No. 8. Upland Aspen Forest

Rex C. Crawford and Jimmy Kagan

Geographic Distribution. Quaking aspen groves are the most widespread habitat in North America, but are a minor type throughout eastern Washington and Oregon. Upland Aspen habitat is found in isolated mountain ranges of Southeastern Oregon, e.g. Steens Mountains, and in the northeastern Cascades of Washington. Aspen stands are much more common in the Rocky Mountain states.

Physical Setting. This habitat generally occurs on well-drained mountain slopes or canyon walls that have some moisture. Rockfalls, talus, or stony north slopes are often typical sites. It may occur in steppe on moist microsites. This habitat is not associated with streams, ponds, or wetlands. This habitat is found from 2,000 to 9,500 ft (610 to 2,896 m) elevation.

Landscape Setting. Aspen forms a "subalpine belt" above the Western Juniper and Mountain Mahogany Woodland habitat and below Montane Shrub-steppe Habitat on Steens Mountain in southern Oregon. It can occur in seral stands in the lower Eastside Mixed Conifer Forest and Ponderosa Pine Forest and Woodlands habitats. Primary land use is livestock grazing.

Structure. Deciduous trees usually <48 ft (15 m) tall dominate this woodland or forest habitat. The tree layer grows over a forb-, grass-, or low-shrub-dominated undergrowth. Relatively simple 2-tiered stands characterize the typical vertical structure of woody plants in this habitat. This habitat is composed of 1 to many clones of trees with larger trees toward the center of each clone. Conifers invade and create mixed evergreen-deciduous woodland or forest habitats.

Composition. Quaking aspen (Populus tremuloides) is the characteristic and dominant tree in this habitat. It is the sole dominant in many stands although scattered ponderosa pine (Pinus ponderosa) or Douglas-fir (Pseudotsuga menziesii) may be present. Snowberry (Symphoricarpos oreophilus and less frequently, S. albus) is the most common dominant shrub. Tall shrubs, Scouler’s willow (Salix scouleriana) and serviceberry (Amelanchier alnifolia) may be abundant. On mountain or canyon slopes, antelope bitterbrush (Purshia tridentata), mountain big sagebrush (Artemisia tridentata ssp. vaseyana), low sagebrush (A. arbuscula), and curl-leaf mountain mahogany (Cercocarpus ledifolius) often occur in and adjacent to this woodland habitat.

In some stands, pinegrass (Calamagrostis rubescens) may dominate the ground cover without shrubs. Other common grasses are Idaho fescue (Festuca idahoensis), California brome (Bromus carinatus), or blue wildrye (Elymus glaucus). Characteristic tall forbs include horsemint (Agastache spp.), aster (Aster spp.), senecio (Senecio spp.), coneflower (Rudbeckia spp.). Low forbs include meadowrue (Thalictrum spp.), bedstraw (Galium spp.), sweetcicely (Osmorhiza spp.), and valerian (Valeriana spp.).

Other Classifications and Key References. This habitat is called "Aspen" by the Society of American Foresters and "Aspen woodland" by the Society of Range Management. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Type ¹²⁷ that would represent this type is aspen groves. Other references describe this habitat ^{2, 88, 119, 161, 222,}.

Natural Disturbance Regime. Fire plays an important role in maintenance of this habitat. Quaking aspen will colonize sites after fire or other stand disturbances through root sprouting. Research on fire scars in aspen stands in central Utah ¹¹⁹ indicated that most fires occurred before 1885, and concluded that the natural fire return interval was 7-10 years. Ungulate browsing plays a variable role in aspen habitat; ungulates may slow tree regeneration by consuming aspen sprouts on some sites, and may have little influence in other stands.

Succession and Stand Dynamics. There is no generalized successional pattern across the range of this habitat. Aspen sprouts after fire and spreads vegetatively into large clonal or multiclonal stands. Because aspen is shade intolerant and cannot reproduce under its own canopy, conifers can invade most aspen habitat. In central Utah, quaking aspen was invaded by conifers in 75-140 years. Apparently, some aspen habitat is not invaded by conifers, but eventually clones deteriorate and succeed to shrubs, grasses, and/or forbs. This transition to grasses and forbs occurs more likely on dry sites.

Effects of Management and Anthropogenic Impacts. Domestic sheep reportedly consume 4 times more aspen sprouts than do cattle. Heavy livestock browsing can adversely impact aspen growth and regeneration. With fire suppression and alteration of fine fuels, fire rejuvenation of aspen habitat has been greatly reduced since about 1900. Conifers now dominate many seral aspen stands and extensive stands of young aspen are uncommon.

Status and Trends. With fire suppression and change in fire regimes, the Aspen Forest habitat is less common than before 1900. None of the 5 Pacific Northwest upland quaking aspen community types in the National Vegetation Classification is considered imperiled ¹⁰.