Description and Ecology: Mountain shrubland is one of the several manifestations ecologists
identify for the Transition Zone between semi-arid pinyon-juniper woodlands and the subalpine
forest above. Mountain shrubland consists primarily of Gambel oak and other associated shrubs,
including serviceberry, mountain mahogany, chokecherry, antelope bitterbrush, and snowberry.
Gambel oak is a large shrub or small tree and is probably the best known of the mountain shrubs
in Colorado. Gambel oak is widely distributed throughout the Southern and Central Rocky
Mountains. It occupies about 3.76 million ha (9.3 million ac) of land in Colorado. Gambel oak
has been described as a climax indicator in a number of habitat types. It reproduces by suckering,
and very large areas--whole mountainsides--can be populated by clones. Gambel oak is
extremely fire tolerant, vigorously re-sprouting from stem bases or from underground tubers and
rhizomes following fire. It can recover to original heights from a fire in 30 to 40 years. A healthy
stand of Gambel oak contains shrubs of varying heights and has robust native bunchgrasses and
forbs growing between them and relatively little bare ground.
Serviceberry is a large mountain shrub that increases in abundance in Colorado from south to
north. Mottes of this species make extremely dense cover. Mountain mahogany has a structure
similar to that of serviceberry, but in Colorado it does not create thickets as dense as serviceberry.
It grows with and adjacent to oak and serviceberry, but on drier sites. Chokecherry is a large
shrub common to mountain shrublands, but it rarely dominates large areas. Snowberry and
antelope bitterbrush are lower stature species that often grow intermixed with sagebrush and
Gambel oak. They are about the same height as sagebrush. Other shrubs occurring in mountain
shrubland communities (e.g., Squaw apple, curl-leaf mountain mahogany, buckbrush, and
mountain spray) do not become widespread dominants. Three species of Neotropical
migrants--Dusky Flycatcher, Virginia's Warbler, and Green-tailed Towhee--are associated with
Gambel oak and other shrub habitat.
Importance and Conservation Status: Mountain shrubland habitat provides valuable food and
cover for many wildlife species. Many shrub species produce edible fruits, and they provide a
large selection of forage types. Often the soil moisture is enough for shrubs to grow densely,
particularly on elk winter ranges.
Gambel oak acorns are an important mast crop in many areas. Birds such as Band-tailed Pigeon,
Wild Turkey, Lewis's Woodpecker, Steller's Jay, Western Scrub-Jay, and Green-tailed Towhee
feed on the acorns. Other birds such as the Virginia's Warbler utilize mountain shrub habitat for
resting, feeding, and nesting. At least 24 species of birds use Gambel oak habitat.
In Colorado, bird populations in mountain shrubland habitat are probably meeting their
requirements fairly well at this time, and impacts do not seem very serious. For Gambel oak this
status could change if firewood harvesting resumes importance or if chip plants discover the fiber
resource in Gambel oak. There is also a growing perception that something must be done to
increase the western mule deer population. Vegetation conversion projects in mountain
shrublands are being proposed and funds to implement them are being raised. Many acres of bird
habitat could be changed. In localized areas in Colorado, mountain shrubland habitat is being
altered, sometimes severely. These local alterations' cumulative impact can be detrimental to
birds over the long-term.
Priority Species Accounts: Two species are identified as high priority in mountain shrubland habitats
in Physiographic Area 62: Virginia's Warbler and Green-tailed Towhee.