This widespread ecological system is more common in the southern and central Rocky Mountains, but occurs in the montane and subalpine zones throughout much of Montana north into Canada. It is similar to the Inter-Mountain Basins Aspen Mixed Conifer Forest-Woodland found in the Big Snowy Mountains, but lacks the conifer component. Distribution of this system is primarily limited by adequate soil moisture required to meet its high evapotranspirative demand, length of growing season, and temperatures. Mean annual precipitation where these systems occur is generally greater than 38 centimeters (15 inches) and typically greater than 51 centimeters (20 inches), except in semi-arid environments where occurrences are restricted to mesic microsites such as seeps or areas below large snow drifts. Stands can occur on gentle to moderate slopes, in swales, or on level sites. At lower elevations, occurrences are found on cooler, north aspects and mesic sites. Soils are usually deep and well developed with rock often absent from the soil. Soil texture ranges from sandy loam to clay loams. This system describes mesic forests and woodlands dominated by quaking aspen (Populus tremuloides) without a significant conifer component (<25% relative tree cover). This aspen system can be stable and long-lived with little encroachment of coniferous species. The understory structure may be complex with multiple shrub and herbaceous layers, or simple, with just an herbaceous layer. The herbaceous layer may be dense or sparse, dominated by mesic grasses or forbs. Occurrences of this system often originate, and are likely maintained, by stand-replacing disturbances such as crown fire, disease, windthrow, elk and beaver activity.
This system includes aspen stands with a relatively closed canopy of trees 5-20 meters (16 to 66 feet) tall. In Montana, most aspen clones are smaller than in the Central Rocky region. Clones can be stable and long-lived or seral to Douglas-fir (Pseudotsuga menziesii), subalpine fir (Abies lasiocarpa) or spruce (Picea species) dominated forests (Habeck, 1967). Stable climax aspen forest also occurs in southwestern Montana (Pfister et al, 1977). Conifers that may be present but are never codominant include subalpine fir, Engelmann spruce (Picea engelmannii), white spruce (Picea glauca), ponderosa pine (Pinus ponderosa), and Douglas-fir. Conifer species may contribute up to 25% of the tree canopy before the occurrence is reclassified as a mixed occurrence. This system can be interpreted in some instances to be the seral phase of the Inter-Mountain Basins Aspen-Mixed Conifer Forest & Woodland. In Glacier County, this system differs in height growth, which is controlled by recurring Chinook winds.
Depending on available soil moisture and other factors like disturbance, the understory structure may be complex with multiple shrub and herbaceous layers, or simple with just an herbaceous layer. The herbaceous layer may be dense or sparse, dominated by graminoids or forbs. Common shrubs include Rocky Mountain maple (Acer glabrum), serviceberry (Amelanchier alnifolia), creeping Oregon grape (Mahonia repens), chokecherry (Prunus virginiana), rose (Rosa spp.), thimbleberry (Rubus parviflorus), and snowberry (Symphoricarpos spp.). The herbaceous layers may be lush and diverse. Common graminoids may include mountain brome (Bromus carinatus), pinegrass (Calamagrostis rubescens), Ross’ sedge (Carex rossii), blue wildrye (Elymus glaucus), slender wheatgrass (Elymus trachycaulus) and bearded fescue (Festuca subulata). Common mesic understory forbs include yarrow (Achillea millefolium), sharptooth angelica (Angelica arguta), Engelmann aster (Eucephalus engelmannii),larkspur (Delphinium species), aspen daisy (Erigeron speciosus), Richardson’s geranium (Geranium richardsonii), common cow parsnip (Heracleum maximum), western sweet cicely (Osmorhiza occidentalis), western meadow rue (Thalictrum occidentale), stinging nettle (Urtica dioica) and western valerian (Valeriana occidentalis). Bracken fern (Pteridium aquilinum) is present in some stands.Exotic grasses such as the perennials Kentucky bluegrass (Poa pratensis), common timothy (Phleum pratense) and smooth brome (Bromus inermis) are often common in occurrences disturbed by grazing.
Occurrences in this ecological system often originate with, and are likely maintained by, stand-replacing disturbances such as crown fire, disease and windthrow, or logging by humans or beaver. Boles are killed by ground fires, but they can quickly and vigorously resprout by root suckers in high densities. Stems are relatively short-lived (70-120 years), and the system will generally succeed to longer-lived conifer forest if undisturbed. Occurrences are favored by fire in the conifer zone (Mueggler, 1988). In Montana, seed production is erratic and infrequent. Natural seedling establishment is limited to years of viable seed production. Seedling recruitment is limited to sites where there is adequate soil moisture following dispersal in early summer. Following the Yellowstone fires of 1988, quaking aspen seedlings established on many suitable sites. These sapling and young tree stands are subjected to heavy elk browsing and will not reach full maturity.
Quaking aspen is dioecious; clones are either male or female. Reproduction is largely clonal. Some clones are thought to be centuries old and have the potential to be large in size. Stems are produced from a common root system; new stems are produced on the outside, advancing in front of the clone, with older trees in the center. The root system persists as stems die and are replaced. Clones can be distinguished by morphological differences in flowering and leaf emergence phenology, leaf size and shape, branching habit, bole character, and gender. Quaking aspen reproduces vegetatively by sprouting from stumps and root crowns, and by forming suckers (adventitious shoots on roots). The ability of Populus to regenerate by suckers can vary widely among clones (Schier et al, 1985).
In recent years, many aspen stands have exhibited mortality from biotic vectors. These pathogens mainly infect clones already stressed by drought, insects, wind damage, heavy livestock and wildlife use and similar factors.
Restoration strategies for will depend on fire severity, grazing or other land impacts. Because lightly burned areas regenerate vegetatively following fire, additional restoration practices are not required. Early successional stages may be dominated by fireweed (Chamerion angustifolium) and other forbs, small amounts of forest graminoids such as mountain brome, blue wildrye, and pinegrass, and by resprouting of dominant shrubs. Aspen will resprout vigorously following fires of low to moderate severity. Some sprouting will occur after higher intensity fires from root suckers that are deeper in the soil profile.
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