In semiarid regions as in open grass lands and steppic areas lichens cover more or less homogeneously soils and rock. Here rainfall may be the main source of moisture for the lichens. According to Rogers (1972) a correlation exists between soil crust formation of lichens and annual rainfall in South Australia. In the more arid desert areas lichen-vegetation is concentrated to certain spots or areas. These patterns are due to totally different moistening conditions in the different deserts. Data of Nash et al. (1977) show a striking correlation (Table 1) between precipitation and biomass of epilithic lichens in North American desert areas as well. On the other hand, in other areas with precipitation less than 100 mm/ year, lichens at least locally can form a much higher biomass than would be expected on the basis of precipitation alone.