Montana Field Guide

This system occurs in north-central Montana in the Big Snowy Mountain range, at elevations of 2,012-2,195 meters (6,600-7,200 feet). Occurrences are typically on gentle to steep slopes on any aspect. Soils in this mountain range are derived from alluvium, colluvium, and residuum from calcareous parent materials. Most current occurrences represent a late-seral stage of aspen (Populus tremuloides) forest changing to a pure conifer forest. Nearly a hundred years of fire suppression and livestock grazing have converted much of the pure aspen occurrences to the present-day aspen-conifer forest and woodland ecological system, with conifers increasing in dominance. Conifers in this system include Douglas-fir (Pseudotsuga menziesii), subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii) and lodgepole pine (Pinus contorta). Common shrubs include serviceberry (Amelanchier alnifolia), creeping Oregon grape (Mahonia repens), chokecherry (Prunus virginiana), Woods’ rose (Rosa woodsii), birch-leaf spiraea (Spiraea betulifolia), and snowberry (Symphoricarpos species).

The tree canopy is composed of a mix of deciduous and coniferous species, co-dominated by aspen (Populus tremuloides) and conifers, including Douglas-fir (Pseudotsuga menziesii), subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta) and ponderosa pine (Pinus ponderosa). Common understory shrubs include serviceberry (Amelanchier alnifolia), creeping Oregon grape (Mahonia repens), chokecherry (Prunus virginiana), Woods’ rose (Rosa woodsii), birch-leaf spiraea (Spiraea betulifolia), and snowberry (Symphoricarpos species). Graminoid composition varies depending on available site moisture, but often includes mountain brome (Bromus carinatus), pinegrass (Calamagrostis rubescens), Geyer’s sedge (Carex geyeri), blue wild rye (Elymus glaucus), and needlegrasses (Achnatherum and Nassella species). Common forbs include yarrow (Achillea millefolium), heart-leafarnica (Arnica cordifolia), aspen daisy (Erigeron speciosus), northern bedstraw (Galium boreale), silver lupine (Lupinus argenteus), starry Solomon’s seal (Maianthemum stellatum), and meadow rue (Thalictrum species). Exotic species such as Kentucky bluegrass (Poa pratensis), timothy (Phleum pratense) and common dandelion (Taraxacum officinale) are frequentin areas impacted by grazing.

Quaking aspen is seral in this system, and in the absence of fire or other disturbance the system will succeed to conifer-dominated forest (Mueggler, 1988). Succession by conifers may occur as a result of a number of processes. Soils may become more acidic and less nutrient-rich as aspen decline in the absence of disturbance, favoring conifer establishment (Cryer and Murray, 1992). Aspen may also facilitate the establishment of conifers due to the shade and higher soil moisture at the base of aspen trees, fostering interspecific competition that favor conifer dominance (St. Clair et al., 2013). Conifer establishment and growth has been found to be greater in aspen and mixed-conifer stands than in conifer dominated stands due to greater nutrient, light, and water availability. Alternatively, aspen mortality tends to increase with increasing conifer abundance due to aspen’s shade-intolerance, and changes to soil chemistry (St. Clair et al., 2013). Light limitation resulting from conifer dominance additionally reduces defense compounds in aspen leaves increasing their susceptibility to herbivory (Wooley et al., 2008).

Fire return intervals in this system generally range from 30-50 years and are mostly either mixed severity or stand-replacing (U.S. Department of Agriculture, 2012). Young conifers are susceptible to fire, but older individuals can withstand low-intensity ground fires. While easily killed by fire, aspen regenerate quickly and will often dominate immediately after fires in this system (Howard, 1996). Shinneman et al., (2013) describe two types of seral mixed aspen-conifer forest types: Montane stands where fire is adequately frequent and severe to allow aspen to persist where conifer competitors are present, and mesic subalpine stands where infrequent, high severity fires promote a short period of aspen dominance before it is replaced by more shade-tolerant conifers. In part due to fire suppression practices since the early 1900s, conifers have become dominant in many seral aspen stands (Howard, 1996).

In Montana, aspen seed production is erratic and infrequent. Natural seedling establishment is rare due to limited years of viable seed dispersal, and the long, moist conditions required for initial germination and first-year establishment. Drought conditions limit natural regeneration, and also may accelerate succession as conifer species are generally more tolerant of drought. Thus, conifers are more likely to persist in this system as drought severity and frequency increase (Bell et al., 2014; St. Clair et al., 2013). Conifers may be additionally favored in stands where heavy use by livestock and wildlife limit aspen regrowth, as conifer species are generally more tolerant of browsing. Conversely, increasingly frequent fires will likely cause conifer mortality and promote aspen regeneration (St. Clair et al., 2013).

Historic fire suppression combined with excessive browsing of young aspen by ungulates is considered to be 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 during late fall months to maintain and enhance aspen regeneration and limit succession by conifers (Hardy and Arno, 1996). 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). Maintaining fire regimes in combination with the management of ungulate use can slow mortality related to succession by conifers, and increase aspen resilience to drought by limiting competition for water with adjacent conifers (St. Clair et al., 2013).