Sky Islands & Montane Communities

The amounts of precipitation and solar radiation received by specific locations on a mountain influence the communities at different elevations. They also influence the communities on the various exposures of its slopes: east and west, north and south. These bands of communities that vary along gradients of precipitation, solar radiation and temperature, referred to as life zones, were correlated with changes in communities at different latitudes from Mexico to Canada by C. Hart Merriam in his 1890 study of the San Francisco Peaks region in northern Arizona. In the Santa Catalina Mountains (on Mount Lemmon, located east of Tucson), we observe these changes in the life zones as we experience changes in elevation.

Heat is lost at higher elevations because of lower air pressure. As air masses move generally from west to east across North America, they cool, and precipitation falls as they rise up the west side of a mountain. As the air descends down the east side, it warms and increases its moisture-holding capacity, creating a drier rain shadow on the leeward side of the mountain. Thus, the western side of a mountain is generally wetter than the eastern side.

Solar radiation influences soil moisture and temperatures, which influence the communities on slopes that face north and south. The tilt of the Earth causes sunlight to be more intense on the south-facing slopes of mountains in the Northern Hemisphere. This in turn causes these slopes to be warmer and drier than the northern exposures.

At the base of the mountain, warm temperatures and low precipitation prevail. The Lower Sonoran Zone is characterized by a desert-scrub community of sparse vegetation, including prickly pear cactus, saguaro, ocotillo, paloverde, and cholla in the Sonoran Desert near Tucson (discussed elsewhere in this web page).

Sonoran Desert, Lower Sonoran Zone, Mount Lemmon, Arizona
Photograph by Matt Withroder, March 2003

Moving up in elevation, the communities of the Upper Sonoran Zone vary with elevation, exposure of the slope, and amount precipitation. On the drier, warmer, south-facing slope of Mount Lemmon (below), the plant community is a desert grassland.

South-facing Slope, Upper Sonoran Zone, Mount Lemmon, Arizona
Photograph by Mark Eberle, March 1999

On the north-facing slope at the same elevation is an oak woodland characteristic of a somewhat cooler, moister environment. In some areas, the oaks are mixed with piñon, juniper, and manzanita (characteristic of a moist chaparral region), with grasses and forbs covering the ground between the larger woody plants.

Manzanita Chaparral (left) and Oak Woodland (right), Mount Lemmon, Arizona
Photographs by Mark Eberle, March 1999

As we enter the Transition Zone, with somewhat greater precipitation and cooler temperatures, we are in an open, park-like pine forest. In southeastern Arizona and southwestern New Mexico, ponderosa pine, which is widespread in the Southern Rocky Mountains, mixes with species of pines (such as Apache, Arizona, and Chihuahua pines) that are characteristic of montane forests of the Sierra Madre Occidental in Mexico.

Pine Forest, Sacramento Mountains, New Mexico
Photograph by William Stark, March 2000

At Inspiration Rock on Mount Lemmon, we see the mixed conifer forest of the Canadian Zone, where temperatures are somewhat lower and precipitation is somewhat higher. In this forest, Douglas-fir and white fir become more numerous and pines become less abundant.

Mixed Conifer Forest, Inspiration Rock, Mount Lemmon, Arizona
Photograph by William Stark, March 1999

Engelmann spruce and Rocky Mountain subalpine fir are the dominate trees in the highest (subalpine) forests of the Hudsonian Zone. Both species occur in the San Francisco Peaks (Humphrey Peak is 3,854 m), but Engelmann spruce is absent from Mount Lemmon (2,791 m). Similarly, the Alpine Tundra Zone is found on Humphrey Peak, but it is absent on Mount Lemmon. Groves of quaking aspen (light brown areas among the conifers in the photograph below) are scattered among the forests. Aspens often colonize areas where fires or other disturbances have opened the canopy of conifers.

Sruce-Fir Forest, San Francisco Peaks, Arizona
Photograph by Connie Chen, March 1999

Sometimes the distinctions between the various plant communities become blurred in mixtures of the characteristic species. We encounter this in Cave Creek Canyon in the Chiricahua Mountains of southeastern Arizona. Because conditions along the streams and adjacent slopes are suitable for growth of a variety of plant species, there is a mix of juniper, Arizona cypress, live-oaks, Douglas-fir, white fir, pines, and other southwestern plants.

Cave Creek, Chiricahua Mountains, Arizona
Photograph by Mark Eberle, March 1999

The Chiricahua Mountains and other isolated ranges in southern Arizona and New Mexico arose in the Basin and Range province that separates the communities characteristic of the Rocky Mountains to the north and the communities of the Sierra Madre Occidental, which extends south through Mexico. Species from both of the larger mountain chains mix in these isolated mountain ranges. The forests and associated communities on these smaller mountain ranges are surrounded by a "sea" of desert-grassland and desert-scrub communities, which is why the mountains are referred to as sky islands. Reliable sources of water, such as Cave Creek, are oases for a variety of animals and excellent places to observe birds not normally found in other parts of the United States.

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