#171: Reindeer Lichen

Reindeer Lichens grow in northern temperate forests, boreal forests, and even in the tundra. They are highly branched, fruticose lichens that are a primary food source for reindeer (also called caribou in North America). These lichens are sometimes called “Reindeer Moss,” even though they are lichens and not moss.^1,2

Description

Reindeer Lichens, like other Cladonia species, form two types of structures. Initially, the lichen grows flat along the ground and produces small, scale-like protrusions (foliose morphology). Eventually, Reindeer Lichen develop upright, hollow, cylindrical, branching structures called podetia (fruticose morphology).^1–3 Growth occurs at the edge of the basal, squamulose region and at the tips of the podetia. As the lichen gets bigger and forms a mat over the ground, resources like light and nutrients become less available to the center and base of the lichen. These areas die back and decompose while the outer edges continue growing.¹ Consequently, the podetia of an old Reindeer Lichen do not necessarily connect to the squamulose base.

There are actually dozens species that go by the common name Reindeer Lichen.² All of these look similar but can be separated by examining subtle morphological differences. However, you’ll probably need a lichenologist to accurately identify the lichens to species. The most common Reindeer Lichens in North America are Cladonia rangiferina, Cladonia arbuscula, Cladonia mitis, Cladonia stellaris, and Cladonia stygia.¹ See the United States Department of Agriculture (USDA) Plant Profile for Reindeer Lichen for a more complete list of North American Reindeer Lichens.

Some species, such as C. rangiferina, produce podetia with a main stalk to which all the many branches connect. Others, such as C. stellaris, do not have a main stalk. Reindeer Lichens can also differ in the texture of the mat surface. For example, C. stellaris forms mats that are not flat but instead feature rounded bulges, somewhat like a carpet of tiny cumulus clouds. C. rangiferina, on the other hand, has a chaotic, bristly surface. Another common morphological difference is color. Reindeer Lichens are colored green to yellowish (e.g. C. mitis) to gray (C. rangiferina).⁵ The mats formed by Reindeer Lichens can be anywhere from 4cm to 12cm. C. rangiferina and C. stygia tend to form the tallest mats while C. stellaris forms the shortest.¹

Ecology

Reindeer Lichens are composite organisms made of fungi and green algae. In this symbiosis, the fungi provide structure, nutrients, and protection that allow the algae to photosynthesize and produce sugars for the fungi.² For more on general lichen biology, see FFF#062 and FFF#063. The Reindeer Lichen fungi can pair with a variety of algal species to successfully form a lichen. The most common algal symbionts in Reindeer Lichen are algae in the genus Trebouxia.¹

The lichens described above are primarily northern species. They grow in northern temperate forests, boreal forests, and in the tundra, although different species prefer different areas.¹ However, there are other North American Reindeer Lichens that grow as far south as the Gulf States. For example, the range of Cladina subtenuis extends from Newfoundland south to Florida and west to Nebraska and Oklahoma. The only areas in North America where Reindeer Lichens do not grow are the American Southwest and a couple Great Plains states.⁴

Reindeer Lichens can grow on a variety of substrates, including rocky expanses as well as the forest floor in both conifer and hardwood forests. Reindeer Lichens are generally latecomers to disturbed forests. They are most often found in old growth forests and forests that have not been burned or logged in at least 40 years. However, they are not tolerant of shade and as a result are most abundant in forests with sparse tree cover and in forest clearings.¹

Mats formed by Reindeer Lichens are not simply a consequence of growth patterns. Rather, they also provide certain benefits to the lichen population. One such benefit is that it helps the lichen retain water. Even when the outer layer of the mat dries out, the spongy branches of the lichen underneath hold moisture for a long time. The lichens’ metabolism is closely tied to humidity, so this helps the lichen keep growing after the forest starts to dry out. Mats also limit the amounts of nutrients, water, and light available to other organisms, which prevent seeds from germinating. The few seeds that do germinate in the soil below the mat can be pulled out of the soil by the lichen as it expands and contracts due to changes in humidity.⁵ These last two actions prevent plants from colonizing the lichens’ area, which keeps the forest canopy open and allows the lichens access to sufficient amounts of sunlight.

