If you look up into the branches of a stand of oak trees on Cape Cod, especially in the winter when the leaves are absent, you’ll likely notice that the tree is still essentially green. The branches are covered with something light green and fluffy. That “fluffy stuff” is a type of lichen called Usnea, or old man’s beard. Usnea is a genus of fruticose lichen that grows anchored to twigs and branches of trees and shrubs using a holdfast. “Fruticose” refers to the lichen’s coral-like shrubby growth form. The disc-shaped structures at the ends of some of stalks are reproductive structures, known as apothecia.

Usnea attach to branches using holdfasts.

Usnea can reproduce asexually, or sexually through production of apothecia. 

Like most other lichens, Usnea is the product of a symbiotic relationship between a fungus and an alga, giving the combined organism a set of properties neither of the individual species would possess on their own. The algal partner provides food through photosynthetic chlorophyll, while the fungal partner prevents the lichen from drying out through it’s network of protective filaments. As a result, lichens can often survive extended dry conditions.

Because Usnea often grows on sick or dying trees, many people mistakenly blame the lichen for the tree’s illness. However, the lichen is simply more likely to colonize dead or mostly leafless branches since the pre-existing loss of canopy leaves allows for greater photosynthesis by the lichen’s algae. Lichens do emit a weak acid that breaks down organic material such as dead wood, providing a vital recycling service in nature, but there is no evidence that Usnea causes harm to living trees.

Usnea is also very sensitive to air pollution, particularly to sulfur dioxide. In poor air quality, Usnea may grow to a length of only a few millimeters, if they survive at all. Where the air is unpolluted, however, individual stalks can grow more than 4 inches long. Due to this sensitivity, Usnea can sometimes be used as a bioindicator.

Long stalks on Usnea are an indicator of good air quality.