Physiognomy and Structure: These forests and woodlands are mostly composed of evergreen conifers, but some broad-leaved cold-deciduous trees are often found, and (very uncommonly) cold-deciduous conifers. They can range from nearly closed-canopy forests to very open or patchy short-statured woodlands, or clumps of tree islands or ribbons with intervening grasslands or shrublands, with stands becoming more open and patchy with increased elevation. At the highest settings or those most exposed to wind, the trees can be flagged or damaged from blowing snow and ice-crystals and severe cold. Shrub and herb layers vary widely, with tall or short cold-deciduous or evergreen shrubs dominating the undergrowth, or in some cases with few or no shrubs and perennial forbs, grasses or sedges the predominant growth forms. Cover of shrubs and herbs varies from none to very high depending on environmental setting, time since disturbance, and microclimate factors. Nonvascular species (mosses, lichens, fungi, or soil cryptogams) also vary, but both dry and mesic forests in this macrogroup can have high cover of nonvascular taxa on the forest floor and on decaying woody debris.

Floristics: These forests and woodlands have a suite of tree species characteristic of the higher elevations (upper montane to subalpine zones), including Abies lasiocarpa, Larix lyallii, Picea engelmannii, Pinus albicaulis, Pinus aristata, Pinus contorta, Pinus flexilis, Pinus longaeva, Populus tremuloides, and Tsuga mertensiana. Other trees may occur but are either long-lived seral taxa (Pseudotsuga menziesii) or are found in transitions to lower montane forests, such as Abies concolor, Juniperus scopulorum, Picea pungens, Pinus ponderosa, Pinus edulis, Pinus monophylla, Tsuga heterophylla, or from adjacent riparian forests, such as Populus angustifolia. Canopies can be mixed or dominated by a single species. Some species are more restricted in range: for example, Tsuga mertensiana and Larix lyallii occur in southern British Columbia, Montana and Idaho west and north, but are not found further south or north; Pinus longaeva is found in the Great Basin and Sierra Nevada, while Pinus aristata is restricted to the central and southern mountains of Colorado and northern New Mexico.

Shrub species are highly variable, and typically are cold-deciduous or sometimes evergreen. Common shrubs in the U.S. part of the range include Acer glabrum, Amelanchier alnifolia, Artemisia tridentata, Ceanothus velutinus, Jamesia americana, Juniperus communis, Lonicera utahensis, Physocarpus malvaceus, Prunus virginiana, Purshia tridentata, Ribes montigenum, Rosa woodsii, Rhamnus alnifolia, Ribes inerme, Rubus parviflorus, Shepherdia canadensis, Symphoricarpos albus, Symphoricarpos oreophilus, and the dwarf-shrubs Arctostaphylos uva-ursi, Linnaea borealis, Mahonia repens and several Vaccinium spp. More mesic sites, across the entire range, can include the shrubs Cornus canadensis, Empetrum nigrum, Ledum glandulosum, Menziesia ferruginea, Phyllodoce empetriformis, Rhododendron albiflorum, Salix brachycarpa, Salix glauca, Spiraea betulifolia, and Vaccinium membranaceum. Examples in the Great Basin and Sierra Nevada might include Arctostaphylos patula, Artemisia arbuscula, Artemisia tridentata ssp. vaseyana, Cercocarpus intricatus, Cercocarpus ledifolius, Chrysolepis sempervirens, Ericameria discoidea, Juniperus communis, Ribes cereum, and Ribes montigenum.

Associated herbaceous species are especially diverse given the wide elevational and latitudinal range, with alpine species occurring near the upper treeline and montane and subalpine species below. Mesic stands include herbaceous species such as Actaea rubra, Calamagrostis canadensis, Carex siccata, Clintonia uniflora, Erigeron eximius, Eucephalus engelmannii (= Aster engelmannii), Gymnocarpium dryopteris, Heracleum maximum, Luzula glabrata var. hitchcockii, Maianthemum stellatum, Oryzopsis asperifolia, Osmorhiza berteroi, Osmorhiza occidentalis, Packera cardamine, Packera sanguisorboides, Pedicularis racemosa, Rubus pedatus, Rudbeckia occidentalis, Saxifraga bronchialis, Senecio triangularis, Thalictrum fendleri, Tiarella spp., Valeriana occidentalis, Valeriana sitchensis, and Xerophyllum tenax. Drier sites close to the alpine might include xeric graminoids, such as Calamagrostis purpurascens, Festuca arizonica, Festuca idahoensis, Festuca thurberi, Leucopoa kingii, Muhlenbergia filiculmis, Muhlenbergia montana, Poa fendleriana, and Trisetum spicatum. Highest elevation stands have a floristic component of typically subalpine and alpine plants, such as Carex elynoides, Carex rupestris, Erigeron peregrinus, Phlox pulvinata, Sedum lanceolatum, or Trifolium dasyphyllum. Other scattered forbs may include species of Achillea, Antennaria, Arenaria, Arnica, Astragalus, Artemisia, Campanula, Erigeron, Hymenoxys, Penstemon, Polemonium, Senecio, and Thalictrum.

