gray-green patch on tree trunk - lichen

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

Updated: February 27, 2023

Key points

  • Lichen are non-destructive, 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, lichen can live and grow in extremely barren areas.
  • They grow in colonies on tree trunks, rocks, and fences, even in Antarctica.
  • Lichen grows in many interesting forms. Those with a flattened and crusty appearance are called crustose lichen. Foliose lichen have raised, leaf-like lobes. Fruticose lichen have branched growths with finger or thread-like projections.
  • Colors range from gray-green to bright orange-red.
  • Lichen 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.
Algae and lichens on tree trunk
Algae, mosses, and lichen growing on a tree trunk
  • Algae and mosses often grow as green patches on the bark of trees.
  • Contrary to popular belief, moss does not grow only on the north side of a tree. Algae and mosses grow in any moist shady site.
  • They live on the tree bark without harming it.
  • Mosses are non-vascular plants, able to colonize on hard, impervious surfaces such as bare rock and brick, as well as tree bark.
  • Moss and algae are primitive plants that derive nourishment through photosynthesis. Some algae may have a moist or slimy appearance. While they are both usually green, colors may range from blue-green to black. No controls are necessary.


  • Lichen, algae, and mosses 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.
  • Lichen rarely develops on rapidly growing trees, probably because the bark is shed before the lichen has time to spread.
  • There may be more lichen on a mature, declining, or less vigorous tree, due to a slower growth rate. Lichen themselves do not cause the decline.

Lichen identification and biology

There is a lot to be said for simply enjoying the natural beauty of lichen without trying to label them. However, lichen 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 lichen 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.  Lichen 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 lichen are most colorful. During dry spells, lichen shrivel up and their surfaces become opaque and faded-looking. This protects the internal, photosynthesizing algae from desiccation. When re-moistened, lichen 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. 

Lichen 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 lichen 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 lichen.  Here are a few examples from animals native to Maryland:

Lichen 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 lichen.
  • Lichen absorb nutrients from the air, and can be used as air quality indicators.
  • Lichen contribute nitrogen and minerals to the ecosystems in which they occur.
  • Historically, humans have used various species of lichen to make dyes and medicines.
  • Lichen 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.