Quaking aspen (Populus tremuloides) has the largest distribution of any tree native to North America. This widespread ecological system occurs throughout much of the western U.S. and north into Canada, although it is more common in the montane and subalpine zones of the southern and central Rocky Mountains. In Colorado this system is most common on the west slope at elevations of 2,290 to 3,350 m (7,500-11,000 ft), with smaller stands represented on the east slope. These are upland forests and woodlands dominated by quaking aspen without a significant conifer component (<25% relative tree cover). The open growth form of this tree allows light to penetrate to the understory. Aspen forest and woodlands usually occur as a mosaic of many plant associations and may be surrounded by a diverse array of other systems, including grasslands, wetlands, or coniferous forests. Understory composition is vairable and may be lush, with multiple shrub and herbaceous layers, or less complex.

These forests and woodlands are characterized by extensive quaking aspen stands and very few conifers. The typically lush understory appears lighter and more open than in aspen-mixed conifer forests, due to the dominance of deciduous trees. Understories are variable, and may be shrub or herbaceous dominated. In general, these forests are more mesic in comparison to the aspen-mixed conifer types.

This widespread ecological system occurs throughout much of the western U.S. and north into Canada, although it is more common in the montane and subalpine zones of the southern and central Rocky Mountains. In Colorado this system is most common on the west slope, with smaller stands represented on the east slope.

Rocky Mountain Aspen Forest and Woodland is a large patch type.

Quaking aspen is able to grow on a wide variety of sites, both dry and mesic, but is limited to areas where annual precipitation exceeds evapotranspiration and annual mean temperature does not exceed 8°C (45°F). In the central Rocky Mountains, quaking aspen distribution is highly correlated with elevation, due to its influence on temperature and precipitation patterns. Occurrences at high elevations are limited by cold temperatures and are found on warmer southern aspects. At lower elevations occurrences are restricted by lack of moisture to cooler northern aspects and mesic microsites. Topography is variable, sites range from level to steep slopes. The soils are typically deep and well developed with rock often absent, and texture ranges from sandy loam to clay loams. Parent materials are variable and may include sedimentary, metamorphic or igneous rocks.

The aspen canopy allows sufficient light penetration for the development of a lush understory that may be dominated by shrubs, graminoids, or forbs. The typically diverse species composition in these woodlands makes it difficult to delineate plant associations with certainty from place to place. In effect, there are groups of associations characterized by understory structure, variable levels of moisture, and disturbance history.
Common shrubs include Rocky Mountain maple (Acer glabrum), Saskatoon serviceberry (Amelanchier alnifolia), mountain big sagebrush (Artemisia tridentata ssp. vaseyana),common juniper (Juniperus communis), chokecherry (Prunus virginiana), Wood's rose (Rosa woodsii), russet buffaloberry (Shepherdia canadensis),mountain snowberry (Symphoricarpos oreophilus), and the dwarf-shrubs creeping barberry (Mahonia repens) and whortleberry (Vaccinium spp.). Common graminoids include mountain brome (Bromus marginatus), wooly brome (Bromus lanatipes), pinegrass (Calamagrostis rubescens), dryspike sedge (Carex siccata), Geyer's sedge (Carex geyeri), Ross' sedge (Carex rossii), blue wildrye (Elymus glaucus), slender wheatgrass (Elymus trachycaulus), and Thurber fescue (Festuca thurberi). Exotic grasses such as the perennials Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), and timothy (Phleum pratense) are often common in occurrences disturbed by grazing. Associated forbs may include common yarrow (Achillea millefolium), monkshood (Aconitum columbianum), heartleaf arnica (Arnica cordifolia), paintbrush (Castilleja spp.), Engelmann's aster (Eucephalus engelmannii), larkspur (Delphinium spp.), Richardson's geranium (Geranium richardsonii), common cowparsnip (Heracleum maximum), Porter's licorice-root (Ligusticum porteri), silvery lupine (Lupinus argenteus), sweetcicely (Osmorhiza berteroi), western brackenfern (Pteridium aquilinum), Fendler's meadow-rue (Thalictrum fendleri), western valerian (Valeriana occidentalis), American vetch (Vicia americana), mule-ears (Wyethia amplexicaulis), and many others.

Aspen forests provide essential wildlife habitat; mesic areas are most diverse. Where aspen is near riparian areas, beaver (Castor canadensis) make extensive use of these trees. Mammals both large and small can be found in these forests, including elk (Cervus elaphus), mule deer (Odocoileus hemionus), black bear (Ursus americanus), coyote (Canis latrans), bats (Myotis spp.), shrews (Family Soricidae), squirrels (Family Sciuridae), pocket gopher (Family Geomyidae), white-footed mice (Peromyscus spp.), and voles (Microtus spp.). Aspen forests are important for cavity-nesting birds but also provide habitat for many songbirds. Species found in these forests include Warbling Vireo (Vireo gilvus), House Wren (Troglodytes aedon), Red-naped Sapsucker (Sphyrapicus nuchalis), Williamson's Sapsucker (Sphyrapicus thyroideus), Hairy Woodpecker (Dryobates villosus), Northern Flicker (Colaptes auratus), Tree Swallow (Tachycineta bicolor), Violet-green Swallow (Tachycineta thalassina), American Robin (Turdus migratorius), Mountain Bluebird (Sialia currucoides), Yellow-rumped Warbler (Setophaga coronata), Western Wood-Pewee (Contopus sordidulus), Dark-eyed Junco (Junco hyemalis), and Purple martin (Progne subis).

Aspen is extremely shade intolerant, but is able to establish quickly in disturbed open areas due to its ability to reproduce by vegetative sprouting. The tufted seed capsules produced by mature aspen trees can be wind dispersed over considerable distances. Although quaking aspen establishment from seed is common in Alaska, northern Canada and eastern North America, this is less true in the western US, probably because germinated seedlings do not receive sufficient moisture for survival.
Colorado's western slope appears to occupy the margins between the seral aspen type found in boreal and montane areas, and the stable type that is characteristic of Colorado Plateau highlands. Seral aspen types (i.e., aspen-mixed conifer) are common and may be intermingled with persistent aspen stands, and are thought to originate with stand-replacing disturbance, especially fire. Rocky Mountain Aspen Forest and Woodland occurrences are generally persistent aspen stands that have remained stable over periods of 80 years or more with little to no conifer development.
Disturbance type, frequency, and extent is important in determining the appearance of these woodlands. Stands that have escaped catastrophic fire may exhibit multi-layered structure with regeneration and/or recruitment as well as mature canopy. Small- to moderate-scale disturbance can create gaps that are filled in by younger stems that are already present in the under or mid-story. Single-layer stands where regeneration patterns have been altered by grazing may also belong to this system.

In the absence of historic predators, elk grazing appears to have increased in many stands, with consequent impacts on aspen regeneration. Grazing management may be needed to reduce impacts on drought-stressed areas. Overall exposure to warmer and effectively drier conditions is low for this system in Colorado; stands at lower elevations are most at risk. These forests are moderately resilient, and in generally good condition. Aspen dynamics are variable across the west, depending on both spatial and temporal scales; as a result there is much uncertainty about the future distribution of this species. Low elevation stands impacted by drought are likely to experience dieback, but in other areas the interaction of changing climate and disturbance regimes may favor aspen. Drier and warmer conditions in the future are likely to increase disease and insect outbreaks, and may reduce the frequency of occasions suitable for aspen seed production and successful recruitment.