Description and Ecology: Aspen is the most widely distributed tree species in North America, found from northern Mexico north to Alaska and east to the Atlantic coast (Eyre 1980). It occurs from sea-level to 600 m (2,000 ft) at its northern limit and at increasing elevations to the south. At its southern limit it occurs from 2000 to 3000 m (7,000-10,000 ft). Aspen-dominated woodlands are best developed and most abundant in Colorado, northern New Mexico and southern Utah at 2000 to 3000 m elevation (Eyre 1980, Mueggler 1989). There are about 2.8 million ha (7 million ac) of aspen-dominated woodlands across the western United States, of which about 1.4 million ha (3.5 million ac, 50%) occur in Colorado. On National Forests in the Rocky Mountain Region, aspen forests comprise almost 20% of the tree-covered woodlands (Torrence 1984).

Aspen grows under a wide variety of environmental conditions and upland sites. Required site conditions include long growing seasons, deep snow, and annual precipitation exceeding 40 to 50 cm (16-20 in) (Jones and DeByle 1985). In the Southern Rockies, the best stand development occurs on well-drained, sandy to silt-loam soils (Eyre 1980), and on southerly to easterly exposures (Mueggler 1989). Aspen-dominated woodlands are highly valued for summer forage for livestock grazing, watershed protective cover, timber harvest, firewood, and scenic beauty. Most aspen in Colorado occurs in Physiographic Area 62, the Southern Rocky Mountains.

Aspen occurs primarily as an early seral species, eventually being replaced by surrounding shade-tolerant late-seral conifers (Mueggler 1989). In Colorado, it is a major seral constituent of Englemann spruce-subalpine fir, Douglas-fir, white fir, blue spruce, and ponderosa pine forests. At lower elevations, it is often found as stringers along riparian corridors, or in small mesic islands surrounded by drier pine uplands. At higher elevations, it functions primarily as a seral dominant species within the spruce-fir ecotone. At intermediate elevations and on deep soils, aspen can occur as pure stands of successionally stable woodlands dispersed within a matrix of coniferous forest types. Mueggler (1989) states "the most valid general indicator of seral aspen is the presence of conifers either in the overstory or as reproduction, suggesting active replacement by the more shade tolerant [conifer] species. A lack of conifers might indicate either a true late-seral aspen situation, or merely the absence of a nearby conifer seed source."

Following severe disturbance, such as stand-replacement fires or clear cutting, aspen usually dominates sites for many decades. On some sites, conversion back to a conifer-dominated stand occurs very quickly (<100 years), while on other sites, conversion may take many aspen generations and extend 300-400 years (Eyre 1980, Mueggler 1989). The rate of stand conversion is determined by site conditions, proximity to conifer seed sources, and rate of conifer seedling growth into the stand canopy.

Most aspen regeneration occurs as root-suckering with little occurring from seed. Therefore, aspen stands are usually a mosaic of clones, within which individual trees are genetically identical and have strong structural uniformity (Eyre 1980). Despite uniformity within clones, multi-clone stands often exhibit wide structural variation due to genetic and site differences among adjacent clones (Johnston and Hendzel 1985). Clone size averages 0.2 ha (0.5 ac) across the species range (Eyre 1980). In Colorado, clone size averages 0.8 to 1.2 ha (2-3 ac) but can exceed 20 ha (50 ac) (Johnston and Hendzel 1985). Aspen stems are typically short-lived, with average stem age being 50-60 years across the species range. An extensive study in Colorado examined 64 aspen stands and found the oldest stem was 137 yrs old, and it was also the largest diameter stem and measured 21.8 cm (8.6 in) dbh (Johnston and Hendzel 1985). The tallest stem found in this study was 26 m (86 ft). Because stem diameter and height are not strongly correlated with stem age, it is difficult to clearly define a structurally distinct mature stage.

Most aspen stands have well-defined overstory layers of relatively uniform height produced by the rapid regeneration of suckers following stand-replacement events (Mueggler 1989). The majority of stems are produced during the first 4-6 years after disturbance; thus, stands with an equal representation of multiple age and size classes are rare (Johnston and Hendzel 1985). Multiple age classes develop when older stands begin to disintegrate or when disease or insects open up the canopy and apical dominance declines, releasing understory suckers.

