Rocky Mountain Aspen Forest

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 the western U.S. and north into Canada. An eastern extension occurs along the Rocky Mountains foothill front and in mountain "islands" in Montana (Big Snowy and Highwood mountains), and the Black Hills of South Dakota. In California, this system is only found on the east side of the Sierra Nevada adjacent to the Great Basin. Large stands are found in the Inyo and White mountains, while small stands occur on the Modoc Plateau. In western Alberta, it occurs only in the Upper Foothills subregion, and north of there transitions to Western North American Boreal Mesic Birch-Aspen Forest (CES105.108). Elevations generally range from 1525 to 3050 m (5000-10,000 feet), but occurrences can be found at lower elevations in some regions, especially in the Canadian Rockies. Distribution of this ecological system is primarily limited by adequate soil moisture required to meet its high evapotranspiration demand. Secondarily, it is limited by the length of the growing season or low temperatures. These are upland forests and woodlands dominated by Populus tremuloides without a significant conifer component (<25% relative tree cover). 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. In California, Symphyotrichum spathulatum is a common forb. Associated shrub species include Symphoricarpos spp., Rubus parviflorus, Amelanchier alnifolia, and Arctostaphylos uva-ursi. Occurrences of this system originate and are maintained by stand-replacing disturbances such as avalanches, crown fire, insect outbreak, disease and windthrow, or clearcutting by man or beaver, within the matrix of conifer forests. It differs from Northwestern Great Plains Aspen Forest and Parkland (CES303.681), which is limited to plains environments. In Texas, this system occurs as small patches within the higher elevation conifer systems of the Guadalupe, Davis, and Chisos mountains. These patches are considered relictual remnants in this southwestern extension of this more commonly encountered type further north.

These are cold-deciduous, broad-leaved upland forests and woodlands dominated by Populus tremuloides without a significant conifer component (<25% relative tree cover). The tree canopy ranges from 5-20 m tall and may be open to closed. Conifers may be present but never codominant and include Abies concolor, Abies lasiocarpa, Picea engelmannii, Picea pungens, Pinus ponderosa, and Pseudotsuga menziesii. Because of the open growth form of Populus tremuloides, enough light can penetrate for lush understory development. Depending on available soil moisture and other factors such as 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 depending on available soil moisture and other factors such as disturbance. Associated shrub species include Amelanchier alnifolia, Arctostaphylos uva-ursi, Artemisia tridentata, Juniperus communis, Prunus virginiana, Ribes montigenum, Robinia neomexicana, Rosa woodsii, Rubus parviflorus, Shepherdia canadensis, Symphoricarpos spp., and the dwarf-shrubs Mahonia repens and Vaccinium spp. Numerous mesic forbs and graminoids may be present to dominant. Common graminoids may include Bromus carinatus, Calamagrostis rubescens, Carex siccata (= Carex foenea), Carex geyeri, Carex rossii, Elymus glaucus, Elymus trachycaulus, Festuca thurberi, Hesperostipa comata, and Muhlenbergia montana. Associated forbs may include Achillea millefolium, Eucephalus engelmannii (= Aster engelmannii), Delphinium spp., Geranium viscosissimum, Heracleum sphondylium, Ligusticum filicinum, Lupinus argenteus, Osmorhiza berteroi (= Osmorhiza chilensis), Pteridium aquilinum, Rudbeckia occidentalis, Thalictrum fendleri, Valeriana occidentalis, Wyethia amplexicaulis, and many others. Exotic grasses such as the perennials Poa pratensis and Bromus inermis and the annual Bromus tectorum are often common in occurrences disturbed by grazing. The over 60 associations included in this system document its heterogeneous nature. The vegetation description is based on several references, including Henderson et al. (1977), Eyre (1980), Hess and Wasser (1982), DeByle and Winokur (1985), Youngblood and Mauk (1985), DeVelice et al. (1986), Mueggler (1988), Powell (1988a), Knight (1994), Shiflet (1994), Reid et al. (1999), Neely et al. (2001), NCC (2002), Comer et al. (2002), Tuhy et al. (2002), Minnich (2007), and NatureServe Explorer (2009).

This widespread montane and subalpine ecological system is more common in the central and southern Rocky Mountains extending south to the Sacramento Mountains of New Mexico, west into the high plateaus of the Colorado Plateau and ranges of the Great Basin into the eastern Sierra Nevada, and north into the Canadian Rockies. Eastern extensions occur along the Rocky Mountains foothill front and in mountain "islands" in Montana (Big Snowy and Highwood mountains), and the Black Hills of South Dakota. Very small occurrences may be found in a few scattered locations of the Trans-Pecos of Texas. Elevations generally range from 1525 to 3050 m (5000-10,000 feet), but occurrences can be found at lower elevations in some regions. Climate is temperate with a relatively long growing season, typically cold winters and deep snow. Mean annual precipitation is greater than 38 cm (15 inches) and typically greater than 51 cm (20 inches), except in semi-arid environments where occurrences are restricted to mesic microsites such as seeps or large snow drifts. Distribution of this ecological system is primarily limited by adequate soil moisture required to meet its high evapotranspiration demand (Mueggler 1988). Secondarily, its range is limited by the length of the growing season or low temperatures (Mueggler 1988). Topography is variable; sites range from level to steep slopes. Aspect varies according to the limiting factors. Occurrences at high elevations are restricted by cold temperatures and are found on warmer southern aspects. At lower elevations occurrences are restricted by lack of moisture and are found on cooler north aspects and mesic microsites. The soils are typically deep and well-developed, with rock often absent from the soil. Soil texture ranges from sandy loam to clay loam. Parent materials are variable and may include sedimentary, metamorphic or igneous rocks, but it appears to grow best on limestone, basalt, and calcareous or neutral shales (Mueggler 1988). In Texas, this system occurs on high mountain slopes, valleys and ridges at higher elevations on Permian limestone (Guadalupe Mountains) and igneous substrates (Davis and Chisos mountains). The environmental description is based on several other references, including Henderson et al. (1977), Bartos (1979), Bartos and Mueggler (1979), Eyre (1980), Hess and Wasser (1982), DeByle and Winokur (1985), Johnston and Hendzel (1985), Youngblood and Mauk (1985), DeVelice et al. (1986), Mueggler (1988), Powell (1988a), Knight (1994), Shiflet (1994), Bartos and Campbell (1998), Reid et al. (1999), Neely et al. (2001), Comer et al. (2002), Tuhy et al. (2002), Minnich (2007), and NatureServe Explorer (2009).

