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. 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. Elevation range from as low as 1,981 meters (6,500 feet) in northwestern Montana to 2,682 meters (8,800 feet) in southwestern Montana. 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. Fire suppression, disease, insects and climate change are changing the structure, distribution and composition of these systems.
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. Historically, stand-replacing fires occurred infrequently in this system, at least where open woodlands limited fire severity and spread (Arno, 1980). These tree species are very susceptible to fire.Whitebark pine and subalpine firhave some tolerance to low and moderate severity fire if old individual trees have developed thick bark.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, especially in areas of steep terrain. In precipitous mountain areas that receive heavy snowfall, avalanches are common and can remove broad swaths of subalpine forest.
Insects and disease can play a major role in the successional direction of these forests. Whitebark pine is affected by white pine blister rust and mountain pine beetle and is experiencing marked decline. Subalpine firis becoming more prevalent in these forests due to high blister rust mortality. Blister rust mortality 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. Throughout Montana, both subalpine fir and spruce are affected by spruce bud worm attacks, and large stands of these subalpine forests can be killed following several years of drought or unusually mild winters. Warming climate patterns can result in increases of tree seedling recruitment and density at the upper elevation limit of this ecological system (Klasner and Fagre, 2002).
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