Mention islands and most of us think of land masses that protrude above lakes and oceans. But, in desert regions, isolated mountain ranges produce the opposite effect, yielding pockets of moisture in a sea of dryness. Reaching high into the atmosphere, these "sky islands" are bathed by cool air and receive "upslope precipitation" from all directions. As a result, the mountain slopes are cloaked by bands of vegetation, corresponding to the change in climate that occurs with increasing elevation.
Such islands of water are common throughout the western U.S., especially in the vast, dry terrain between the Sierra Nevada and Rocky Mountain chains. Requiring an annual precipitation of 15 inches or more, a zone of juniper and pinyon pine begins above elevations of 5500 feet. Nearing 7000 feet, one encounters open woodlands of ponderosa pine and, on shaded slopes, forests of Douglas fir. Aspen and lodgepole pine begin to appear above 8000 feet, mixing with Englemann spruce, limber pine and subalpine fir at higher elevations. At timberline, which ranges between 10,500 and 12,000 feet (depending on latitude) pockets of bristlecone pine are often found.
Of course, the wildlife population varies with the life zones and many species demonstrate vertical migration, moving higher during the warmer months and descending to lower elevations in the winter. Some of the better areas to explore sky island ecosystems include the La Sal Mountains (east of Moab), Mt. Lemmon (east of Tucson), and Mt. Wheeler in Great Basin National Park, on the border of Utah and Nevada.
