Arizona Mountains forests
Arizona’s mountain forests are a unique and diverse ecological zone, shaped by the state's varied geology, elevation, and climate. Spanning from arid desert landscapes to higher elevations, these forests are characterized by distinct vegetation zones influenced by moisture availability and temperature. The elevation gradient creates two significant tree lines: a lower line where water scarcity limits growth, and an upper line defined by cooler temperatures. Typical forest types vary by region, with oak-pine woodlands in the southeast, piñon-juniper woodlands in the north, and extensive pine and spruce-fir forests at higher elevations.
These ecosystems are home to a variety of tree species, including ponderosa pine and Douglas fir, along with associated flora such as oaks and junipers. The rich biotic diversity supports a range of wildlife, including apex predators like bears and cougars, as well as numerous smaller mammals. However, Arizona’s mountain forests face challenges from climate change, with rising temperatures and frequent droughts leading to increased wildfire risks and pest infestations. Such environmental stresses threaten the resilience and recovery of these forests, prompting concerns about their future sustainability.
Arizona Mountains forests
- Category: Forest Biomes.
- Geographic Location: North America.
- Summary: Arizona's geology, elevation, and climatic diversity produces a landscape rich in montane habitats.
Arizona, the sixth-largest state in the United States with an area of 114,000 square miles (295,259 square kilometers), offers one of the most spectacular and diverse landscapes in North America. It spans nearly six degrees of latitude and longitude, with elevations ranging from 72 feet (22 meters) to 12,637 feet (3,852 meters). Arizona comprises two physiographic provinces: the Basin and Range in the south and west, and the Colorado Plateau, including the Grand Canyon, in the north and east. In the center is a transition zone whose main feature is the Mogollon Rim, which marks the southern edge of the Colorado Plateau. Portions of four deserts—Chihuahuan, Great Basin, Mojave, and Sonoran—fall within its borders.
The incredible geologic and climatic range produces great biotic diversity. The primary gradients influencing the composition of Arizona mountain forests include latitude, elevation, and moisture. Other factors that influence forest competition are disturbance (such as fire) and underlying geology. Predominant climate types are hot desert, hot steppe, and cold steppe, but a variety of warm and cold mid-latitude climate types are found at higher elevations. There is rather striking differentiation in forest composition along elevation gradients. In arid environments with large topographic diversity here, one may notice two tree lines on mountain slopes. A lower tree line occurs where lack of water limits tree growth. An upper tree line occurs where low temperatures and short growing seasons likewise limit tree growth.
Upper-Tree-Line Vegetation
Moisture patterns add another layer of complexity to vegetation zonation in these mountains. Different exposures—the direction that a slope faces—are exposed to different patterns of shading, heating, and cooling, which in turn influence moisture supply. Southwest-facing slopes tend to be driest; northeast-facing slopes wettest. The effect is that boundaries between vegetation belts tend to be lower on the wetter slopes and higher on the drier ones.
Many of Arizona's mountain ranges rise from desert- or steppe-like environments where trees are scattered—often confined to ravines—and shrubs, grasses, or succulents like cacti are the dominant residents of the landscape. The dominant plant in parts of the Sonoran Desert, for example, is the giant saguaro (Carnegiea gigantea), a cactus that may reach more than 40 feet (12 meters) in height—twice as high as the palo verde (Parkinsonia microphylla), a tree with which the saguaro is commonly associated.
Lower-Tree-Line Vegetation
Trees are uncommon at lower elevations because of the lack of precipitation, coupled with high evapotranspiration demand—loss of water through evaporation and transpiration, and loss of water through stomates (openings) in their leaves. Higher up the mountain slopes, as water becomes less limiting, woodlands begin to replace desert scrub or grassland as the dominant vegetation type.
The type of woodland found at lower elevations varies according to location. In the southeast, oak–pine woodland is typically found from 4,000 to 7,000 feet (1,219 to 2,134 meters). Emory oak (Quercus emoryi), Arizona oak (Q. arizonica), and blue oak (Q. oblongifolia) dominate, with alligator juniper (Juniperus deppeana) and Mexican piñon pine (Pinus cembroides) being common associates. The open canopy of these woodlands leaves room for succulent and semi-succulent species, including Agave, Opuntia, and Yucca, as well as for small trees and shrubs such as acacias (Mimosa), manzanita (Arctostaphylos), mountain mahogany (Cercocarpus), and sumac (Rhus).
In the northern part of the state, the oak–pine woodland is replaced by piñon–juniper woodland; oaks are a minor component here. The dominant species are piñon pines and junipers. Mexican piñon pine, more abundant in the south, is replaced by Colorado piñon (Pinus edulis) in the north. Likewise, alligator juniper is replaced by other juniper species, such as oneseed juniper (Juniperus monosperma), Rocky Mountain juniper (J. scopulorum), and Utah juniper (J. osteosperma).
