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Sky Islands

The topography of the area surrounding Tucson in southeastern Arizona, in southwestern New Mexico, and in northern Sonora, Mexico is of great interest to biologists because there are many "islands" of raised land that project upwards out of the Sonoran Desert. These islands are “stepping stone” from the Rocky Mountains of the United States to the Sierra Madre mountains of Mexico. The “islands” are often widely separated from each other by many miles of dry and hot desert with many species of cactus and arid adapted plants. The islands themselves, rising to nine or 10 thousand feet, are wetter, cooler, and have lush vegetation zones from oak and juniper up through pines, to fir and sometimes spruce at the top. Because each island is ecologically isolated, some species cannot easily move from one island to another (for example plants, lizards, non-flying insects, and small mammals). However, birds, bats, flying insects, and plants that propagate by seeds that are carried by the wind, may be the same in all neighboring islands. Madera Canyon, while only one of many canyons in the archipelago of desert sky islands, is the most studied and the most easily accessible canyon to the public. At the same time, its environment and its flora and fauna are protected and available for all visitors to enjoy.


Santa Rita Mountains - a Sky Island - in winter

MADERA CANYON - A SKY ISLAND OASIS IN THE DESERT

Madera Canyon descends from the peaks of the Santa Rita Mountains, one of southeast Arizona’s forested Sky Island ranges. Sky Islands rise high enough above the desert to produce a decrease in temperature as elevation increases from base to top. The lower temperatures allow communities of plants and animals to live in the Sky Islands that cannot survive in the desert below.

As islands in an ocean are isolated by the surrounding water, Sky Islands are mountains with plants and animals isolated by surrounding hot, dry desert. Madera Canyon, with active springs and a seasonal creek, is a lush Sky Island oasis supporting an amazing diversity of life.

Life zones of the Santa Rita Mountains and Madera Canyon

From Green Valley to the 9,453-foot summit of Mt. Wrightson, the mountains rise nearly 7,000 feet. Moisture increases and temperature decreases 3 - 5°F for every 1,000 feet of elevation gain, creating a succession of four life zones. Each life zone has communities of plants and animals adapted to the environmental conditions found in the zone. Life zones extend into lower elevations on a cool north-facing slope than on a warm south-facing one. The edges of life zones overlap, creating a transition or ecotone. Ecotones contain a blend of the plants and animals from each life zone.

Lower Sonoran Zone (0 to 4,500-feet) - This life zone extends from above the mouth of Madera Canyon down to the Santa Cruz Valley. Sonoran Desertscrub with desert trees, shrubs, and cactus grows in hot, dry lower elevations. A transition to Desert Grassland with grasses, shrubs and Velvet Mesquite occurs as temperature and moisture moderate higher up.

Upper Sonoran Zone (4,500 to 6,500-feet) - Arising around the canyon mouth, this zone is cooler and more moist than the Lower Sonoran below. It is composed primarily of two woodland plant communities and represented by plants such as evergreen oaks, Alligator Juniper, Mexican Piñon Pine, shrubs, and bunchgrasses.

Transition Zone (6,500 to 8,000-feet) - With mild summers and cold winters, the temperate Transition Zone is characterized by many plants common to the Rocky Mountains. Ponderosa Pine, deciduous Gambel Oak, Smooth Sumac, maples, and other mountain species grow in the upper canyon.

Canadian Zone (8,000 to 9,500-feet) - The highest and coolest life zone in the Santa Rita Mountains occurs on the lofty peaks above the canyon. Plants such as Douglas-fir, White Fir, Quaking Aspen, and Kinnikinnick grow in this high altitude zone.

Riparian Corridor:
Madera Canyon is a bowl-shaped watershed. Side-canyons funnel water from springs and runoff in seasonal streams that feed Madera Creek. This stream system and the abundant plants along its banks form a riparian corridor. The corridor descends through all the canyon life zones and creates excellent wildlife habitat.

MADERA CREEK - ECOLOGY OF A SEASONAL STREAM

Beneath the shade of the trees, Madera Creek tumbles over bedrock and boulder. Water and stream-borne sediment gradually grind rocks to gravel, gravel to pebbles, and pebbles to sand. Over millennia, this stream and its tributaries have carved the canyon into the form that we see today.

Madera Creek is a seasonal stream. It does not flow year-round. But at certain times of the year, water from springs and seasonal run-off drain down the tributaries and feed the main creek in the large bowl-like watershed named Madera Canyon.

Water does not always flow in the creek - Water in the southwest deserts is seasonal. Creeks flow from snow melt and rain in winter and from summer monsoon rains. In these seasons water fills pools and spills over waterfalls. At other times of the year the streams appear bone dry, although water may be trickling out of sight under rock and gravel.

Life in the stream - Water in pools and riffles create aquatic habitat for a variety of life. Because the creek is seasonal and dries up, no fish live in Madera Canyon. Life in this stream must be able to cope with drying out!

Much of the stream life is minute or microscopic. Simple algae, water molds, one-celled animals, and tiny crustaceans live out complex life-cycles invisible to the human eye.

Aquatic insects are numerous. Water striders, water beetles, Ferocious Water Bugs, backswimmers, and dragonfly larva live in pools. Long-jawed orb weaver spiders spin webs over the water surface.

