Most mountain ranges of the northern hemisphere grade in elevation from one forest type to another in a steady progression from less cold-hardy to more cold-hardy trees, eventually giving way to alpine tundra where trees can no longer survive the extremely cold temperatures. The Great Basin mountain ranges of Nevada provide a notable exception to this rule. In most of these ranges there is no continuous forest zone above the pinyon-juniper woodland. Instead, there typically is a belt of mountain shrubs over extensive areas between 7,500 and 10,000 feet in elevation. This simply may be a result of slow dispersal—there has not yet been enough time for subalpine tree species, whose tolerances for moisture and temperature conditions are appropriate for this elevation zone, to migrate into the region following Pleistocene glaciations. Mountain peaks of the Great Basin are widely separated by broad sagebrush valleys that, under current climate conditions, provide a barrier to tree migration.