(Photograph = Three different species of desert lichens on rock)
Lichens are made from two different organisms, a fungus and algae. The two organisms play different roles. The fungus gives structure to the lichen (provides a scaffold), and the algae provide food for themselves and the fungus through photosynthesis.
Lichens generally grow in areas that are moist for at least part of the year. These habitats might include woodlands, meadows, and even tundra. But desert lichens have adapted to life in hot dry places. Though they can take the heat some desert lichens also live in areas with freezing winters (e.g., the Great Basin of the USA).
Desert lichens grow very slowly because they are limited by extreme conditions. They become dormant in extreme heat, drought, or cold. Though some lichens can grow several inches tall, desert lichens may be only 1 mm in height. Lichens in deserts often form colorful crusts on rocks. Lichens can grow in rocks, soils, and even on plants. Though they may grow on plants, lichens are not parasites.
Additional Reading: Lichens on Woody Plants