Word of the Week: Lichen

lichen [ lahy-kuhn ] noun: a complex life form that is a symbiotic union of two separate life forms, a fungus and an alga


A variety of crustose and foliose lichens colonize the trunk and branches of Amelanchier arborea (downy serviceberry) in the Sunny Garden in Bon Air Park, Arlington, Virginia. In this case, their presence was symptomatic of an unhealthy tree that was later removed.
Photo © Mary Free

There are about 3,600 known species of lichens in North America. A lichen anchors itself to a substrate like a rock or tree with mostly non-penetrating fungal filaments called rhizines or a central peg-like extension from its thallus (body) called a holdfast. Lichens appear on healthy trees, but also can be symptomatic of a tree under stress or in decline when other signs such as dying foliage are present.

The fungus and alga of a lichen have a mutually beneficial relationship. A lichen does not feed on other organisms even though the fungus is incapable of producing its own food. Instead, it relies on its alga partner to photosynthesize the food and in return it provides structural support and protection (fungal filaments collectively called the medulla comprise most of the lichen thallus) as well as water and minerals for the alga.

Lichens are poikilohydric, which means that they have no means to prevent desiccation. However, they are desiccation-tolerant and can withstand “repeated periods of hydration and dehydration without injury.” (Smith 1997) Lichens enter a dormant phase when they are dry; their structures are brittle and their colors are pale. They rehydrate by absorbing the moisture of fog, rain, melted snow, or even water vapor during periods of high relative humidity; their structures become supple and their hues intensify, becoming brighter or darker, indicating that they are photosynthesizing.

A variety of crustose and foliose lichens colonize the trunk and branches of a declining Amelanchier arborea in the Sunny Garden. In this case, their presence is symptomatic of an unhealthy tree. Photo © 2019 Mary Free

Both foliose and fruticose lichens have attached themselves to this birch tree at the Fort Shantok Archaeological District in Montville, Connecticut. Crustose and foliose lichens cover the boulder in the foreground.
Photo © Mary Free

There are three main types of lichens, which vary in color from off-whites to grays to black, and from differing tints and shades of greens to bright yellows, oranges, and reds:

  • Crustose lichens, as their name suggests, are crust-like in appearance and attach themselves firmly to tree barks, rocks, soil, and rooftops.
  • Foliose lichens are flat and can be leaf-like, or convoluted with ridges and bumps.
  • Fruticose lichens have a hair-like appearance, similar to bushy branches.

Learn more about these fascinating organisms in Are You Lichen the Bark? Part 3. Lichens and Mosses.


Smith SD, Monson RK, Anderson JE. 1997. Poikilohydric Plants. Physiological Ecology of North American Desert Plants, Adaptations of Desert Organisms. Berlin, Heidelberg: Springer. 191–198. https://doi.org/10.1007/978-3-642-59212-6_10.

Lichens. U.S. Department of Agriculture, Forest Service Rangeland Management & Vegetation Ecology—Botany Program. [accessed 2021 Jan 9].

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