In the Great Basin and Sierra Nevada common herb species may include Antennaria rosea, Aquilegia scopulorum, Arabis drummondii, Arenaria congesta, Arenaria kingii, Astragalus kentrophyta, Astragalus platytropis, Calamagrostis rubescens, Carex rossii, Cirsium eatonii, Cymopterus cinerarius, Cymopterus nivalis, Elymus elymoides, Eriogonum gracilipes, Eriogonum holmgrenii, Eriogonum ovalifolium, Erigeron pygmaeus, Erigeron tener, Festuca brachyphylla, Koeleria macrantha, Linanthus pungens (= Leptodactylon pungens), Packera werneriifolia, Penstemon leiophyllus, Poa fendleriana, Phlox pulvinata, Trifolium gymnocarpon, and Trisetum spicatum. Selaginella watsonii is common in some high-elevation stands.

In aspen forests, at least in the U.S. portion of the range, the herbaceous layers may be lush and diverse. Common graminoids may include Bromus carinatus, Calamagrostis rubescens, Carex siccata (= Carex foenea), Carex geyeri, Carex rossii, Elymus glaucus, Elymus trachycaulus, Festuca thurberi, and Hesperostipa comata. Associated forbs may include Achillea millefolium, Aconitum columbianum, Delphinium spp., Eucephalus engelmannii (= Aster engelmannii), Geranium viscosissimum, Heracleum sphondylium, Ligusticum filicinum, Lupinus argenteus, Osmorhiza berteroi (= Osmorhiza chilensis), Pteridium aquilinum, Rudbeckia occidentalis, Thalictrum fendleri, Valeriana occidentalis, Wyethia amplexicaulis, and many others. In California, Symphyotrichum spathulatum (= Aster occidentalis) is a common forb. Exotic grasses such as the perennials Poa pratensis and Bromus inermis and the annual Bromus tectorum are often common in occurrences due to grazing disturbance.

Dynamics:  Occurrences in this macrogroup often originate from, and are likely maintained by, stand-replacing disturbances such as avalanche, crown fire, insect outbreaks, disease, occasional windthrow, or clearcutting by man or flooding by beaver. Fire regimes are generally mixed-severity or stand-replacing, and of long return intervals, occurring from 150 to 500 years. Insect outbreaks are more frequent, every 30-50 years in some types, and can alter both the structure and composition of stands.

In spruce-fir forests, Picea engelmannii can be very long-lived, reaching 500 years of age. Abies lasiocarpa decreases in importance relative to Picea engelmannii with increasing distance from the region of Montana and Idaho where maritime air masses influence the climate. Abies lasiocarpa increases in importance northward into British Columbia. In northern stands. Abies lasiocarpa makes up most of the biomass, and also increases in stand composition with increasing elevation. Disturbance includes occasional blowdown, insect outbreaks and fire. Fire is an important disturbance factor, but fire regimes have a long return interval and so are often stand-replacing. Picea engelmannii can rapidly recolonize and dominate burned sites, or can succeed to other seral species such as Pinus contorta or Populus tremuloides. Due to great longevity, Pseudotsuga menziesii may persist in occurrences for long periods without regeneration. Old-growth characteristics in Picea engelmannii forests will include treefall and windthrow gaps in the canopy, with large downed logs, rotting woody material, tree seedling establishment on logs or on mineral soils unearthed in root balls, and snags.

Pinus contorta is an aggressively colonizing, shade-intolerant conifer which usually occurs in lower, drier subalpine forests in the major ranges of the western United States and Canada. Establishment is episodic and linked to stand-replacing disturbances, primarily fire. The incidence of serotinous cones varies within and between varieties of Pinus contorta, being most prevalent in Rocky Mountain populations. Closed, serotinous cones appear to be strongly favored by fire and allow rapid colonization of fire-cleared substrates (Burns and Honkala 1990a). Hoffman and Alexander (1980, 1983) report that in stands where Pinus contorta exhibits a multi-aged population structure, with regeneration occurring, there is typically a higher proportion of trees bearing nonserotinous cones. The dominance of Pinus contorta in associations in this macrogroup is related to fire history and topo-edaphic conditions (Pfister et al. 1977, Hoffman and Alexander 1980, Steele et al. 1981, Mauk and Henderson 1984). Following stand-replacing fires, Pinus contorta will rapidly colonize and develop into dense stands of even-aged trees. These stands, while frequently persistent for more than 100 years, may succeed to spruce-fir forests or woodlands. Most (but not all) forests in this macrogroup are early- to mid-successional forests which developed following fires.

