In the forests of Nahuelbuta, many branches seem to have a beard. Grey-green threads hang from oaks, lengas and araucarias, and in the fog they make the forest look like a fairy tale. People in southern Chile call it **barba de viejo**, old man's beard.
## It isn't a moss
Old man's beard is a **lichen**, almost always of the genus *Usnea*. A lichen isn't a plant: it's a partnership between **a fungus and an alga** living together. The fungus provides structure and protection; the alga photosynthesizes and makes food for both.
## It isn't a parasite either
It uses trees only as support. It takes water and nutrients from the air, rain and fog, not from the bark. That's why it thrives in humid, foggy forests like those of the coastal range.
## A clean-air meter
Because it absorbs everything the air carries, old man's beard is very sensitive to pollution, especially **sulfur dioxide**. Where the air is dirty, it stays a few millimeters long or disappears. Where it's clean, it can hang 10 to 20 cm or more. Scientists use it as a **bioindicator**: if a forest is full of old man's beard, the air you're breathing there is excellent.
## A natural antibiotic
Most *Usnea* species produce **usnic acid**, a substance with antibiotic and antifungal effects. It has been used as a remedy for centuries, for example to dress wounds when there was no gauze or antibiotics. Careful, though: taking it as a supplement isn't safe, and some products have been linked to liver damage.
## The trick to recognize it
Pick up a strand that has fallen to the ground and stretch it gently. If it's *Usnea*, the outer layer splits and reveals a **white, elastic central cord**, like an elastic band inside a hose. Look-alike lichens don't have it.
## Look after it
It grows very slowly, so don't pull it off trees or take it home as a souvenir. If you want to try the cord trick, use a piece that has already fallen.
Next time you walk through Nahuelbuta in the fog, look at the branches: every beard hanging there is telling you you're breathing some of the cleanest air in the country.
