Montana Field Guide

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 of 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. Fire return intervals vary from approximately 30-165 years and range from mixed severity to stand replacing (U.S. Department of Agriculture, 2012). The diversity of fire regimes is attributable to the variability of stand structure, topography, and climate found within this system (Shinneman et al., 2013). 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 stands (Shinneman et al., 2013). Boles 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 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 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 et al., 2014), making stands with low levels of regeneration particularly susceptible to drought-related mortality.