Montana Field Guide

This widespread group occurs on both sides of the Continental Divide but is more common east of the Divide. It occurs in the montane and subalpine zones throughout much of Montana. Elevations range from approx 3,000ft in northwest Montana to 8,000ft in southwest Montana with stands mostly between 4,500-7,000ft. Stands are dominated by Quaking Aspen (Populus tremuloides) or in limited areas of northwest Montana by Paper Birch (Betula papyrifera). Stands of aspen in Montana are generally relatively small in extent but occasionally form large patches. Sites within this group are upland sites and do not include riparian or wetlands sites that are dominated by aspen or birch, nor aspen communities on the Great Plains. Distribution of this group is primarily limited to areas with relatively high soil moisture and occurs on gentle to moderate slopes, in swales, or on level sites. At lower elevations, occurrences are found on cooler, north aspects and mesic sites. Stands can be stable and long-lived or seral to Douglas-fir (Pseudotsuga menziesii), Subalpine Fir (Abies lasiocarpa) or Engelmann Spruce (Picea engelmannii, Picea x albertiana) dominated forests (Habeck 1967, Pfister et al, 1977). In this group, conifers that may be present, forming up to 25% of the tree canopy but are never codominant. 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.

Aspen stands within riparian or wetland sites are part of either G505 Montane Forested Wetland or G506 Montane - Subalpine Riparian Forest. Aspen stands in the Great Plains Region not including stands within the island ranges are often part of G328 Great Plains Aspen Forest & Woodland.

This Group encompasses the Aspen Forest and Woodland Ecological System and a portion of the Aspen and Mixed Conifer Forest Ecological System.

Quaking Aspen (Populus tremuloides) or Paper Birch (Betula papyrifera) in limited areas of northwest Montana; Montane and Foothills Zones; Deciduous Forest and Woodlands.

In Montana, this Group is found throughout the mountainous portion of the state on both sides of the Continental Divide, including in the island ranges of central Montana. It is most common in areas such as along and within the mountains of the Rocky Mountain Front and in the island ranges such as the Highwoods and the Little Belt Mountains. It is extensive along portions of the Rocky Mountain Front where it grades into G328 Great Plains Aspen Forest & Woodland.

In MT, G222 occurs within these Level III Ecoregions: 15 (Northern Rockies), 16 (Idaho Batholith), 17 (Middle Rockies) and 41 (Canadian Rockies).

In Montana, G222 occurs within these Major Land Resource Areas: 43A-Northern Rocky Mountains; 43B-Central Rocky Mountains and 46 - Northern and Central Rocky Mountain Foothills.

Large Patch

Aspen stands in this group occur in the montane and subalpine zones. Elevations range from approx 3,000ft in northwest Montana to 8,000ft in southwest Montana with stands mostly between 4,500-7,000ft. Climate is temperate with a relatively long growing season, typically cold winters and deep snow. Mean annual precipitation is greater than 15 inches and typically greater than 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. In Glacier County, aspens stands differ in height growth, which is controlled by recurring Chinook winds.

This group includes Quaking 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 Mountain Region to the south. Clones can be stable and long-lived or seral to Douglas-fir (Pseudotsuga menziesii), Subalpine Fir (Abies lasiocarpa) or Engelmann Spruce (Picea engelmannii, Picea x albertiana) dominated forests (Habeck 1967, Pfister et al, 1977). Stands of Paper Birch (Betula papyrifera) in upland setting also occur within this group. These are restricted primarily to northwest Montana and are very minor in extent and abundance.

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), Sreeping Oregon-grape (Berberis 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), Angelica arguta, Engelmann Aster (Eucephalus engelmannii), Larkspur (Delphinium species), Fleabane (Erigeron speciosus), Richardson’s Geranium (Geranium richardsonii), Cow-parsnip (Heracleum maximum), Western Sweet-cicely (Osmorhiza occidentalis), Western meadowrue (Thalictrum occidentale), Stinging Nettle (Urtica dioica) and Small-flower Valerian (Valeriana occidentalis). Bracken fern (Pteridium aquilinum) is present in some stands. Exotic grasses such as Kentucky Bluegrass (Poa pratensis), Common Timothy (Phleum pratense) and Smooth Brome (Bromus inermis) are often common in occurrences disturbed by grazing.

