Life on a Sky Island:

The Catalina Mountain Range

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In southern Arizona, the topography is called Basin and Range.

The Basins, or desert valleys of about 2,500 feet elevation, alternate with the Ranges, mountains of about 9,000 - 10,000 feet elevation. A colorful phrase credited to Major C.E. Dutton, an early explorer of the West, describes the mountain ranges as an "army of caterpillars crawling northward out of Mexico." (King, 1977)

Unlike northern and central Arizona, these mountain ranges have formed by a geologic process known as block faulting, caused by stretching of the earth's crust in this region in the last 15 million years. The result is the mountains are isolated from each other. The mountains have been called sky islands, or mountain islands, surrounded by a vast sea of desert. The isolated flora and fauna on one mountain have had an opportunity to evolve differently than related flora and fauna on neighboring mountains. An example is the Mt. Graham red squirrel, which is somewhat different from the red squirrel of central Arizona. This small population of squirrels, occupying a tiny region on the top of Mt. Graham in the Pinalenos Mountains, almost threatened to wipe out the University of Arizona's plans to build a complex of telescopes on the mountain. In the end, the University won, and it is the Mt. Graham red squirrel that may be wiped out.

We visited Mt. Lemmon, in the Catalina Mountain range north of Tucson. Mt. Lemmon has its own isolated population of squirrels; the Arizona Gray squirrel here and in the Rincon Mountains differs slightly from the Arizona Gray squirrel in the Santa Rita, Huachuca, and Patagonia Mountains. In order to visit each other, the squirrels would have had to scamper down their respective mountains and across twenty miles of hot desert. The last time their populations might have intermingled might have been during the last glaciation, 11,000 years ago, when cooler temperatures might have lead to expanded forests and a grassland, instead of a desert, divide.

On a weekend in February, we did what the squirrels could not: we traveled from the desert around Tucson to the top of Mount Lemmon. Along the way, we traveled through four different life zones (as classified by Merriam): from desert upland, we traveled through desert grassland-chaparral, oak woodland, Ponderosa pine forest, and mixed conifer forest.

Beginning the drive up Mt. Lemmon Highway, we ascended through thick stands of saguaros until we reached the turnoff at Molino Basin at about 4000 feet. Getting out, we found a trail meandering among ocotillo, sotol, cane cholla, agave and yucca. There is a little more water in grassland-chaparral biomes than in pure desert-- about 12 to 15 inches a year-- but the rocky soil inhibits dense grass growth. Grasslands and chaparral burn periodically from fires caused by lightning strikes. Grasses recover quickly by resprouting from their roots. The chaparral community, dominated by shrubs growing close together, have small, leathery, resinous leaves to inhibit water loss, and the resins make the plants extremely flammable. The shrubs are adapted to periodic fires; the seeds of many chaparral plants cannot germinate until they have been scorched by fire or at least baked in full sun in barren soil. Here at Molino basin, the chaparral shrubs are scrub oak, manzanita, turpentine bush, and Agave lechugilla (the evil Shin Dagger). We heard more birds than we saw, but I would guess the ones we heard were Mourning Dove, Curve-billed Thrasher, Say's Phoebe, and Cactus Wren. I spotted a whiptail lizard, one of the few that comes out in the middle of the day.

Manzanita Yucca elata Agave lechugilla Turpentine bush
Back in the car, we drove up a little farther to about 5,000 feet elevation, where we noticed the grasses and shrubs give way to oaks and pines. Instead of small shrubs, this land is dominated by trees. There is open space between the tree canopies, so it would be classified as woodland instead of forest. Here the summers aren't quite as hot and the rainfall may get to 15 or 20 inches a year. The dominant oaks are Arizona white oak, Emory oak, silverleaf oak and Mexican blue oak. The oaks are evergreen, but cope with the semiaridity by producing thick leaves with a fuzzy coating-- which makes the silverleaf look silver. There are also small pines, such as Chihuahuan pine and Apache pine, and alligator juniper. It is noticeably cooler now; with each 1,000 increase in elevation, the temperature drops about three degrees Fahrenheit.
Juniper Oak Pine Woodland Silverleaf Oak
This is a different community than the chaparral-grassland we were in before. In the oak-pine woodland are gray-breasted jays instead of scrub jays, white tailed deer instead of mule deer, and turkeys, coatis, and the Sonoran mountain kingsnake.The kingsnake, which kills by constriction instead of venom, can be distinguished from the smaller, venomous coral snake by remembering the rhyme "Red on yellow can kill a fellow; red on black is a friend of Jack."

