High Rise Forests?

From reading about the Brooklyn Grange, I learned that “green roofs” have become desirable features for urban developers, not for food production, but for the pleasure of the tenants. Indeed, the Brooklyn Grange people utilize their farm as an event space, providing a critical income stream. So, I totally get it, everyone loves gardens, and who wouldn’t love to have a great garden in their urban tower?

This desire is not lost on architects. As Kurt Kohlstedt comments, more and more high rises are being designed to include not only plantings, but trees, and not only on the roof, but on every balcony. Not an urban farm, an urban forest.

As Kohlstedt remarks, it is one thing to draw trees in your renderings, it is another thing to actually build such a building. In this case, the CAD system is making promises that it may be difficult to meet.

One such project has been completed, Bosco Verticale (Vertical Forest) in Milan. It is certainly an interesting looking building, completely coated in fully grown trees, with most units having a nice little grove on the balcony. It doesn’t look like a forest to me, but it certainly has more habitat than most residential towers.

Le torri residenziali Bosco verticale viste dalla Torre UniCredit. https://en.wikipedia.org/wiki/File:Bosco_Verticale_from_UniCredit_Tower,_Milan_%2817591709258%29.jpg

Kohlstedt makes a number of sensible points about these projects. Any kind of plant scaping adds weight and complexity to the building. Once installed, they require non-trivial amounts of skilled care to keep them healthy and attractive. (I’m pretty sure that the ultra-wealthy clients of Bosco Verticale are not interest in DIY garden work, though they can surely afford to pay for a gardening staff.) As he notes, these costs add to the energy and carbon footprint of the building, which most likely will not be recovered from the benefits of the greenery.

He also points out that, unlike cooperative steel, glass, and concrete, trees are living things, and do not look like the renderings. They grow and change with the seasons, dying and spreading, and generally looking like trees. Kohlstedt notes that these effects will be very noticeable because tall buildings aren’t really good environments for trees (any more than rocky hillsides are), and different sides and levels of the building will have different microclimates, where the foliage will grow differently.

He also points out that the renderings do not show the wind, which is significant at higher floors. The trees up top will be thrashing around constantly, and may well be damaged or stunted by the constant wind.

In the end, Kohlstedt hits the nail on the head, when he calls them “green ornaments”, which he laments “up where they can be seen by many but enjoyed by few.:

I’ll add one additional point. Forests age. Trees build up layers of leafy mulch, drop branches, and eventually die. Trees also grow, pushing roots down and out and over the edge of cliffs. Forests fill with living and dead animals, and everything is recycled by fungus and insects. All these natural processes are not necessarily “ornamental”, and I’m pretty sure that the wealthy tenants will have their minions remove these signs of death and decay.

But eventually, the people will leave or will stop cleaning up. The forest will add a new, and potentially dramatic way for the building to age. In twenty years, as the structure wears out, roots will find and enlarge cracks, water will seep into the structure in interesting ways. Sections of the space will be filled with fallen trees. Debris will fall down the outside and inside, animals will nest and range through the “forests”.

An decrepit or abandoned building is dangerous enough, an abandoned vertical forest will be even more dangerous!  And they will fall down sooner, and in different ways than conventional buildings.


  1. Kurt Kohlstedt (2016) Rendering vs. Reality: The Improbably Rise of Tree-Covered Skyscrapers. 99% Invisible, http://99percentinvisible.org/article/renderings-vs-reality-rise-tree-covered-skyscrapers/

 

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