Occurrences in this ecological system often originate, and are likely maintained by, stand-replacing disturbances such as crown fire, disease and windthrow, or clearcutting by man or beaver. The stems of these thin-barked, clonal trees are easily killed by surface fires, but they can quickly and vigorously resprout in densities of up to 30,000 stems per hectare (Knight 1994). As dbh increases beyond 15 cm, Populus tremuloides stems become increasingly resistant to fire mortality, and large stems may survive low-severity surface fire but usually show fire damage (Brown and DeByle 1987). The stems are relatively short-lived (100-150 years), and the stand will succeed to longer-lived conifer forest if undisturbed. Occurrences are favored by fire in the conifer zone (Mueggler 1988). With adequate disturbance a clone may live many centuries. Although Populus tremuloides produces abundant seeds, seedling survival is rare because the long moist conditions required to establish them are rare in the habitats that it occurs in. Superficial soil drying will kill seedlings (Knight 1994).

Although many diseases and insects attack Populus tremuloides (DeByle and Winokur 1985), under presettlement conditions, disease and insect mortality did not appear to have major effects; however, older aspen stands would be susceptible to outbreaks every 200 years on average (LANDFIRE 2007a, BpS:1210110). Sudden aspen decline (SAD) results in root mortality with subsequent effects on tree canopy and clone persistence. It appears to be triggered by severe drought (Worrall et al. 2010).

This system is also important habitat and browse for many species of wildlife, including various birds, beaver, snowshoe hare and large ungulates such as deer, elk and moose (DeByle and Winokur 1985). Concentrated use by elk can significantly impact stands (DeByle and Winokur 1985).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has three classes in total (LANDFIRE 2007a, BpS 2810110). These are summarized as:

A) Early Development 1 All Structures (5% of type in this stage): Aspen suckers less than 6 feet tall and abundant. Grasses and forbs resprout vigorously with high cover. Often densely vegetated.

B) Mid Development 1 Closed (pole-sized tree-dominated - 35% of type in this stage): Tree cover is 21-100%. Aspen over 6 feet tall dominate. Canopy cover highly variable, but usually dense. Understory also usually dense.

C) Late Development 1 Closed (tree-dominated - 60% of type in this stage): Tree cover is 21-100%. Aspen trees 9+ inches dbh. Canopy cover is highly variable, but usually dense. Understory dense. Lots of dead and downed material.

Fire, insects and disease. In absence of disturbance, may stay aspen. Fire will generally come from adjacent systems. Surface fire would generally affect the margins of stands as a result of fire on adjacent vegetation types. Mixed fire may occur, but is undocumented (LANDFIRE 2007a, BpS 2810110).

In the western U.S., Populus tremuloides forests have been utilized primarily for livestock grazing and to a lesser extent harvested for wood products. Stands typically have lush understory because tree canopy allows significant light to pass through, and sites tend to be relatively mesic (DeByle and Winokur 1985, Howard 1996). Heavy grazing by livestock can deplete or convert an understory dominated by shrubs and forbs to an understory dominated by grazing-tolerant grasses. Degraded stands were often seeded to grazing-tolerant introduced forage species such as Bromus inermis, Dactylis glomerata, Phleum pratense, and Poa pratensis (DeByle and Winokur 1985). Excessive browsing by livestock or wildlife can also significantly impact regeneration by suckers (DeByle and Winokur 1985, Howard 1996).

Harvesting Populus tremuloides trees greatly stimulates regeneration by suckering. Stand structure is obviously affected depending on silviculture treatment (clearcut versus partial cut) and management objectives (DeByle and Winokur 1985). Prescribed burning can also regenerate stands (DeByle and Winokur 1985, Howard 1996). Introduced species can be brought in during logging operations and other management actions that disturbed soil.

Human development has impacted many locations throughout the ecoregion. High- and low-density urban and industrial developments also have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

This system is more common in the central and southern Rocky Mountains extending south to the Sacramento Mountains, however, it occurs in the montane and subalpine zones throughout much of the western U.S. and north into Canada, as well as west into California. Elevations generally range from 1525 to 3050 m (5000-10,000 feet), but occurrences can be found at lower elevations in some regions. Very small occurrences may be found in a few scattered locations of the Trans-Pecos of Texas.