A few years ago Phil and I visited the Chiricahua Mountains, another sky island in southeastern Arizona. In the oak woodland there we saw a troop of chittering coatis. Tails held erect, they crossed the road and ran up into the trees.
Coati photo by John C. Stevenson Sonoran Mountain Kingsnake
photo by Howard Lawler

On our journey up the mountain, we had passed through two biotic communities; we weren't at the top yet. So we hopped back in the car and drove up another 1,000 feet to about 6,000 feet elevation. Here we began to see tall Ponderosa Pines. The trees' canopies overlapped here, allowing less sunlight to reach the ground. As I got out to take a picture, I realized that it was time to put on the jacket I had brought. We were now in Ponderosa Pine forest, and there were patches of snow on the ground. Precipitation, in the form of snow now as well as rain, can be from 18 to 26 inches per year. The tall stands of Ponderosa pine are home to the Stellar's Jay, black bear, mountain lion, turkeys, the thick billed parrot, and the endangered Twin-spotted Rattlesnake.


Ponderosa pines live above the junipers and below the aspens. They can grow to be 150 feet in height and 500 years of age. The Ponderosa pine has three long needles in a bundle at the end of its twigs, but there is a variety called the Arizona pine which has five needles in a bundle.

Continuing up the road, we reached the top of the mountain. There was a sign, congratulating us on traveling through the same life zones as if we had gone from Mexico to Canada. Mt. Lemmon is only about 9,500 feet in elevation, but other mountains in southern Arizona reach above 10,000 feet. Here is mixed conifer forest, with Douglas fir, white fir, and Quaking aspen mixed with Ponderosa pine.
Ponderosa pines are tall and straight. Pine forest has some snow on the ground.
Mixed Conifer Forest We might as well be in Canada OK, I'm cold now



Thick billed parrots are a species native to the mountain islands of Arizona. From a population of 1000 only seventy years ago, their numbers had dwindled due to habitat destruction and hunting until there were none left in the Chiricahua Mountains by the 1930s.

Birds still remained in Mexico's Sierra Madres, however, so the U.S. Fish and Wildlife Service began a program to reintroduce the parrot to the Chiricahuas. With their bright green feathers, showy red face, and loud squawking, the parrots call attention to themselves when a hiker passes nearby. Even non-birders such as Phil and I can identify a thick billed parrot. They inhabit conifer forests and eat seeds from pinecones (that thick bill is useful for ripping pinecones apart). The reintroduction program had a few false starts, but there appears to be at least a few mated pairs of birds in the Chiricahuas, raising their young in their native mountains.

photo by Bob Miles

At the top of our mountain, we read the sign describing the zones we had traveled through, looked around at the trees, and tried to identify the conifers. Douglas fir is common at the top of the mountain. Instead of long needles attached to the end of twigs, as the Ponderosa pine, fir needles are scaly and attach directly to the branches. In areas where fire has swept through, clearing a space for new trees, there are aspen and lodgepole pine.

Here there is also snow. There is much more shade because of the forest canopy over us, and I'm cold. Our shoes crunch through the snow and we get back into our car to drive back down the mountain, through pine forest, woodland, and grassland, and back into the vast desert sea that separates the sky islands.

Pictures were taken on Mt. Lemmon, in the Catalina Mountain Range.
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