This ecological system is restricted to the highest elevations of the Rocky Mountains, from Alberta and British Columbia south into New Mexico, and west into the highest mountain ranges of the Great Basin. It is composed of barren and sparsely vegetated alpine substrates, typically including both bedrock outcrop and scree slopes, with lichen- dominated communities. In Montana, alpine bedrock and scree are well represented throughout the northern Rocky Mountains and island mountain ranges. Elevations range from as low as 2,285 meters (7,500 feet) in northwestern Montana to 3,500 meters (11,500 feet) in southern Montana. Exposure to desiccating winds, rocky and sometimes unstable substrates, and a short growing season limit plant growth. Typically, there is sparse (less than 10%) cover of forbs, grasses, and low shrubs, with exposed, unstable scree, talus and bedrock constituting the remainder of cover. Diverse crustose and foliose lichen cover is high (often greater than 50%) on exposed talus and bedrock Soils on these windy, unproductive sites are very poorly developed, often only occurring in fractures of bedrock. This system is characterized by a very cold climate during winter, high winds, high UV radiation and high surface daytime temperatures during summer months on south and west facing aspects, and a very short growing season. Most scree- and bedrock-inhabiting plants are highly adapted to this xeric environment and occur as singular plants among the exposed rocks or in bedrock fractures. These species are typically cushioned, matted or succulent, or grow as flat rosettes, often with thick leaf cuticles or a dense cover of hairs. This system often occurs adjacent to or immediately below North American Alpine Ice Fields and intermingles with Rocky Mountain Alpine Fell Fields.
Plant cover is usually less than 10% with exposed, unstable scree, talus and bedrock constituting the remainder of cover. Most scree and bedrock inhabiting plants are highly adapted to this xeric environment and occur as singular plants among the exposed rocks or in bedrock fractures. These species are typically cushioned, matted or succulent, or grow as flat rosettes, often with thick leaf cuticles or a dense cover of hairs. Diverse crustose and foliose lichen cover is high (often greater than 50%) on exposed talus and bedrock. Common lichen genera include Rhizocarpon, Xanthoria, Lecidea, and Umbilicaria species. Mosses are typically found in bedrock fractures and the leeside of bedrock slabs and chutes below the summits, especially on the north and east facing aspects.
Forbs occur singly or in small patches among the exposed talus and scree and in fractures of the bedrock or the leeside of bedrock where organic matter has accumulated. In northwestern Montana, common forbs include yarrow (Achillea millefolium), elliptic leaf penstemon (Penstemon ellipticus), phacelia (Phacelia species), alpine sky pilot (Polemonium viscosum), alpine spring beauty (Claytonia megarhiza), alpine sandwort (Minuartia species), cut-leaf daisy (Erigeron compositus), draba (Draba spp.), boreal crazyweed (Oxtropis borealis), silky crazyweed (Oxytropis sericea), wooly groundsel (Senecio canus), alpine arnica (Arnica alpina), moss campion (Silene acaulis), spotted saxifrage (Saxifraga bronchialis), alpine buckwheat (Eriogonum ovalifolium), alpine forget-me-not (Myosotis alpestris) and Siberian aster (Symphyotrichum sibiricus). Woody species such as arctic dryad (Dryas octopetala), shrubby cinquefoil (Dasiphora fruticosa) and rock willow (Salix vestita) occur in bedrock fractures or the lee side of bedrock and boulders. Saxifrages and ferns also occur in these protected microsites.
Several northern Rocky Mountain endemic species, Montana species of concern, and potential species of concern inhabit this system. In Montana, some arctic species reach their southernmost range limit within this system, while some middle and southern Rocky Mountain species reach their northernmost range limit, particularly in southwestern ranges of the state.
Graminoid cover is usually very low and often occurs within patches and mats of forbs or woody species. In northwest Montana, common species include Dunhead sedge (Carex phaeocephala), spike sedge (Carex nardina), curly sedge (Carex ruprestris), single spike sedge (Carex scirpoidea), black and white sedge (Carex albonigra), spiked woodrush (Luzula spicata), Drummond’s rush (Juncus drummondii), alpine blue grass (Poa alpina), spike trisetum (Trisetum spicatum) and slender wheatgrass (Elymus trachycaulus). In southwestern Montana, other species are common, such as blackroot sedge (Carex elynoides), Parry’s rush (Juncus parryi), Idaho fescue (Festuca idahoenisis), spike fescue (Festuca kingii), and sheep fescue (Festuca ovina) (Cooper et al. 1999).
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