Is it a reindeer or caribou? Reindeer and caribou share the same scientific name, Rangifer tarandus. Generally, the term caribou is used for the animals in North America and reindeer for the ones in Europe and Asia. Reindeer is also the term used for referring to domesticated caribou (many of the herds in Europe and Asia are domesticated or semi-domesticated).
Caribou are primarily grazing herbivores. Their diet is most variable during the summer, when they consume the leaves of willows and birches, mushrooms, cotton grass, sedges, and numerous other ground dwelling species of vegetation. Lichens are an important component of the diet, especially in winter, but are not eaten exclusively. One of the things that they eat is called Caribou Moss (Cladonia rangiferina), and it also has a special ‘friendship.’
Caribou moss grows in the arctic and northern regions around the world. It grows on the ground and on rocks. It looks like a foamy, gray-green spongy mass, and grows to be 1 to 4 inches high. The stems, or stocks, are hollow, and branch out many times. Although it is called caribou moss, it is actually a lichen. Lichens are two separate organisms. They are made up of fungi and algae, which live and grow together. The spongy threads of lichens support and protects the algae. The algae has chlorophyll which can make food. Each has something the other needs. This is called a symbiotic relationship. Lichen can make food when the temperature gets very low and there is little light. The tissues of lichens aren't easily damaged by frost. This makes it a great plant for the tundra.
Animals such as reindeer/caribou feed on lichen during the coldest periods of the season. They do this because it is one of the only things that they have for food when the weather is cold and there is little other vegetation left. It has lots of carbohydrates that give the caribou energy to make body heat. Caribou have special microorganisms in their stomachs which let them digest lichen. Very few other animals eat lichens. Some scientists think that the caribou evolved to fill the tundra's food niche that other animals couldn't fill.
Reference: https://www.blueplanetbiomes.org/caribou_moss.php