No, you will never have skyscrapers with trees on them

When we imagine the future of environmentally sustainable cities, it's common to depict them as forests of skyscrapers with, well, forests on them. But environmental writer Tim De Chant says that architects and futurists need to get real. Skyscrapers will never support trees. Here's why.

Though green roofs are not a pipe dream, the problem arises when we start talking about skyscrapers with trees on top. TreeHugger blames the popular designs of architect Stefano Boeri, whose concept design "Vertical Forest," above, has become many people's blueprint for what they expect to see in cities in 50 years. It's a pretty picture, but not accurate.

On his blog Per Square Mile, De Chant writes:

There are plenty of scientific reasons why skyscrapers don’t—and probably won’t—have trees, at least not to the heights which many architects propose. Life sucks up there. For you, for me, for trees, and just about everything else except peregrine falcons. It’s hot, cold, windy, the rain lashes at you, and the snow and sleet pelt you at high velocity. Life for city trees is hard enough on the ground. I can’t imagine what it’s like at 500 feet, where nearly every climate variable is more extreme than at street level.

Wind is perhaps the most formidable force trees face at that elevation. Ever seen trees on the top of a mountain? Their trunks bow away from the prevailing winds. That may be the most visible effect, but it’s not the most challenging. Wind also interrupts the thin layer of air between a leaf and the atmosphere, known as the boundary layer. The boundary layer is tiny by human standards—it operates on a scale small enough that normally slippery gas particles behave like viscous fluids.

For plants, the boundary layer serves to control evapotranspiration, or the loss of gas and water through the tiny pores on a leaf’s underside, known as stomata. In calm conditions, a comfortably thick boundary layer can exist on a perfectly smooth leaf. But plants that live in hot or windy places often have adaptations to deal with the harsh conditions, including tiny hairs on their leaves which expand each leaf’s surface area and thus its boundary layer. Still, plants in these environments aren’t usually tall and graceful. In other words, not the tall trees we see in architectural drawings.

And then there is the issue of extreme cold at skyscraper altitudes, as well as the pragmatic question of who would care for all these trees that are dangling hundreds of feet above the ground. Read more on Per Square Mile.