The extensive Iranian plateau constitutes a clearly defined physiographic and geologic unit; yet from the biogeographical and ecological points of view–that is, as a place to live for plants and animals, including man—the plateau displays marked variation. This is brought out very well by the regional changes in distribution and character of its natural vegetation cover, which in turn may be said to stem from four main factors:

First, climatic situation. Along its length of approximately 2,000 miles the Iranian plateau crosses—very much as a gigantic bridge—the full width of the great desert belt of the Old World; it does so at the point where this belt bends north-east to connect its Saharan-Arabian portion with its Central Asian section. With this geographical location, the Iranian plateau, while highly arid in its large and depressed central part, is exposed to a variety of climatic influences, some of which just touch its edges, while others penetrate or even submerge it for a restricted period. Of this latter kind is a temperate regime, with the westerlies and wandering depressions which in summer just impinge on the north-western fringe of the plateau in Armenia and Transcaucasia, but which become more established during most of the winter half of the year. Also at this season the highly continental, anticyclonic regime of Central Asia and Siberia makes itself felt by sending its icy air masses against and over the plateau. The southern fringe of Iran, however, is exposed to the tropical or subtropical climatic conditions of the southern desert, which in the summer extend over all the lower parts of the plateau.