Summary: This
ecological system occurs in dry mountains and foothills of the Colorado
Plateau region including the Western Slope of Colorado to the Wasatch
Range, south to the Mogollon Rim and east into the northwestern corner
of New Mexico. It is typically found at lower elevations ranging from
1500-2440 m. These woodlands occur on warm, dry sites on mountain
slopes, mesas, plateaus, and ridges. Severe climatic events occurring
during the growing season, such as frosts and drought, are thought to
limit the distribution of pinyon-juniper woodlands to relatively narrow
altitudinal belts on mountainsides. Soils supporting this system vary in
texture ranging from stony, cobbly, gravelly sandy loams to clay loam or
clay. Pinus edulis and/or Juniperus osteosperma dominate
the tree canopy. In the southern portion of the Colorado Plateau in
northern Arizona and northwestern New Mexico, Juniperus monosperma
and hybrids of Juniperus spp may dominate or codominate the tree
canopy. Juniperus scopulorum may codominate or replace Juniperus
osteosperma at higher elevations. Understory layers are variable and
may be dominated by shrubs, graminoids, or be absent. Associated species
include Arctostaphylos patula, Artemisia tridentata, Cercocarpus
intricatus, Cercocarpus montanus, Coleogyne ramosissima, Purshia
stansburiana, Purshia tridentata, Quercus gambelii, Bouteloua gracilis,
Pleuraphis jamesii, or Poa fendleriana. This system occurs at
higher elevations than Great Basin Pinyon-Juniper Woodland (CES304.773)
and Colorado Plateau shrubland systems where sympatric.
Classification
Approach: International Terrestrial Ecological Systems
Classification (ITESC)
Figure 2.2 Provinces of Arizona and New Mexico; sections within provinces
are delimited with solid lines; National Forests are indicated by solid fill
pattern.province
Figure 2.3 Life zones of Arizona and New Mexico as characterized by biotic
communities.biotic communities map
LIFE ZONES OF THE SOUTHWEST
The General Ecosystem Survey (Carleton et al. 1991) groups Southwestern
ecosystems into life zones characterized by biotic community types including
desert, grassland, chaparral, evergreen oak woodland, coniferous woodland,
ponderosa pine, mixed conifer, spruce–fir, tundra, and riparian wetlands.
Because this an assessment of forest, woodland, and associated riparian
ecosystems, desert, grassland, chaparral, and tundra life zones are excluded
from further discussion. The concept of a life zone is derived from a taxonomic
classification system described first by Merriam (1898), revised by UNESCO
(1973), and applied in the Southwest by the Terrestrial Ecosystem Survey. The
General Ecosystem Survey life zones (Table 2.2) can be cross-referenced to the
biotic communities described by Brown and Lowe (1977, 1980) and Brown (1994).
Aspen is a component of the montane forest found mostly in the mixed conifer
zone but also in the ponderosa pine and the spruce–fir zones. Riparian
wetlands occupy little area but like aspen perform special and very important
ecological and landscape functions within their life zone. Because of their
uniqueness and value, aspen and riparian wetlands are treated here along with
evergreen oak and coniferous woodlands, ponderosa pine, mixed conifer, and
spruce–fir forests as forest biotic communities of the Southwest (Figure
2.3).
Table 2.2 Correspondence between General Ecosystem
Survey Life Zones and Southwestern Biotic Communities.
Life Zone
Biotic Community
Evergreen oak woodland
Madrean evergreen woodland
Coniferous woodland
Great Basin conifer woodland
Ponderosa pine and mixed conifer
Rocky Mountain/Madrean montane coniferous forest
Spruce-fir
Rocky Mountain subalpine coniferous forest.
Life zones defined by Carleton et al.
(1991); biotic communities described by Brown and Lowe (1977, 1980) and
Brown (1994).
A specific ecosystem can be located with reference to a geographic province;
and if it is defined by the dominant vegetation, it can also be associated with
other ecosystems of similar biotic composition (life zone). Whereas the province
scale is the correct perspective for examining landscape dynamics, the life zone
is the appropriate scale for describing aspects of community development such as
disturbance regime and successional pattern. Every ecosystem is a unique entity
with its own particular history, composition, structure, and potential. Although
ecosystems of a common life zone tend to respond in similar ways, each is
different. Some of this difference can be explained by location within a
landscape and province. Because most of the available information for past and
current ecosystems is cataloged or identified by life zone, this assessment
primarily describes Southwestern ecosystems by life zone and notes differences
by province where they are known.
Evergreen Oak and Coniferous Woodlands
Woodlands generally include evergreen oak and conifer species that occupy
certain areas along an elevational gradient from low-elevation desert
shrub/grasslands and short-grass prairies to high-elevation montane coniferous
forests of ponderosa pine and Gambel oak. Oak woodlands occur within the range
of 4,000– to 9,000–feet elevation. Gambel oak occurs at higher elevations,
and wavyleaf oak (Quercus undulata) occurs either below Gambel oak or
intermingled with it in a transition zone. Woodlands were used extensively by
prehistoric and historic populations for habitation and subsistence. Uses today
include grazing, fuelwood
harvest, and recreation.
