to include trees on the vertical forest, you could restore at least 2,125 acres of horizontal forest. In contrast, the Bosco Verticale will host 2.5 acres.

Here’s an alternate plan: Instead of planting trees on buildings, let’s focus on preserving and restoring places that already have, or desperately need, trees.

So the case for skyscraper trees is a little more complicated than the lovely architectural renderings would have us believe. That isn't to say these projects should be stopped — if there's a point of universal agreement here, it's that cities absolutely need more green life — but perhaps some middle ground can be explored in advance. What if some of the premium these developers can charge goes toward local reforestation, or better yet, the creation of public urban parks? Upward and outward, we say.

Flickr user EliasSchewel via creative commons.

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