Montana Field Guide

This system includes all subalpine and treeline forest associations of the Montana Rocky Mountains and island ranges. It is characteristically a high-elevation mosaic of stunted tree clumps, open woodlands, and herb- or dwarf-shrub-dominated openings, occurring above closed forest ecosystems and below alpine communities. It includes open areas with stands of whitebark pine (Pinus albicaulis) occurring most commonly on south-, east-, and west-facing aspects, or less commonly, alpine larch (Larix lyallii) on north-facing aspects and in basins with shallow rocky soils. Subalpine fir (Abies lasiocarpa) is the co-dominant in these systems and is often the most prevalent tree species. Engelmann spruce (Picea engelmannii) is usually associated with subalpine fir and occurs as either a climax co-dominant or as a persistent, long-lived seral species in most upper elevation subalpine fir habitat types. Whitebark pine may initiate tree islands at treeline sites, facilitating the establishment and growth of subalpine fir and Engelmann spruce which are less tolerant of harsh climatic conditions (Callaway, 1998; Tomback et al., 2014). Long-term absence of disturbance may enable these more shade tolerant species to succeed whitebark pine and alpine larch at these sites (Arno and Hoff, 1989). Elevations range from as low as 1,981 meters (6,500 feet) in northwestern Montana to 2,682 meters (8,800 feet) in southwestern Montana. At or above treeline, subalpine fir, spruce, and whitebark pine will often exhibit krummholz form and reproduce by layering as a result of the harsh climate and considerable snow accumulation typical of these sites.

The climate is typically very cold in winter and dry in summer. Landforms include ridgetops, mountain slopes, glacial trough walls and moraines, talus slopes, landslides and rockslides, and cirque headwalls and basins. Snow accumulation is high in basins, but ridgetops have little snow accumulation because of high winds and sublimation. In this harsh, often wind-swept environment, trees are usually stunted and flagged from damage associated with wind, blowing snow and ice crystals, especially at the upper elevations. Tree species that occur in the otherwise sparsely vegetated communities of this system are particularly important for wildlife and watershed hydrology as they provide important food sources and slow spring snowmelt, thereby increasing the duration of summer runoff and streamflow (Arno and Hoff, 1989). Fire suppression, disease, insects and climate change are changing the structure, distribution and composition of this system.

Whitebark pine-subalpine fir forests and parklands occur throughout the Montana Rocky Mountains and east into the mountain island ranges. This is the most common forest alliance in the drier mountain ranges east of the Continental Divide. It is especially well represented in the Yellowstone Basin and surrounding mountain ranges. The distribution pattern of alpine larch (Larix lyalli) is patchy; the best stands occur near the treeline in the Anaconda-Pintlar, Bitterroot, Cabinet, Whitefish, Swan, southern Mission, Sapphire, and Flint Creek ranges, and in scattered locations in Glacier National Park and at the top of the headwaters of the Teton and Sun River drainages (Arno and Habeck, 1972). Mountain hemlock (Tsuga mertensiana)/subalpine fir is a minor subalpine forest type present in some high mountain cirques along the Montana-Idaho Divide from Lolo to the Cabinet Gorge. Mountain hemlock stands are much more prevalent immediately to the west in Idaho; however, they also occur in small isolated stands in the Whitefish, Mission, and Bitterroot ranges. They are absent from Glacier National Park.

These forests or patches often originate when Engelmann spruce, alpine larch, or whitebark pine colonize a sheltered site. Alpine larch/subalpine fir communities are prevalent on cool, north- and east-facing exposures west of the Continental Divide. Whitebark pine/subalpine fir communities occur on adjacent, warmer exposures and aspects. Subalpine fir colonizes in the shelter of these speciesand may form a dense canopy by branch layering. This species is capable of remaining dominant within these subalpine and treeline forests due to its longevity and ability to regenerate vegetatively. In the absence of disturbance, it continues to regenerate under shaded conditions. Seed crops are erratic at the lower elevational limit of this system and are virtually absent at treeline. The most common subalpine forest association in Montana is whitebark pine-subalpine fir.

The understories of these forests are usually sparse, but moister sites support mats of ericaceous plants, such as tall huckleberry (Vaccinium membranaceum), dwarf bilberry (Vaccinium myrtillus), or most often, grouse whortleberry (Vaccinium scoparium). Mountain heath (Phyllodoce species) and white mountain heather (Cassiope mertensiana) are commonon sites with more organic matter accumulation. A few taller shrubs such as alpine currant (Ribes montigenum), short fruited willow (Salix brachycarpa), and planeleaf willow (Salix planifolia) may also be present.The herbaceous layer is sparse under dense shrub or tree canopies, but may be dense where the shrub canopy is open or absent. Purple mountain hairgrass (Vahlodea atropurpurea), Hitchcock’s woodrush (Luzula glabrata var. hitchcockii), alpine bluegrass (Poa alpina), Sandberg’s bluegrass (Poa secunda), alpine timothy (Phleum alpinum), pinegrass (Calamagrostis rubescens), Parry’s rush (Juncus parryi) and sedges (Carex species) are the most common graminoids. A wide diversity of forbs are present in open meadows among or adjacent to these forests, typically including species such as arnica (Arnica species), subalpine wandering daisy (Erigeron peregrinus), arrowleaf groundsel (Senecio triangularis), aster (Symphyotrichum species), sibbaldia (Sibbaldia procumbens), glacier lily (Erythronium grandiflorum), rhexi-leaf paintbrush (Castilleja rhexifolia), western windflower (Anemone occidentalis), alpine St. John’s wort (Hypericum formosum), diverse leaf cinquefoil (Potentilla diversifolia), and penstemon (Penstemon species).