Reindeer Lichens primarily reproduce through asexual means. Most often, that means a piece of one lichen breaks off and is carried by wind, water, an animal, etc. to a new location.¹ Reindeer Lichens also produce special reproductive structures called soredia that bundle together the fungal and algal cells. The soredia are powdery and can be blown by the wind to novel areas.^1,6

Reindeer Lichens do produce sexual spores in structures called apothecia that form at the tips of the podetia. However, the Trebouxia symbiont has not been found in nature and the sexual spores do not carry it with them. It is therefore unlikely that the sexual spores will be able to successfully form a new lichen.¹

Reindeer

Reindeer (Rangifer tarandus), also known as caribou in North America, have a large range that extends to the tundra all the way around the north pole. These animals are highly specialized for life in the arctic. If you want to read about the adaptations that allow reindeer to survive in such an extreme environment, see this page from the San Diego Zoo.*

The normal diet of a reindeer consists of ferns, grasses, mosses, and any other leaves or shoots they can find. However, these die back during the winter and are not available for the reindeer. When their primary sources of food become scarce, reindeer turn to their alternate food: Reindeer Lichen.⁷ Over the winter, reindeer rely on these and other lichens for roughly 90% of their food. C. stellaris is most commonly eaten species, followed by other Reindeer Lichens and finally other lichens.⁸ An adult reindeer has to eat 4 – 8kg of food a day, so they have to spend a lot of time looking for lichens.⁷ Reindeer Lichens are a good source of carbohydrates (which are extracted by special enzymes in the reindeers’ digestive system), but a poor source of protein. Even reindeer who eat their fill of lichens will lose weight by the end of the winter.^5,6

Before they can eat the lichens, the reindeer must first find them. This is difficult in the winter months when the ground is covered in snow and ice. Luckily, reindeer have excellent noses that allow them to smell Reindeer Lichen through snow.⁸ Once they find a patch of lichens, they dig through the snow (creating a “crater”) to reach the food.^2,6 Competition over a patch of lichens can be fierce and reindeer will fight for control over the best patches.^2,8

Human activity is one of the main dangers to reindeer that rely on lichens for food. Lichens are very sensitive to pollution and can be killed by relatively low levels of common pollutants such as heavy metals and sulfur dioxide. This can cut the amount of food available to reindeer during the winter, especially around urban areas.²

Lichens also absorb radioactive materials, which was a problem for the area around Chernobyl after the 1986 meltdown. The lichens absorbed the radioactive atoms, which were eaten by reindeer and later found in reindeer meat and milk.^2,9 Even 30 years after the incident at Chernobyl, reindeer meat from Norway is often deemed too radioactive for human consumption. In 2014, an exceptionally large amount of reindeer meat was thrown out because of radioactivity. Apparently that was due to excessive rain that year, which produced lots of mushrooms (which are also good at picking up radioactive atoms). Reindeer therefore ate more mushrooms and consumed more radiation than usual in 2014.⁹

Taxonomy

Mycologists have not quite decided where Reindeer Lichens should be classified. They are currently placed in the genus Cladonia, but some sources still classify them under Cladina (including the USDA’s PLANTS Database). More recent research suggests that Cladina is best used as a subgenus of Cladonia. The phylogenetic relationships within this group are far from settled, however. Don’t be surprised if the names change in the near future.¹

Note the similarities in the taxon name endings for Reindeer Lichen and the green alga. This is due to the fact that fungi were once classified as plants. Even though the fungi received their own kingdom, mycologists still use the botanical endings for taxon names.

Common Name	Reindeer Lichen	Reindeer (Caribou)	Trebouxia green alga
Kingdom	Fungi	Animalia	Plantae
Subkingdom		Bilateria	Viridiplantae
Phylum	Ascomycota	Chordata	Chlorophyta
Subphylum	Pezizomycotina	Vertebrata	Chlorophytina
Class	Lecanoromycetes	Mammalia	Trebouxiophyceae
Subclass	Lecanoromycetidae	Theria
Order	Lecanorales	Artiodactyla	Trebouxiales
Family	Cladoniaceae	Cervidae	Trebouxiaceae
Genus	Cladonia Cladina	Rangifer	Trebouxia Puymaly10
Species	Cladonia rangiferina (L.) Weber ex F.H. Wigg.11 Cladonia arbuscula (Wallr.) Flot.12 Cladonia mitis Sandst.13 Cladonia stellaris (Opiz) Pouzar & Vezda14 Cladonia stygia (Fr.) Ruoss15 Cladina subtenuis (Abbayes) Hale & W.L. Culb.16	Rangifer tarandus (Linnaeus, 1758)17

 

* Side Note (not included on Zoo page): Arctic reindeer are one of the few animals to lack a circadian clock. Consequently, they do not have regular sleep-wake cycles during summer and winter, when it is constantly light or dark, respectively. See this scientific paper and this WIRED article for more information.