gray-green patch on tree trunk - lichen

Common greenshield lichen. Photo: Anita Rose, USDA Forest Service,

Updated: April 6, 2021

Key points

  • Lichens are living organisms composed of a fungus and algae living in a symbiotic relationship. In a symbiotic relationship, two organisms function in a way that is mutually beneficial. Because the algae derive nutrients through photosynthesis, and the fungi protect the algae from drying out, lichens can live and grow in extremely barren areas.
  • They grow in colonies on tree trunks, rocks, and fences, even in Antarctica.
  • Lichens grow in many interesting forms. Those with a flattened and crusty appearance are called crustose lichens. Foliose lichens have raised, leaf-like lobes. Fruticose lichens have branched growths with finger or thread-like projections.
  • Colors range from gray-green to bright orange-red.
  • Lichens are often blamed for killing a tree or shrub but this is not true. They do, however, grow on slow-growing and sometimes declining trees and shrubs.  


  • Lichens grow harmlessly on tree trunks and no control is necessary.
  • They are considered an indicator of good air quality. They are extremely sensitive to sulfur dioxide and are not usually found in industrial areas.
  • Lichens rarely develop on rapidly growing trees, probably because the bark is shed before the lichens have time to spread.
  • There may be more lichens on a mature, declining, or less vigorous tree, due to slower growth rate. Lichens themselves do not cause the decline.

Lichen identification and biology

There is a lot to be said for simply enjoying the natural beauty of lichens without trying to label them. However, lichens are easier to identify, at least to the family level, than you might think.  

If you can distinguish soil, rock and tree bark, you are off to a running start.  Different lichen species specialize in growing on these three types of substrate, so this is an important first clue to a lichen’s identity. Hunting for lichens in the winter is facilitated by the general lack of foliage, making bare soils, rock outcrops, and tree trunks more visible than usual. As a special bonus, storms litter the ground with lichen-encrusted branches, revealing treasures that would normally be above our reach. Even small branches can host several species, each with its own unique color and shape.  

Body shape is another clue that is easily understood by beginners.  Lichens occur as either:

  • Powdery, crusty colors on surfaces, reminiscent of spray paint.
  • Flat leafy shapes, usually rounded in outline.
  • Three-dimensional shapes reminiscent of shrubs, beards, cups, etc. 

Color is another easy taxonomic clue, and it is during gray, wet weather (typical of March) when lichens are most colorful. During dry spells, lichens shrivel up and their surfaces become opaque and faded-looking. This protects the internal, photosynthesizing algae from desiccation. When re-moistened, lichens expand and their surfaces become transparent again. Light and moisture can reach the internal algae, and photosynthesis resumes. The algal colors, which are often brighter than that of the fungal surface, shine through. 

Lichens produce special structures for dispersing their progeny, and these result in interesting changes in shape, texture and color which are further clues to a lichen’s identity.  The reproductive lives of lichens are unique, involving asexual methods, as well as sexual reproduction of the fungal symbiont.  As with many things botanical, these structures have intimidating names like insidia, soredia, and apothecia.  Fortunately it is not necessary to remember the names of the dispersal structures to use them successfully as taxonomic clues.

The life cycles of many native animals are intricately tied to lichens.  Here are a few examples from animals native to Maryland:

Lichens also have many other stories to tell, intertwining their presence in almost all aspects of ecology and human endeavor. Here are but a few examples that illustrate their importance:

  • Approximately 8% of terrestrial earth is covered by lichens.
  • Lichens absorb nutrients from the air, and can be used as air quality indicators.
  • Lichens contribute nitrogen and minerals to the ecosystems in which they occur.
  • Historically, humans have used various species of lichens to make dyes and medicines.
  • Lichens produce unique biochemicals to fend off herbivores, prevent freezing, and stop seeds from germinating in their soft, moist tissue.  These chemicals hold promise for the development of new medicines and agricultural chemicals.

Additional resources

Take a photo tour on the Maryland Biodiversity Project website. Click on the icon for thumbnail images, and then click on the icon for slideshow. 

Lichens of the North Woods: A Field Guide to 111 Northern Lichens by Joe Walewski is an affordable field guide for beginners. The introduction includes a very readable overview of lichen ecology, reproductive biology, and human uses. A simple system of three substrates and three basic shapes allows you to quickly start keying out lichens.

Lichens of North America, by I. Brondo, S.D. Sharnoff, and S. Sharnoff is a big, beautiful picture book. It contains in-depth descriptions of lichen biology and a detailed key to the 3600 species found in North America.

Sara Tangren, Ph.D., former Agent Associate, University of Maryland Extension, contributed to this page.