A storm brought down this decayed tree limb recently. It supports a large colony of lichens. I have always been amazed by these growths.
Lichens do not cause harm to a tree, but they can be a sign of poor health. They are not parasitic but are opportunistic, taking advantage of poor growth and open light from thinning branches. They get their nutrients from the atmosphere, rather than from their hosts or from the soil. Lichens grow slowly, most only a few millimeters a year. Those that grow upright, such as the coral-like lichen in these photos, can grow up to a couple of centimeters, or a bit less than an inch a year. Given the right host and environmental conditions, they can survive on old trees and rocks for prolonged periods of time, hundreds and even thousands of years!
Lichens are susceptible to air pollution, especially sulfur dioxide. The presence of shrubby, hairy and leafy types of lichen is an indicator of good air quality, because these types of lichen are particularly vulnerable to pollution.
Some lichens in forests are important food sources for birds and other animals, and some lichens are useful for humans, providing food and used in producing dyes and perfumes. Currently, research is studying their potential use in antibiotics and sunscreens.