Each type of lichen is actually a composite of two different organisms: a fungus and an alga (or less commonly a fungus and a yeast or a cyanobacterium). The fungal partner within each species of lichen is unique, but a single species of algae can be found in many different lichens. It is therefore the fungal partner, also known as the mycobiont, from which the lichen’s name is derived. To help differentiate lichens from each other, it’s helpful to consider their growth form and the substrate on which they grow. Structurally, lichens are typically described as fruticose (branching or shrubby and more or less circular in cross section), foliose (having leaf-like structure with a clearly defined upper and lower surface), or crustose (flakey or embedded in the substrate), although some other less common growth forms also exist. In terms of substrate, check whether your lichen of interest is growing on a tree, a rock or from the soil. Although some lichen species can be found on more than one substrate, most can only be found on one type of surface. For example, bushy beard lichen (Usnea strigosa), also known as Old Man’s Beard, will only grow on trees. 

Last week the Upper Cape Naturalists Club had two lichen-themed events led by the incredibly knowledgeable Tom Walker. Both his presentation and his outdoor walk focused on types of lichen that can be found locally on Cape Cod. Below is a run down on some of my favorites:

Powdered ruffle lichen (Parmotrema hypoleucinum) is a light green foliose lichen common on pine trees. It is easily distinguishable from other fluffy green tree lichens by the prominent black cilia (hair like structures) along its margins, which are thought to deter predators. Powdered ruffle lichen also have marginal soralia, bulbous granular-looking structures at the edges of each ruffle that release reproductive spores.  

Gray reindeer lichen (Cladonia rangiferina) is a grayish fruticose lichen that grows on the ground. The species name, rangiferina, comes from the genus name of caribou/reindeer: Rangifer. In areas where caribou are common, reindeer lichen can comprise a large percentage of their diet in the winter. Consumption of lichen in large mammals is unique to caribou. They are the only large mammal able to break it down and assimilate its nutrients. Although reindeer lichen can reproduce by spores, they more commonly reproduce by fragmentation. Therefore, herbivory and trampling by caribou, which break apart small pieces of the lichen’s structure, can actually help the lichen spread. 

Common toadskin lichen (Lasallia papulosa) is an umbilicate foliose lichen found on rocks. The term umbillicate refers to the fact that is attached to the rock by a single central point, called an umbilicus. The upper surface of toadskin lichen has a warty appearance reminiscent of its namesake. Although it is typically a dull light brown when dry, it becomes a brighter green when wet. 

Next time you take a walk in the woods, see how many different lichens you can find.

Cover photo: Mealy pixie cup lichen (Cladonia chlorophaea)