What’s the Crust On My Trees?
There is a whole community of organisms that can take up residence on the surface of trees and shrubs. Some of these are beneficial, some are harmful, and others have no effect on the plant either way.
Lichens are organisms that generally fit this latter category. Lichens are made of a symbiotic association between a fungus and an alga or blue-green bacterium. The algal partner, which lives inside the fungus, photosynthesizes and provides nutrients for the fungus, and the fungus provides shelter, water and minerals to the alga.
Lichens are often grayish-green with a crusty flattened or lobe-like appearance or some forms may even be hair-like. Some species can be yellowish or orange-red in color. They can be found growing on many solid structures both living and nonliving, such as fences, rocks, tree trunks, shrubs, house siding, and so forth. They tend to colonize areas that are sunny and, since they are sensitive to air pollutants, their presence can be an indicator of good air quality.
People may become concerned when they see lichens on trees in their yard and think that they must be harmful to the plant. This belief is heightened by the fact that dead branches on trees and shrubs may often be covered by lots of lichens. However, it’s not the lichens that caused the problems with the host plant in the first place.
Lichens are not parasites, and they don’t feed on the tissue of the tree! Rather the death of leaves and thinning of the canopy and possibly changes in the structure of the bark on the diseased or stressed plant produced the conditions ripe for lichen growth. Excessive lichen growth and dead branches on the tree or shrub may be a warning that some other factor such as a disease organism or pest or poor environmental conditions such as waterlogged or compacted soil is at work. Further investigation will help to determine what might be causing the problems. Although you will want to prune out dead branches covered with lichens, there is no need to control or destroy lichens growing on healthy plants.
Lichens are ancient organisms that can survive in some fairly harsh environments. They are used for food, shelter, and nesting material by various wildlife. Humans have eaten them and used them to monitor air pollutants and to make dyes, fabrics, medicine, and perfume. There are more than 13,000 species of lichens throughout the world. If you’d like to learn more about these fascinating creatures, there are numerous field guides and websites devoted to them.