Montana Field Guide

These forests are generally dominated by western hemlock (Tsuga heterophylla), western red cedar (Thuja plicata), and grand fir (Abies grandis). They are found in areas influenced by incursions of mild, wet, Pacific maritime air masses west of the Continental Divide in Montana. Occurrences are found on all slopes and aspects but grow best on sites with high soil moisture, such as toeslopes and bottomlands. At the periphery of its distribution, this system is confined to moist canyons and cooler, moister aspects. Generally, these are moist, non-flooded or upland forest sites that are not saturated yearlong. In northwestern Montana, western hemlock and western red cedar forests occur on bottomland and northerly exposures between 609-1,585 meters (2,000-5,200 feet) on sites with an average annual precipitation of 635 millimeters (25 inches). These forests are common in extreme northwestern Montana, and extend eastward to the Continental Divide in the Lake McDonald drainage of Glacier National Park. Isolated stands of western hemlock occur in the Swan Valley, but are found most commonly in the Libby and Thompson Falls vicinities, west to the Idaho border. Western red cedar occurs extensively in the Mission Mountain range south to Missoula, and on lower flanks of the Swan Range north of Lion Creek. It is confined to the riparian zone of major streams on the east face of the Bitterroot Mountain Range. Grand fir, being less moisture dependent, occurs in more southerly and easterly sites than western red cedar and western hemlock. This system is similar to Rocky Mountain Dry-Mesic Mixed Montane Conifer Forest, which can be described as a seral phase of this system on appropriate sites west of the Continental Divide.

Forest and woodland; udic soils; Tsuga heterophylla and Thuja plicata.

These forests are common in extreme northwestern Montana, and extend eastward to the Continental Divide in the Lake McDonald drainage of Glacier National Park. Isolated stands of western hemlock (Tsuga heterophylla) occur in the Swan Valley, but are found most commonly in the Libby and ThompsonFalls vicinities west to the Idaho border. Western red cedar (Thuja plicata) occurs extensively in the Swan and Mission Mountain ranges, south to Missoula, and forms small riparian forests along major streams in the Bitterroot Mountain Range. This system is confined to the riparian zone of major streams on the east face of the Bitterroot Mountain Range as far south as Hamilton. Grand fir (Abies grandis), being less moisture dependent, extends to drier sites and occurs in more southerly and easterly sites than western red cedar and western hemlock.

These forests are generally dominated by western hemlock, western red cedar, and grand fir. Disturbed sites can and occasionally do return directly to dominance by the climax tree species, but other stands are often a mixture of the climax species with seral tree species such as Douglas-fir (Pseudotsuga menziesii), western white pine (Pinus monticola), lodgepole pine (Pinus contorta), western larch (Larix occidentalis) and paper birch (Betula papyrifera). Engelmann spruce (Picea engelmannii) and subalpinefir (Abies lasiocarpa) may be present on the coldest sites,and ponderosa pine (Pinus ponderosa) may be present on the warmest and driest sites. In Glacier National Park and the Selway-Bitterroot Wilderness in Montana, western red cedar is dominant in wet ravines, well-drained flats and poorly drained depressions. Both western red cedar and western hemlock are shade-tolerant conifers that occur in similar mesic environments; however, western red cedar extends locally into slightly drier sites and occurs further south and east in Montana. Shade-intolerant grand fir is seral in western hemlock or western red cedar dominated forests and has lower moisture requirements than its associates.

Common shrubs include mountain boxwood (Paxistima myrsinites), thinleaf alder (Alnus incana), Rocky Mountain maple (Acer glabrum), birch leaf spiraea (Spiraea betulifolia), common snowberry (Symphoricarpos albus), bunchberry dogwood (Cornus canadensis), thimbleberrry (Rubus parviflorus), rusty leaf menziesia (Menziesia ferruginea), and mountain huckleberry (Vaccinium membranaceum). Pacific yew (Taxus brevifolia) can occur in a tree form in the understory on some occurrences in westernmost Montana and as a prevalent shrub in occurrences in the Swan Valley.

Composition of the herbaceous layer reflects local climate and degree of canopy closure; it is typically highly diverse in all but closed-canopy conditions. Queen’s cup beadlily (Clintonia uniflora), western foamflower (Tiarella trifoliata), pioneer violet (Viola glabella), Canadian white violet (Viola canadensis), dark woods violet (Viola orbiculata) and beargrass (Xerophyllum tenax) are most the most common forbs in these forests. Other forbs include baneberry (Actaea rubra), pathfinder (Adenocaulon bicolor), false sarsaparilla (Aralia nudicaulis), lanceleafarnica (Arnica latifolia), fragrant bedstraw (Galium triflorum), rattlesnake plantain orchid (Goodyera oblongifolia), twinflower (Linnaea borealis), liverleaf wintergreen (Pyrola asarifolia) and western trillium (Trillium ovatum). In extreme northwestern Montana, wild ginger (Asarum caudatum) is a component on mesic sites with a mild temperature regime.

Ferns and fern allies also form an important component of the understoryand are indicative of the most mesic sites. Species include American ladyfern (Athryium filix-femina), western swordfern (Polystichum munitum), male fern (Dryopteris filis-mas), oak fern (Gymnocarpium dryopteris) and horsetails (Equisetum species). Bracken fern (Pteridium aquilinum) can occur in relatively high coverage (20% or greater) in mature stands, however it can form dense (up to 100%) cover in early seral stands, retarding forest regeneration. Graminoids may be absent or form a very minor component, and may include forest brome (Bromus vulgaris), fringed brome (Bromus ciliatus), Geyer’s sedge (Carex geyeri), pinegrass (Calamagrostis rubescens), blue wildrye (Elymus glaucus), and rough leaf ricegrass (Oryzopsis asperifolia).

