Montana Field Guide

These mesic forests are dominated by Engelmann Spruce and Subalpine Fir, either in mixed stands or individually. Sites in northwestern Montana dominated or co-dominated by Mountain Hemlock (Tsuga mertensiana) are also included here. These forests are similar to G218 Dry Spruce-Fir Forest but occur on more mesic and generally cooler sites such as northerly-facing slopes, areas with cold-air drainage or ponding, or where snowpacks linger late into the summer. They are most abundant in the Flathead, Lolo, Bitterroot and Kootenai National Forests. Elevations range from 3,000-6,500 feet west of the Continental Divide, and 5,000-9,000 feet east of the Continental Divide. This group is more common and extensive west of the Continental Divide but occurs east of the divide in smaller patches in suitable sites such as cool, northerly slopes, near drainage bottoms or areas with cold air drainage. Other conifers are often present within these stands and may include Lodgepole Pine, Whitebark Pine, or Grand Fir, with Lodgepole Pine more typical of earlier successional stands, Grand Fir at mesic, lower elevations west of the divide and Whitebark Pine at higher elevations. The forest understory may be composed of low to tall shrubs, graminoids and/or forbs. Composition varies widely by geography, site characteristics and Association. The herbaceous understory contains mesic forbs, graminoids, and ferns and fern allies on the wettest sites. Moss cover is high in some communities Stand-replacing fires are less common in mesic spruce-fir forests than in dry spruce-fir forests.

This Group is equivalent to the Rocky Mountain Subalpine Mesic Spruce-Fir Forest and Woodland Ecological System.

Subalpine Fir (Abies lasiocarpa), Engelmann Spruce (Picea engelmannii), Mountain Hemlock (Tsuga mertensiana) Dominated; Mesic Conifer Forest and Woodlands; Upper Montane and Subalpine Zones; Moderate to Dense Canopies; Long Fire Return Intervals.

The Group occurs both east and west of the Continental Divide in the upper montane and subalpine zones. It is most common in northwest Montana extending east to Glacier NP and the Bob Marshall Wilderness Complex and south to the Bitterroot Mountains but also occurs in sw MT mountains, east to the Beartooth Mountains and the Big Snowy Mountains.

In MT, G218 occurs within these Level III Ecoregions: 15 (Northern Rockies), 16 (Idaho Batholith), 17 (Middle Rockies) and 41 (Canadian Rockies).

In Montana, G218 occurs within these Major Land Resource Areas: 43A-Northern Rocky Mountains, 43B - Central Rocky Mountains, and 44A - Northern Rocky Mountain Valleys.

Large Patch-Matrix

This group occurs at relatively low to high elevations west of the Continental Divide and mid to high elevations east of the Divide. Elevations range from 3,000-6,500 feet west of the Continental Divide, and 5,000-9,000 feet east of the Continental Divide. Cold winter temperatures with heavy snowpack are common at these sites. Stands are often found in locations with cold-air drainage or ponding, or where snowpacks linger late into the summer, such as north-facing slopes and high-elevation ravines. They can extend lower in elevation below the subalpine zone in places where cold-air ponding occurs, especially on north and east aspects. Soils are derived from a variety of parent materials. They are usually rocky or gravelly with good aeration and drainage and are usually acidic.

These forests are dominated by Engelmann Spruce and Subalpine Fir, either in mixed stands or individually. Sites in western Montana dominated or co-dominated by Mountain Hemlock (Tsuga mertensiana) are also included here. Other conifers are often present and may include Lodgepole Pine, Whitebark Pine, or Grand Fir, with Lodgepole Pine more typical of earlier successional stands, Grand Fir at mesic, lower elevations west of the divide and Whitebark Pine at higher elevations. This group encompasses the wetter end of the Spruce-fir forest type and site characteristics and the understory vegetation should reflect that in relation to G219 Dry Spruce-Fir.