Canyon Tree Frog and Black-Necked Gartersnake live along the creek. Toads come to the water to mate and lay their eggs. Wildlife in the canyon depends on the stream and springs for water.

Streamside Vegetation - Several varieties of trees are dependent upon the stream. Fremont Cottonwood and Arizona Sycamore grow only where they can keep their roots in the water. Willows, Arizona Ash, Netleaf Hackberry, and Box Elder also require lots of water. Grasses like Deer Muhly and sedges are common on stream banks. Some beautiful wildflowers such as Yellow Columbine, Common Monkey Flower, Coral Bells, and Lemon Lily also grow along the stream.

Why are there giant boulders and huge piles of rock along the stream?
Ancient flood events washed the boulders and debris down into the canyon when the climate was much wetter than it is today. Recent smaller rock and debris flows seen along the creek result from flash-flooding. Strong monsoon rains falling on land damaged by the 2005 Florida Fire triggered some of the most recent erosion.

PLANTS - DIVERSITY AT A GEOGRAPHIC CROSSROADS

Madera Canyon supports an amazing variety of plants. This plant diversity is the result of location, elevation and regional climate history.

Four geographic regions influence Sky Island plant diversity - The Sierra Madre Occidental to the south, the Rocky Mountains to the north, the Sonoran Desert to the west, and the Chihuahuan Desert to the east. Plants from each region create the unique flora of these isolated ranges:

Southern Influence - Many species from the Sierra Madre Occidental and sub-tropical Mexico (called Madrean plants) reach their northern limit in the Sky Islands. With origins in a warmer, moist climate, their range becomes restricted by freezing and drought. Protected canyons, like Madera Canyon, provide refuge for species such as Southwestern Coral Bean, Wait-a-minute Bush, and Kidneywood.

Northern Influence - Freezing is not a problem for most Rocky Mountain plants, but they do require moderately high temperatures and adequate moisture. In the Sky Islands, temperate North American plants like Ponderosa Pine, Douglas-fir, and Quaking Aspen grow essentially stranded on isolated mountaintops above the desert.

Desert Influence - Southeast Arizona lies between the hot, dry Sonoran Desert and the cooler, higher Chihuahuan Desert. Plants adapted to the Sonoran Desert such as Saguaro and Paloverde disappear as elevation increases and winter low temperatures drop to the east of the Santa Rita Mountains. Likewise, Chihuahuan Desert species like Soaptree Yucca and Beargrass vanish west of the Sky Islands in the dry heat of southern Arizona. Plants of both deserts grow in the desert grasslands and oak woodlands of the Sky Islands.

How did plants from these different regions end up here?
Since the formation of the Sky Islands, the climate has warmed and cooled numerous times. During cool periods, northern plants extend their ranges south. With warming trends, these temperate plants retreat north or into higher altitudes and southern species advance northward. Entire plant communities advanced or retreated across the region with the climate, blending floras.

The climate has warmed and dried in recent times. Many species have withdrawn to higher elevation and sheltered canyons resulting in the unique plant communities found in the Sky Islands and Madera Canyon today.

Plant communities in Madera Canyon:

Desert Grassland - In the upper Lower Sonoran Life Zone, scattered Velvet Mesquite, Ocotillo, and shrubs like Fairy Duster and Velvet-pod Mimosa, grow among grasses. Soaptree Yucca and cacti are also present. Leaf succulents, cactus, and grasses from the Desert Grassland blend extensively into the Madrean Evergreen Woodland above.

Madrean Evergreen Woodland - Mild winter/wet summer woodland occurs in the Upper Sonoran Life Zone. At lower elevations is open-growing Oak Woodland (or Encinal, from the Spanish “encino” for oak) with evergreen oaks, Alligator Juniper and Mexican Piñon Pine, shrubs, and grasses.

At higher elevations the vegetation shifts to Pine-Oak Woodland of evergreen oaks, Apache Pine, Chihuahua Pine, shrubs, and bunchgrasses. Interior Chaparral, with Pointleaf Manzanita, Wright Silktassel, Birch-leaf Cercocarpus, and Fendler Ceanothus dominates thinner, eroded soils of exposed slopes on both sides of the canyon.

Madrean Montane Conifer Forest - This high altitude Sky Island forest is composed of two major vegetation types. At lower elevations, Ponderosa Pine Forest occupies the Transition Life Zone. Arizona Ponderosa Pines tower over deciduous Gambel Oak, New Mexico Locust, mountain grasses, and shrubs.

Mixed Conifer Forest is found in the Canadian Life Zone of the highest elevations. Douglas-fir is the dominant tree, mixed with Southwestern White Pine, and White Fir. Stands of Quaking Aspen and Bracken Fern colonize forest openings; grasses, shrubs, and wildflowers thrive in meadows.

Riparian Woodland - These communities ascend from the foothills into the canyon along the creek and tributary drainages. Low in Madera Canyon is the Interior Deciduous Forest, recognized by large deciduous trees like Arizona Sycamore, Freemont Cottonwood, Netleaf Hackberry, and Velvet Ash. At higher elevations the forest transitions to Montane Riparian Forest with trees such as maples, Box Elder, and Chokecherry growing among shrubs like Red-osier Dogwood, hawthorns, and wild roses.