Western red cedar and western hemlock are capable of remaining dominant within these forests due to their longevity. Because they are highly shade-tolerant, they can reproduce under their own canopy. In the absence of fire, both species can reproduce vegetatively (Pfister et al 1977; Minore 1990). Closed-canopy conditions generally favor vegetative reproduction, and this strategy is common in old-growth stands (Tesky, 1992b). Western red cedar can reproduce vegetatively by branch layering, rooting of fallen branches, and by branch development on fallen trees (Parker,1979). Trees of this species have reached 2,000 years in northern Idaho (Parker, 1986); trees in northwestern Montana have attained ages of at least 500 years.

Fire return intervals in this system average 80 years and are either moderate or high severity (U.S. Department of Agriculture, 2012). Typically, stand-replacement fire-return intervals are 150-500 years, with moderate-severity fire intervals ranging from 50 to 100 years (Arno, 1979). Western red cedar, western hemlock and grand fir are characterized by having thin bark, shallow root systems, low dense branching habits, and heavy lichen growth, making these species susceptible to fire damage (Fischer and Bradley, 1987). All but the largest trees are typically killed in fires of moderate intensity. With vigorous fire suppression, longer fire-return intervals are now common, and multi-layered stands of conifers provide fuel "ladders," making these forests more susceptible to high-intensity, stand-replacing fires.

A variety of biotic disturbance agents interact with this system. Defoliation of western hemlock by the western hemlock looper (Lambdina fiscellaria lugubrosa) causes mortality in mature and old growth stands, with previous looper outbreaks increasing risk of future defoliation (Tesky, 1992a; McCloskey et al., 2009). Warmer and drier growing season conditions may increase stand susceptibility to looper attack, and outbreaks are expected to be increasingly frequent and severe in the future (McCloskey et al., 2009). Western blackheaded budworm (Acleris gloverana) and hemlock sawfly (Neodiprion tsugae) also cause western hemlock defoliation, and may cause severe damage to even-aged juvenile stands (Nealis and Turnquist, 2010). The gall midge (Mayetiola thujae) infects western redcedar, causing damage to their seeds. Fungi additionally attack western redcedar in the form of root butt and trunk rots. In the eastern portion of western redcedar’s range in northwestern Montana, Poria asiatioa and Phellinus weiri are most damaging, and are increasingly common in old-growth stands. Roots injured by fire may be more vulnerable to fungal infection (Tesky, 1992b).

Wind throw can occur during unusually intense wind storms as the dominant species typically possess shallow, spreading root systems. Trees are particularly susceptible to uprooting in areas with shallow soils or a high water table (Tesky, 1992a). Individual trees that have been damaged by root diseases are especially prone to windthrow. Frost damage also affects western hemlock in northwestern Montana (Tesky, 1992a). In the past, these forests have been priorities for timber production in this region. Today, many old-growth stands occur in protected areas.

In the absence of natural fire, periodic prescribed burns can be used to maintain this system. Fire is generally infrequent in this system due to moist conditions, however, intense summer drought may lead to severe burning as this system is highly productive with heavy fuel loading (Fischer and Bradley, 1987). Prescribed burning will therefore be most effective following summer drought when fuels are driest. Burning may be a valuable strategy to control dwarf mistletoe outbreaks (Tesky, 1992a), promote nutrient cycling and species diversity, and create mineral seedbeds that favor natural regeneration of the species in this system (Fischer and Bradley, 1987). However, fire may also have consequences when root damage occurs, leading to chronic stress, growth loss, and increased susceptibility to fungal infection (Tesky, 1992b).

When thinning is utilized for silvicultural or fire risk reduction purposes, increased risk of windthrow should be considered. Both dominant species in this system are vulnerable to blowdown, in part due to their shallow rooting systems. Maintaining windfirm borders when thinning is necessary to avoid widespread blowdown (Tesky, 1992a). Thinning may likewise increase susceptibility to insect defoliation, particularly for stands dominated by western hemlock, and vulnerability to insect attack should be considered prior to thinning (Nealis and Turnquist, 2010).

Post-fire restoration strategies will depend largely on the severity of the fire. Early successional stages may be dominated by fireweed (Chamerion angustifolium) and other forbs, graminoids and understory shrubs. If seed trees are present, both dominant species are good seed producers and are capable of regenerating well following fire. However, of all conifers in the northern Rocky Mountains, western red cedar and western hemlock seedlings have the slowest growth rates and are highly susceptible to drought. Both species are capable of regenerating well on bare mineral soils if adequate moisture is present during the first two years of growth. Drought and high soil temperatures damage seedlings growing in full sunlight, with western red cedar being particularly sensitive (Tesky, 1992a; Tesky, 1992b).

Intense fires that occur during summer months cause considerable damage to native perennial grasses, forbs and shrubs, and may completely destroy existing seed banks, especially on steep facing slopes. In some cases, severely burned sites will require replanting with seedlings. In the Northern Rocky Mountain region, containerized nursery stock is outplanted in spring. Generally, larger container volume of nursery stock results in higher outplanting success than bareroot nursery stock, especially if spring and early summer precipitation patterns are unpredictable, and exposed mineral soil temperatures are high during the first year of establishment. Generally, 6-8 cubic inch container stock types are used on milder sites with good site preparation, and 10, 15 or 20 cubic inch container stock is used on the hotter, drier aspects or sites. Conifer stocking rates must be developed on a site-by-site basis to meet management objectives.