Alpine larch stands generally occur at or near upper treeline in north-facing cirques or on slopes where snowfields persist until June or July (Arno and Habeck, 1972). Typical stands are often isolated pockets of open, parklike groves. Alpine larch is considered a pioneer species in these high, north-facing aspects on rocky sites with little soil development, and due to its longevity (up to 1,000 years), is persistent on these sites. Typically, undergrowth in alpine larch stands can be limited due to high rock cover and limited soil development, but will often includes pink mountain heath (Phyllodoce empetriformis), Hitchcock’s woodrush and subalpine fir.

Major disturbances in this system include fire, avalanches, and biotic vectors. Fires in this system have average return intervals of 170 years and are either mixed severity or stand replacing (U.S. Department of Agriculture, 2012). In general, stand-replacing fires occur less frequently, at least where open woodlands limit fire severity and spread (Arno, 1980). In stands where fire return intervals are longer or where fires have been suppressed, shade-tolerant subalpine fir becomes increasingly dominant. Tree species that occur in this system vary in their susceptibility to fire. Subalpine fir has thin bark and experiences high mortality from low-intensity burns (Uchytil, 1991), whereas whitebark pine has moderately thick bark, and mature trees can withstand low severity fires (Fryer, 2002). Lightning damage to individual trees is common, but sparse canopies and rocky terrain historically limited the spread of fire. More recently, stand-replacing fires caused by lightning strikes are becoming more common in part because woodlands have become increasingly dense due to fire suppression. Post-fire regeneration is dominated by shade-intolerant whitebark pine and alpine larch. Clark’s nutcrackers (Nucifraga Columbiana) are the primary disperser of whitebark pine seeds post-fire (Tomback, 2005). Near treeline, establishment of subalpine fir may take decades or longer to establish after fire in part due to the harsh climate characteristic of this system. Often, stand-initiating whitebark pine facilitate the establishment of later-successional species like subalpine fir and Engelmann spruce by ameliorating the harsh climatic conditions otherwise found at treeline sites (Callaway, 1998; Tomback et al., 2014).

Insects and disease are major forces of disturbance within this system. Whitebark pine is affected by white pine blister rust and mountain pine beetle (Dendroctonus ponderosae) and is experiencing marked decline. Infection rates of white pine blister rust, caused by the non-native pathogen Cronartium ribicola, have increased in recent decades. Blister rust infections are more likely to occur in microsites (Smith-McKenna et al., 2013) or years with relatively cool and humid conditions (Sturrock et al., 2011). Mortality due to blister rust is especially high in northwestern Montana, where the moister Pacific maritime climate at high elevations is more conducive to infection than the drier air in the southern mountain ranges (Smith et al., 2012). Blister rust has a tendency to kill cone producing branches in whitebark pine crowns. The resulting reduction in cone production strongly limits whitebark pine reproduction potential (McCaughey and Tomback, 2001). Mountain pine beetles, which historically occurred at endemic levels in lower subalpine forests, are causing an increased incidence of outbreak in treeline communities, causing mortality of mature whitebark pine. Additionally, beetles preferentially select trees with weakened defenses, such as those affected by blister rust or severe drought (Bockino and Tinker, 2012). Subalpine fir is becoming more prevalent in these forests due to high mortality caused by blister rust and mountain pine beetles.

Throughout Montana, both subalpine fir and spruce are affected by western spruce bud worm (Choristoneura occidentalis) attacks, although outbreaks are less common in high-elevation stands (Uchytil, 1991). Large stands of subalpine forests, however, can be killed following several years of drought or unusually mild winters. Subalpine fir in the northern Rocky Mountains is also susceptible to the fungus (Heterobasidion annosum) which causes root rot and may increase vulnerability to other biotic pathogens (Uchytil, 1991). Unlike other species in this system, alpine larch is relatively unaffected by insects or disease (Habeck, 1991).

Climate can also influence ecological processes in this system. Warming climate patterns may result in increased seedling recruitment and tree density at the upper elevation limit of this ecological system (Klasner and Fagre, 2002). Climate may have additional effects on biotic vectors. Mountain pine beetles, which were historically rare in this system due to cold temperatures, have become increasingly common as a result of changing temperature regimes (Bockino and Tinker, 2012). In addition, avalanches, which are common in precipitous mountain areas that receive heavy snowfall, cause stand-initiating disturbances that remove broad swaths of subalpine forest.