In Montana, this group is currently represented by 6 Alliances and 20 Associations within the National Vegetation Classification. These likely represent the diversity of types found within the state.

Stands often originate with, and are likely maintained by, stand-replacing disturbances such as crown fire, disease and windthrow, or logging by humans or beaver. Fire return intervals vary from approximately 30-165 years and range from mixed severity to stand replacing (U.S. Department of Agriculture 2012). In general, aspen stands are relatively resistant to fire due to high fuel moisture content, however, favorable fire weather conditions can result in the spread of fire within aspen stands (Shinneman etal. 2013). Stems are killed by ground fires but can quickly and vigorously resprout by root suckers in high densities (Howard 1996). Stems are relatively short-lived (70-120 years), and the system will generally succeed to longer-lived, shade-tolerant conifer forest if undisturbed, although seemingly stable fire-independent stands appear to also exist where edaphic and topographic conditions favor the dominance of aspen over conifers (Shinneman et al. 2013). Occurrences are often 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, however, re-sprouting was greater in stands that had burned (Romme et al. 1995), and seedlings were found only in burned forests (Turner et al., 2003). These seedling and sapling stands are subjected to heavy elk browsing and may not reach full maturity (Hessl and Graumlich 2002).

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 outer edge, 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, stem 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 aspen to regenerate by suckers can vary widely among clones (Schier et al, 1985) and suckering response may increase with fire severity (Keyser et al., 2005).

In recent years, many aspen stands have exhibited mortality from biotic vectors. Some examples include Cytospora canker which is not highly destructive in healthy trees, and bronze poplar borer (Agrilus liragus) (Marchetti et al., 2011). These pathogens infect and proliferate in aspen stands already stressed by drought, insects, past fires, wind damage and heavy livestock and wildlife use (Shinneman et al. 2013; Marchetti et al. 2011). In addition to the tendency of drought to increase susceptibility to attack by biotic vectors, drought-related damage may also increase aspen vulnerability to future droughts (Anderegg et al., 2013). Large, older trees are generally more sensitive to water stress (Bell etal. 2014), making stands with low levels of regeneration particularly susceptible to drought-related mortality.

Historic fire suppression combined with excessive browsing of young aspen by ungulates is a primary cause of aspen decline in the Northern Rockies (Shinneman et al. 2013). In the absence of natural fire, periodic prescribed burns can be implemented to maintain and enhance regeneration in declining stands with low productivity. The best conditions for burning generally occur in the early spring or late fall when surface fuels are dry and frozen when aspen stands are more vulnerable (Howard, 1996). Brown and Simmerman (1986) describe methods for determining appropriate timing of prescribed burning in aspen stands. Aspen will typically reproduce prolifically post-burning, however in areas where livestock or wildlife browsing is severe, ungulate management to reduce damage to new growth may be necessary (Shinneman et al. 2013; Durham and Marlow 2010). A study in southwestern Montana found that aspen regeneration increased after prescribed burning, and that ungulate browsing did not limit regeneration due to the low elk density and management of cattle grazing in this region (Durham and Marlow 2010).

Restoration strategies will depend on fire severity, grazing or other land impacts. Because burned areas regenerate vegetatively following fire, additional restoration practices are generally not required. When supplemental planting is necessitated, seedlings preferable to vegetative cuttings, and seed germination and seedling survival are highest on well-drained, moist mineral seedbeds (Howard 1996). 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 re-sprouting of dominant shrubs. Aspen will resprout vigorously following fires of low to moderate severity. Sprouting will also occur after higher intensity fires from root suckers that are deeper in the soil profile. In areas with high elk densities or heavy livestock use, aspen regeneration may require protection from browsing until crowns can grow high enough to avoid excessive browsing damage (6-8 years) (Durham and Marlow 2010; Howard 1996). Restoration of aspen stands may be additionally valuable as the species is unique in its ability to stabilize soil and protect watershed-wide water quality (Howard 1996).

M.E. Hall; F.H. Eyre (1980)

S. Mincemoyer

11/25/2024