Montana Field Guide

Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) make up a substantial part of the montane and lower subalpine forests of the Montana Rocky Mountains and mountain island ranges of north-central and west-central Montana. Spruce is usually associated with fir and occurs as either a climax co-dominant or as a persistent, long-lived seral species in most upper elevation fir habitat types. Dry to mesic spruce-dominated forests range from 884-1,585 meters (2,900-5,200 feet) west of the Continental Divide, and 1585-2,073 meters (5,200-6,800 feet) east of the Continental Divide in the northern and central portions of the state. This system can be found at elevations up to 2,896 meters (9,500 feet) in southwestern Montana. Forests are found on gentle to very steep mountain slopes, high-elevation ridge tops and upper slopes, plateau-like surfaces, basins, alluvial terraces, well-drained benches, and inactive stream terraces. Tree canopy characteristics are relatively uniform. In northern Montana, Engelmann spruce hybridizes with its boreal counterpart, white spruce (Picea glauca). Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), and western larch (Larix occidentalis) (west of the Continental Divide) are seral but often present in these forests. The understory is comprised of a mixture of shrubs, forbs and graminoids tolerant of warmer and drier soil conditions than those found on the more mesic to wet spruce-fir system. The drier occurrences of this system are especially common on steep slopes at upper elevations throughout the eastern Rocky Mountains, whereas the more mesic occurrences form substantial cover west of the Continental Divide in the Flathead, Lolo, Bitteroot and Kootenai river drainages.

Forest and Woodland, ustic and acidic soils, long persistence, Picea engelmannii, Abies lasiocarpa

Tree canopy characteristics are relatively uniform, with Engelmann spruce and subalpine fir dominating, either mixed or alone. In northern Montana, Engelmann spruce hybridizes with its boreal counterpart, white spruce. Spruce is more tolerant of extreme environmental conditions than subalpine fir, and is usually more dominant in the drier and wettest habitats within this system. Douglas-fir, lodgepole pine, and western larch (west of the Continental Divide) are seral but often present in these forests. Mountain hemlock (Tsuga mertensiana) occurs as small to large patches within the matrix of this mesic spruce-fir system, but only in the coldest and wettest environments of northwestern Montana.

The understory of these forests often supports diverse stands of ericaceous shrubs, such as rusty leaf menziesia (Menziesia ferruginea), dwarf huckleberry (Vaccinium caespitosum), mountain huckleberry (Vaccinium membranaceum), dwarf bilberry (Vaccinium myrtillus) and mountain heath (Phyllodoce species). Grouse whortleberry (Vaccinium scoparium) is common on mesic sites. Cascade azalea (Rhododendron albiflorum) occurs in association with mountain hemlock and subalpine fir in some mesic occurrences in northwestern Montana.Other common shrubs include Rocky Mountain maple (Acer glabrum), serviceberry (Amelanchier alnifolia),Utah honeysuckle (Lonicera utahensis), ninebark (Physocarpus malvaceus),currant (Ribesspecies), thimbleberry (Rubus parviflorus), birch leaf spiraea (Spiraea betulifolia) and common snowberry (Symphoricarpos albus). On the driest sites in the Bighorn Mountains,big sagebrush (Artemisia tridentata) may be present.Smooth woodrush (Luzula glabrata var. hitchcockii)is the most common graminoid on mesic sites at higher elevations. Pinegrass (Calamagrostis rubescens), Geyer’s sedge (Carex geyeri), and Ross’ sedge (Carex rossi) are common on drier sites. Forb diversity varies depending on moisture conditions. Species includebaneberry (Actaea rubra), arnica (Arnica species),Columbia clematis (Clematis occidentalis), queen’s cup beadlily (Clintonia uniflora), bunchberry dogwood (Cornus canadensis), fragrant bedstraw (Galium triflorum), twinflower (Linnaea borealis), clasp-leaf twisted stalk (Streptopus amplexifolius), western meadow rue (Thalictrum occidentale) and beargrass (Xerophyllum tenax).

Major disturbances include occasional blowdown, insect outbreaks (30-50 years), and fire. Fire return intervals average approximately 130 years and are either mixed severity or stand replacing (U.S. Department of Agriculture, 2012). Fires in this system are generally more frequent and less intense than those in the Rocky Mountain Subalpine Mesic Spruce-Fir Forest and Woodland system. Both subalpine fir and Engelmann spruce are highly susceptible to fire, in part due to their shallow roots, thin bark, and dense stand growth habits (Alexander and Shepperd, 1990; Uchytil, 1991). Following fire, spruce is more successful at establishing on mineral soils. Subalpine fir, in contrast, is better at establishing in the shade and on organic substrates. Post-fire, subalpine fir establishment may be immediate if nearby seed sources exist and seral lodgepole pine is not present. In forests undisturbed by fire or subjected to spruce budworm attacks, subalpine fir assumes greater dominance. Over a period of 500 years, subalpine fir will largely replace spruce within most habitats of this system.