Along the Mogollon Rim in the central part of the state, chaparral is the primary type of woodland at lower elevations. Sonoran scrub oak (Quercus turbinella) is the dominant tree species. Other important species of the chaparral include pointleaf manzanita (Arctostaphylos pungens), hairy and birchleaf mountain mahogany (Cercocarpus breviflorus and C. betuloides), skunkbush sumac (Rhus trilobata), desert ceanothus (Ceanothus greggii), Wright's silktassel (Garrya wrightii), and Apache plume (Fallugia paradoxa).
Pine and Spruce–Fir Forests
Above these low-elevation woodlands is a pine-forest belt from 6,000 to 9,000 feet (1,829 to 2,743 meters). Ponderosa pine (Pinus ponderosa) is the dominant species in the north, and Arizona pine (P. arizonica) is dominant in the south. Associated species include Apache pine (P. engelmannii), Chihuahua pine (P. leiophylla), Mexican white pine (P. strobiformis), alligator juniper, Gambel's oak (Quercus gambelii), silverleaf oak (Q. hypoleucoides), and madroño (Arbutus arizonica) in the south. In the north, Gambel's oak, quaking aspen (Populus tremuloides), and Douglas fir (Pseudotsuga menziesii) are the most common associates of the pines.
Spruce–fir forests occur above the pine-forest zone at elevations ranging from about 7,000 feet (2,134 meters) to the upper tree line, approaching 11,400 feet (3,475 meters). Douglas fir and white fir (Abies concolor) are the dominant species in this belt south of the Mogollon Rim, with Mexican white pine being a common associate. Engelmann spruce (Picea engelmannii) and corkbark fir (Abies lasiocarpa var. arizonica) dominate stands at the upper limits of the spruce–fir zone in the south.
North of the Mogollon Rim, the spruce–fir zone is more diverse, with Douglas fir, white fir, subalpine fir (Abies lasiocarpa var. lasiocarpa), blue spruce (Picea pungens), and Engelmann spruce being important members of the community. The spruces are typically found at higher elevations. They are joined by limber pine (Pinus flexilis) in isolated locations, such as the San Francisco Peaks.
On the Kaibab Plateau, which forms part of the North Rim of the Grand Canyon in northern Arizona, blue and Engelmann spruce dominate. Quaking aspen occurs in scattered stands. Other common trees include Gambel's oak, water birch (Betula occidentalis), and box elder (Acer negundo).
Effects of Substrate and Slope
Substrate (geology and soils) modifies these general patterns. Soils derived from granite, for example, support vegetation characteristic of more mesic (wet) conditions, whereas soils derived from limestone support vegetation characteristic of more xeric (dry) conditions.
Likewise, slope position has a significant effect. The riparian zone supports a richer assortment of species because of the increased water supply and sometimes greater shelter. Species found in canyons and along streams in Arizona's mountains include blue spruce, box elder, white fir, bigtooth maple (Acer grandidentatum), narrowleaf cottonwood (Populus angustifolia), Arizona walnut (Juglans major), and several alder (Alnus) and willow (Salix) species.
Global Warming and Climate Change
Global warming has had a significant negative impact on Arizona's Mountain forests, particularly those in the southwestern part of the state. According to the National Oceanic and Atmospheric Administration's National Centers for Environmental Information, temperatures in Arizona have risen 2.5 degrees Fahrenheit since the beginning of the twentieth century. Frequent droughts have fueled forest fires, such as the Tunnel Fire, which broke out near Flagstaff in April 2022 and burned 26,532 acres. Fire have become a frequent occurence in Arizona. Arizona's Department of Forestry and Fire Management reported 1,774 wildfires in 2021 that burned a reported 524,428 acres. Fires have destroyed large portions of forests, particularly those with pinon juniper trees.
Warmer temperatures and drier conditions have made pests a larger problem within the forests. Black beetles have damaged the trees. Experts note that pinon junipers grow slowly, so it will take many years to replace the forests in this area. Bark beetles have also caused damage and infested over 100,000 acres of forests.
Fauna
Apex predators across each of these zones include grizzly and black bears, cougars, gray wolves, and raptors. Their prey consists of a wide range of mammals—including both wild species such as antelope, black-tailed deer, bighorn sheep and various domesticated herd animals—as well as reptiles such as the docile western box turtle and the considerably more feisty rattlesnake. These in turn are supported by a full panoply of smaller mammals such as the kangaroo rat, white mouse, and squirrels, which depend ultimately on a diet of invertebrates and vegetation.
Bibliography
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