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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 … Read more... 

The Central Negev Desert and the Arava Valley, two closely situated geographical regions in the southern part of Israel, are areas of extreme desert characteristics. Both are exposed to sporadic, as well as regularly fluctuating environmental condi tions. They differ, however, with respect to the amount of precipitation, average temperature, average relative humidity, number of dew nights and light intensity (Table 1). From Table 1 it is obvious that, although both regions are inhospitable habitats, … Read more... 

The Antarctic botanical zone has been defined as all land south of 60° latitude together with the South Shetland, South Orkney and South Sandwich rslands (Skottsberg 1960, Lindsay 1978). This zone is divided into two regions (Wace 1960, Holgate 1964), the maritime Antarctic region consisting of the west coast the of An tarctic Peninsula and the Antarctic Islands and the continental Antarctic region com prising the east coast of the Antarctic Peninsula and the rest of the Antarctic Continent. The … Read more...