Evergreen oak woodland, characterized by wet summers and mild winters,
extends from the Sierra Madre of Mexico into southeastern Arizona and
southwestern New Mexico. In the United States, a variety of oak species such as
Emory oak (Quercus emoryi), Arizona white oak (Q. arizonica),
Mexican blue oak (Q. oblongifolia), gray oak (Q. grisea),
silverleaf oak (Q. hypoleucoides), and netleaf oak (Q. rugosa) are
found in conjunction with the following Madrean pine species–Apache pine (Pinus
engelmannii), Chihuahua pine (P. leiophylla var. chihuahuana),
and Arizona pine (P. arizonica) (Brown 1994).
Pinyons and junipers, together or alone, dominate coniferous woodland
communities. These woodlands occupy approximately 23 million acres in New
Mexico, about 13 percent of which are on national forest lands, and 4.1 million
acres in Arizona, 34 percent on national forest lands. The pinyons include Pinus
edulis, the most common pinyon pine throughout the type, border pinyon (P.
discolor), and Arizona singleleaf pinyon (P. californarium subsp. fallax).
Junipers are frequently found at lower elevations than pinyons and typically
occupy sites with deep soils. The most common junipers in the Southwest are
one-seed juniper (Juniperus monosperma) found in central and southern New
Mexico and much of Arizona below the Mogollon Rim, the Rocky Mountain juniper (J.
scopulorum) in the higher and colder woodlands of northern New Mexico and
Arizona, Utah juniper (J. osteosperma) in northwestern New Mexico and
northern Arizona, and alligator juniper (J. deppeana) associated with the
Madrean woodlands of southern Arizona and New Mexico (Brown 1994, Gottfried
1992).
Ponderosa Pine
Ponderosa pine (yellow pine or blackjack pine) is found from 6,500 to 8,000
feet elevation. At lower elevations, the ponderosa pine forest meets woodlands
and at higher elevations transitions into the mixed conifer zone. Ponderosa pine
forests of central and northern New Mexico and Arizona cover about 8.4 million
acres. The predominant form throughout the Southwest is the three-needled, Rocky
Mountain ponderosa pine (P. ponderosa var. scopulorum). In lower
elevations of southern Arizona, however, the five-needled, Arizona pine is more
common. Other species associated with ponderosa pine at low elevations are
Gambel oak and New Mexico locust (Robina neomexicana); at high elevations
associates are southwestern white pine (Pinus strobiformis), Rocky
Mountain Douglas-fir (Pseudotsuga menziesii var. glauca), Rocky
Mountain white fir (Abies concolor var. concolor), and quaking
aspen (Populus tremuloides) (Brown 1994). Uses include timber harvest,
grazing, camping, and other types of recreation offering cool relief from hot
urban areas.
Mixed Conifer
Mixed conifer forests dominated by Douglas-fir, white fir, and blue spruce (Picea
pungens) occur at elevations from 8,000 to 9,500 feet. There are about 1.5
million acres of mixed conifer forest in the Rocky Mountain and Madrean montane
forests of southern Colorado, New Mexico, Arizona, and the Sierra Madre
Occidental of Mexico (Brown 1994). Ponderosa pine, southwestern white pine,
aspen, and a number of other tree species may occur in these forests. Uses are
similar to the ponderosa pine community.
Spruce–Fir
Spruce–fir forests are found at high, subalpine, elevations in the
Southwest from approximately 8,000 feet to over 12,000 feet. Spruce–fir
forests are typically restricted to areas receiving more than 25 inches of
precipitation from winter snows and summer thunderstorms. The predominant spruce
is Engelmann spruce which is found as far south as the Pinaleno Mountains in
Arizona and the Sacramentos in New Mexico. The co-dominant species is subalpine
fir (Abies lasiocarpa). Some populations of subalpine fir possess a
distinctive outer cortex and are called corkbark fir (A. lasiocarpa var. arizonica).
Small stands of aspen or blue spruce are found within the spruce–fir forest
(Brown 1994). Uses include wilderness recreation, skiing, and the enjoyment of
high places. Mountain peaks have special cultural and religious significance for
many Southwestern Indian tribes.
Aspen
Quaking aspen occurs at elevations above 6,000 feet as small, transient
patches in ponderosa pine, mixed conifer, or spruce–fir forests. There are
close to 500,000 acres of aspen in the Southwest, seventy-five percent in
northern New Mexico and the remainder in the Mogollon Rim-White Mountains of
Arizona (Brown 1994). Aspens can reproduce by cloning from an established root
system and establish a new stand of trees quickly after a fire or other
disturbance. Aspens, however, are intolerant of shade and eventually lose out to
competition when they become overtopped by re-invading conifers. Aspen stands
are especially valued for their scenic quality and use by traditional
communities.
Riparian Wetlands
Riparian wetlands including cienegas
make up less than 2 percent of the land of New Mexico and Arizona, but they are
the most biologically diverse and productive ecosystems in the Southwest. Over
65 percent of Southwestern animals depend on riparian habitats during all or
part of their life cycles. Millions of Southwestern residents use these areas
for recreation and agriculture. The most important species of Southwestern
riparian wetlands are the Fremont cottonwood (Populus fremontii) and the
narrowleaf cottonwood (P. augustifolia) (Brown 1994).
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