The forest understory may be composed of low to tall shrubs, graminoids and/or forbs. Composition varies widely by geography, site characteristics and Association. Common shrubs in these mesic Spruce-fir forests include False Huckleberry (Menziesia ferruginea), Common Huckleberry (Vaccinium membranaceum), Grouse Whortleberry (Vaccinium scoparium), Dwarf Huckleberry (Vaccinium cespitosum), Bristly Black Currant (Ribes lacustre), Rocky Mountain Maple (Acer glabrum), Spiraea (Spiraea betulifolia), Thimbleberry (Rubus parviflorus) and Snowberry (Symphoricarpos albus). Subshrubs such as Twinflower (Linnaea borealis) and Oregon-Grap (Berberis repens) are common in some habitats. Forb diversity can be high on some sites and common ones include Arnica (Arnica cordifolia, Arnica latifolia), Beargrass (Xerophyllum tenax), Fragrant Bedstraw (Galium triflorum), One-Sided Wintergreen (Pyrola secunda), Queen Cup Beadlily (Clintonia uniflora) among many others. Graminoids are generally few with Bluejoint Reedgrass (Calamagrostis canadensis) and Smooth Woodrush (Luzula hitchcockii) being two of the most consistently present within certain Associations. Mosses may be common in some understories. Non-native species are typically absent or uncommon in these communities.

Engelmann spruce is more tolerant of extreme environmental conditions than subalpine fir and is usually more dominant in the drier and wettest occurrences within this system. Mountain hemlock occurs as small to large patches within the matrix of mesic spruce-fir forest but only in the more maritime-influenced climate of northwestern Montana.

The understory in Spruce-fir forests of northwestern Montana often supports diverse stands of ericaceous plants. Beyond those mentioned previously, Labrador Tea (Ledum glandulosum), White-flowered Rhododendron (Rhododendron albiflorum) and Pink Mountain-heath (Phyllodoce empetriformis) may be abundant in some understories. In the wettest subalpine fir forests in northwestern Montana, Devil's-club (Oplopanax horridus) is a major shrub associate. These sites are usually restricted to ravine bottoms near streams and seeps where the water table remains near the surface all year.

In Montana, this group is represented by 34 Associations grouped into 5 Alliances within the National Vegetation Classification. These appear to represent the diversity of vegetation types within this group in the state.

Major disturbances include occasional blowdown, insect outbreaks (30-50 years), and fire. Fire return intervals are longer in these habitats than in the Dry Spruce-Fir Forest and range between 170 to more than 300 years (U.S. Department of Agriculture 2012). The majority of fires are stand-replacing, although mixed severity fires also occur (U.S. Department of Agriculture 2012). The cool, mesic environment favors greater fuel loading and infrequent fires, however when dry conditions persist over long periods, these characteristics promote intense, stand-replacing fires (Reinhardt and Holsinger 2010). 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). Mountain hemlock is also highly susceptible to fire due to its low-hanging branches, although its bark is relatively thick and may provide some protection to low-intensity burning (Tesky 1992). Following fire, spruce is more successful at establishing on mineral soils while subalpine fir is comparatively better at establishing in the shade and on organic substrates. Both mountain hemlock and spruce are generally slow to establish after fire (Alexander and Shepperd 1990; Tesky 1992). In Montana, subalpine fir will often form pure stands with lesser dominance by Engelmann spruce, although Engelmann spruce often outlives subalpine fir in these habitats (Uchytil 1991). Over time, in the absence of fire or in the presence of spruce budworm attacks, subalpine fir will largely replace spruce within most habitats, with the exception of the wettest sites.

Insects and disease influence species composition and successional direction of this system. Throughout Montana, subalpine fir and spruce are affected by western spruce budworm (Choristoneura occidentalis) attacks. Spruce and subalpine fir in this system may be comparatively less vulnerable to spruce budworm outbreak than those in the Dry Spruce-Fir Forest due to higher energy reserves associated with trees on more mesic sites that increase defense potential against spruce budworm attack (Dupont et al. 1991). 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, also contribute to increases in local beetle populations (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). 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). Root decay is especially problematic for mature subalpine fir in the Northern Rockies (Uchytil 1991).

In the absence of natural fire, periodic prescribed burns can be used to maintain this system, however, fire return intervals are generally quite long where site conditions are more mesic. Maintaining historic fire return intervals in this system 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 mature individuals that are vulnerable to attack. Moderate to severe fires change the age structure of a stand thereby decreasing susceptibility to future 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 prevent beetle population growth. In general, increasing forest heterogeneity limits beetle spread and extent of outbreak (Jenkins et al. 2014).

The dominant species in this system 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 and mountain hemlock colonize both sites with 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, whereas at lower elevations, mineral seedbeds may be more conducive to seedling establishment (Uchytil 1991).

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 of 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 may be greater than in the Dry Spruce-Fir Forest since many sites occur on north aspects with cool air ponding where seedling establishment is favored and droughty conditions are less likely to occur (Alexander and Shepperd 1990).

K.A. Schulz 2013

S. Mincemoyer, L. Vance, T. Luna, M. Hart

12/4/2024