Singapore's aspiration to become the world's garden city notwithstanding, most metropolises are the antithesis of green. If we're going to become more energy efficient, cities must become denser still, which means even more concrete and fewer plants. But what if we could turn the stone and brick faces of all those buildings into a habitat for one of nature's most tenacious symbiotes?
Lichen often lives on the bare face of rocks and trees, inhabiting territory where no other living thing can even get a toehold. It's a unique collaboration of fungi and algae, in which the fungus provides structure and protection to the algae, which feeds itself and its host through photosynthesis.
Lichen is light colored, and plants that respire help cool anything they're attached to, so it's not so far-fetched to imagine that if you could pull it off, a lichen-encrusted building wouldn't just look like a ship you just pulled off the ocean bottom: It would also help keep itself and the city around it cool.
In order to tease out whether this is even possible, Elizabeth Demaray, a professor of fine arts at Rutgers-Camden, figured out how to turn lichen into a slurry she can "paint" onto buildings . One building on 14th street in New York City is even contemplating covering its entire facade with the stuff.
"Lichen can live where plants cannot, and it can thrive without human intervention or maintenance. Also, anyone can "plant" it--all you have to do is smear it on the outside of your building," Demaray said via email.
In nature, lichen thrives in arid, exposed environments like mountaintops. The granite and sandstone facades of New York City's buildings are reasonable facsimiles for these environments. Nothing in nature cultivates lichen on its surface deliberately, but for plants and animals in general, the most efficient way to keep cool (or warm up) is to change color. Lichen should reflect even more of the sun's light and heat than conventional substrates .
Lichen attaches to buildings and other surfaces using an enzyme, says Demaray, and if it doesn't "take," it simply dries up and blows away, leaving the building unharmed and potentially spreading its spores to other structures.
To make sure that the lichen she used would be suitable for buildings in New York City, Demaray collected Flavoparmelia baltimorensis, a rock lichen, from boulders within a five mile radius of the city. "In addition to rocks, I actually found it growing on blacktop," says Demaray.
Demaray isn't a scientist, but her work is often informed by science. She does experiments, just not the sort that a conventional scientist would ever attempt. (Her effort to feed an ant colony on nothing but McDonald's  is not to be missed.) For the lichen project, she had to convince residents and landlords on 14th street to let her paint goopy lichen precursor on their buildings, which was "No easy feat, I assure you," she says.
Demaray doesn't know how well her current crop of lichen--which was painted onto a handful of spots and buildings--will hold up, but that's all right. This was an art project , and as with science, it works mostly as a hypothesis. Lichen are already growing in profusion on 14th street, she discovered, but exclusively on trees. It's possible there is a tree lichen that could make the leap to buildings, or a species of rock lichen from a harsher environment that would be even more suitable for buildings.
There is no shortage of methods for "greening" the faces of buildings , but conventional plants probably require far more upkeep than the humble, and hardy, lichen. Demaray's solution exemplifies one of the dictates of biomimicry that is too often overlooked: Nature generally goes for the simplest and most robust solution.
[Image Courtesy Elizabeth Cheviot]