Because of the ecological amplitude of aspen, overstory and understory vegetation composition varies widely across its range, resulting in equally diverse breeding bird assemblages. The value of aspen habitats to wildlife is directly related to the structural diversity of the canopy and undergrowth (Mueggler 1989). Stands with a predominantly aspen overstory allow sufficient light to reach the forest floor to support multilayered herb and shrub understories, and are often more lush than adjacent conifer stands (Eyre 1980). As aspen dominance gives way to conifer dominance, less light reaches the forest floor, and understory diversity and abundance declines. In Colorado, the most common understory shrubs are snowberry, western serviceberry, chokecherry, and rose. The most common forbs include geranium, valerian, yarrow, and dandelion.

Importance and Conservation Status: Because aspen is seral to and is usually mixed with adjacent conifer types, the importance of aspen-dominated woodlands to birds and other wildlife far exceeds the aerial extent of the stands themselves. DeByle (1985) listed 134 species of birds that use aspen-dominated habitats. This list includes 34 cavity nesters, 7 canopy nesters, 10 shrub nesters, and 10 ground nesters. Few species are limited to aspen, but some reach their highest breeding densities within this habitat type. Bird communities within aspen stands are often composites of aspen-associated species along with many species found in the surrounding conifer habitats. However, the exact species mix depends on the relative amounts of aspen and conifer in the stand.

Perhaps the most important contribution of aspen-dominated woodlands to avian nesting habitat is as a structural substrate for primary cavity excavators and secondary cavity nesters. False tinder rot is a major source of heartwood decay in live aspens; it produces a hard sapwood shell surrounding a soft interior that is ideal for cavity excavation (Kalcounis and Brigham 1998). In conifers, sapwood and heartwood decay at equal rates, precluding the formation of a hard outer shell. Aspen stems with heartrot can be readily detected by the presence of shelf fungi or conks on the trunk. Habitat preferences of primary cavity excavators and the decay characteristics of aspen combine to produce much higher cavity densities in aspen than in surrounding conifer habitats.

Most of today's seral aspen woodlands were produced by stand-replacement fires that occurred within the previous 150 years. Fire suppression during the past 75 years has dramatically reduced the rate of conversion of conifer stands back to early-seral aspen. As a result of continuing fire suppression and successional processes, the extent of aspen across the Colorado landscape is probably declining and will continue to do so unless direct management actions are taken. Controlled burning of aspen stands is difficult and expensive, and currently encompasses less than 0.009% of aspen woodlands in the interior west (DeByle et al. 1987). Although loss of aspen woodlands to conifer succession at the landscape scale has yet to be conclusively demonstrated, most land managers agree that it is occurring and that management actions are insufficient to maintain current aspen dominance in many areas.

Wood fiber production is becoming increasingly important as a tool for stand regeneration of aspen woodlands in western Colorado. Clearcut harvesting is the most common harvest mechanism. Significant production of aspen chipboard, indoor paneling, and excelsior has recently developed in western Colorado. This production may provide a viable economic tool for small-scale aspen stand regeneration. There have been few studies on the effects of aspen harvesting on breeding bird communities. Scott and Crouch (1987) found that breeding bird density and species composition did not change significantly following small patch clearcutting of 25% of an aspen analysis area in Colorado, and made specific recommendations for harvest patch size and rotation age. These results concurred with an earlier study by DeByle (1981) in Utah. However, in a Minnesota study, Merrill et al. (1998) found significant differences in bird species composition between clearcuts with residual patches and clearcuts without patches.

Grazing by both domestic livestock and native ungulates, particularly elk, can have significant impacts on aspen regeneration and shrub-forb structure of aspen-dominated woodlands (Shepperd and Fairweather 1994). Because of the comparatively high production of grass and forbs in aspen understories compared to surrounding conifer stands, aspen woodlands frequently provide excellent summer range for domestic livestock, and in some cases they have been severely overgrazed, especially in the early 1900s. Aspen-dominated woodlands continue to provide a significant amount of the available forage for domestic livestock on public land grazing allotments. Grazing on vegetative structure of aspen stands may change the composition and relative abundance of understory vegetative species and change the structure of shrub and forb communities; intense repeated browsing on apical stems can severely damage aspen suckers and halt regeneration. Fire suppression has allowed dense conifer regeneration to survive, reducing available forage in some conifer types such as ponderosa pine and increasing browsing pressure in adjacent aspen stands where forage is more readily available (Shepperd and Fairweather 1994). Browsing by elk is thought to be partly responsible for an estimated 50% loss of aspen in Yellowstone National Park (Mueggler 1989).

Priority Species Accounts: Priority birds in aspen-dominated habitats in Colorado include Broad-tailed Hummingbird, Red-naped Sapsucker, Purple Martin, and Violet-green Swallow.