Insects and disease can play a major role in the successional direction of these forests. Throughout Montana, subalpine fir and spruce are affected by western spruce budworm (Choristoneura occidentalis) attacks. The spruce beetle (Dendroctonus rufipennis) also causes extensive damage to spruce in this system. Severe wind events that cause extensive blowdown are often followed by spruce beetle outbreaks, as downed trees provide an abundant food supply and are favored by the beetles (Lindemann and Barker, 2001). Other disturbances that cause an abundance of downed material, including landslides and avalanches, may also contribute to increases in beetle populations within a region (Jenkins et al., 2014). When outbreak conditions occur, or when downed material is unavailable, spruce beetles will also attack live trees, favoring large size classes and overmature individuals (Alexander and Shepperd, 1990). Beetle outbreaks have implications for watershed function, wildlife habitat, recreation, and stand species composition (Jenkins et al., 2014).

Historically, spruce beetle outbreaks have been related to drought conditions as drought weakens spruce resistance mechanisms to beetle attack (DeRose and Long, 2012; Jenkins et al., 2014). Drought has additionally been found to have lag effects on mortality for spruce, subalpine fir and lodgepole pine in this system, with drought having the most substantial and long-lasting effects on subalpine fir, and the smallest impact on lodgepole pine (Bigler et al., 2007). Large stands of these subalpine forests can be killed following several years of drought or unusually mild winters.

Spruce broom rust (Chrysomyxa arctostaphyli) also occurs in this system causing deformation, and increased vulnerability to windbreak (Alexander and Shepperd, 1990). Subalpine fir is additionally affected by western balsam bark beetle (Dryocoetes confuses), balsam wooly adelgid (Adelges piceae), and the fir engraver beetle (Scolytus ventralis). Root and wood rots also affect the dominant species in this system by weakening their defenses to insect attack and increasing vulnerability to windfall (Jenkins et al., 2014).

In the absence of natural fire, periodic prescribed burns can be used to maintain this system. Maintaining historic fire return intervals may decrease stand susceptibility to spruce beetle outbreak (Bebi et al, 2003; Kulakowski and Veblen, 2006). Old stands have a greater abundance of downed material and overmature individuals that are vulnerable to attack. Moderate to severe fires change the age structure of a stand thereby decreasing susceptibility to future beetle attack (Kulakowski and Veblen, 2006).

All species in this system are vulnerable to windthrow. Mechanical thinning for silvicultural or fire risk reduction purposes should therefore consider stand blowdown as a potential result of thinning treatment. Risk of windfall increases in stands with shallow soils and poor drainage, high degree of root and wood rot, and old, dense stand structure (Alexander and Shepperd, 1990). Alternatively, thinning may be useful to promote natural regeneration and reduce abundance of overmature individuals in a stand, altering stand age structure, and thereby decreasing susceptibility to future spruce beetle outbreaks. However, logging residue may contribute to beetle population increases, and downed woody material may therefore need to be removed from the site to reduce potential for future beetle outbreak (Jenkins et al., 2014).

Post-fire restoration strategies will be largely dependent on the severity of the fire. Because lightly burned areas recover quite quickly from fire, reseeding is usually not necessary if an intact, native understory was present before the fire. Early successional stages may be dominated by fireweed, arnica, aster, pearly everlasting (Anaphalis margaritacea), mountain hollyhock (Iliamina rivularis) and other forbs, and small amounts of forest graminoids. Both dominant species are good seed producers and are capable of regenerating well following fire. Spruce is capable of regenerating well on bare mineral soils if adequate moisture is present during the first two years of growth. Subalpine fir colonizes sites with both mineral soil and those with some organic matter. At the higher elevation occurrences of this system, seedling survival may be greater where duff seedbeds are present as they protect seedlings from harsh climatic conditions (Uchytil, 1991).

Large, prescribed, stand-replacement fires are not recommended in areas where spruce is in severe decline. Small-scale prescribed burning during late fall after several hard frosts can facilitate regeneration and increase stand heterogeneity in terms of age structure and species composition, thereby decreasing susceptibility to insect outbreaks (Jenkins et al., 2014). In some cases, nursery stock may be used to expedite regeneration on severely burned areas if seed rain from adjacent stands is not likely to occur, or if bare mineral soil following severe insect outbreak is limited (Jenkins et al., 2014). When supplemental planting is required, cold, moist stratification is necessary for germination of subalpine fir and spruce (Uchytil, 1991). Success of seedling establishment will be greatest when spring and early summer conditions are relatively moist and late summer drought is not intense (Alexander and Shepperd, 1990).