08 – Upland Aspen Forest
Rex C. Crawford & 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 the isolated mountain
ranges of southeastern Oregon, e.g., Steens Mountain, 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, but 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 and Alpine Grasslands on Steens Mountain in
southern Oregon. It can occur in seral stands in the lower Eastside Mixed
Conifer Forest and Ponderosa Pine Forest and Woodland habitats. Primary land use
is livestock grazing.
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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
two-tiered stands characterize the typical vertical structure of woody plants in
this habitat. This habitat is composed of one 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
Project126 and Oregon Vegetation Landscape-Level Cover
Type127 that would represent this type is aspen
groves. Other references describe this habitat2, 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 Utah119 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 four 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 are considered
imperiled.10
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