Some Pinus contorta forest associations occur, and will persist, on sites that are too extreme for other conifers to establish. These include excessively well-drained pumice deposits (Volland 1976), glacial till and alluvium on valley floors where there is cold-air accumulation (Steele et al. 1981), warm and droughty shallow soils over fractured quartzite bedrock (Mauk and Henderson 1984), well-drained to xeric stabilized sand dunes (Jenny et al. 1969, Kumler 1969), and shallow moisture-deficient soils with a significant component of volcanic ash (Cooper et al. 1987). Some Pinus contorta forests can be persistent for hundreds of years, a result of a lack of seed source or the competitive exclusion of other conifer species (Moir 1969a, Despain 1973b, Pfister et al. 1977, Hoffman and Alexander 1983, Cooper et al. 1987), or the frost tolerance of Pinus contorta seedlings and mature trees, which allows the development of monotypic stands in frost-prone areas (Steele et al. 1981, Burns and Honkala 1990a).

Both Pinus longaeva and Pinus flexilis are slow-growing, long-lived trees that are intolerant of shade. Pinus longaeva may attain nearly 4900 years in age and 12 m in height, whereas Pinus flexilis may live 1000 years and attain 18 m in height. Bristlecone pine branches retain needles for as long as 30 years, whereas limber pine needles are lost after several years. Bristlecone pine trees produce dense, resinous wood that is resistant to rot and disease. Mature trees have massive, contorted trunks with mostly dead and gnarled wood (Sawyer et al. 2009). Natural regeneration of Pinus flexilis appears to be closely associated with caching of the large wingless seeds, primarily by Clark''s nutcracker (Nucifraga columbiana) (Lanner and Vander Wall 1980). Germination of cached seeds often results in the multi-stemmed clumps characteristic of these sites, although the species may produce multiple stems from boles damaged near the ground. Germination and rooting will sometimes be restricted to crevices in rock. Pinus longaeva has smaller winged seeds and should be wind-disseminated. However, caching by nutcrackers does take place, especially when other Pinus species are also available (Dr. R. Lanner pers. comm.). Fires seldom destroy these woodlands due to the sparse nature of the canopy cover of trees and abundant bare ground. Peet (1978a, 1981) notes that Pinus aristata is dominant at higher elevations in much of the southern Rocky Mountains, where Pinus flexilis is restricted to lower elevations. This is attributed to apparent competitive exclusion, because Pinus flexilis is dominant at high elevations in northern Colorado, Wyoming and Montana.

Pinus albicaulis is a slow-growing, long-lived conifer that is common at higher elevations in the upper subalpine zone over much of the central and northern range of this macrogroup. It typically occurs in a mosaic of tree islands and meadows where it often colonizes sites and creates habitat for less hardy tree species. In lower subalpine forests, it is a seral species, establishing after a large disturbance such as stand-replacing fire or avalanche, or it is restricted to dry, rocky ridges where it competes well with shade-tolerant tree species. Without disturbance, it will be overtopped in 100-120 years by faster growing, shade-tolerant species such as Abies lasiocarpa, Picea engelmannii, Pseudotsuga menziesii, or Tsuga mertensiana. Although crown fires and high-intensity surface fires kill Pinus albicaulis, it tolerates low-intensity surface fires that will kill the shade-tolerant understory. Fire intervals range from 30-300 years.

Birds and small mammals often eat and cache the large, wingless seeds of whitebark pine and are responsible for the dispersal of this species. Most important is the Clark''s nutcracker, which can transport the seeds long distances and cache them on exposed windswept and burned-over sites. This results in the regeneration of pines in clumps from forgotten caches (Eyre 1980, Steel et al. 1983, Burns and Honkala 1990a, Schmidt and McDonald 1990).

Pests include the mountain pine beetle (Dendroctonus ponderosae), which has killed many mature trees in the past, during epidemics where populations of the beetle build up in lower elevation Pinus contorta stands, then move up into the Pinus albicaulis regions (Steel et al. 1983, Burns and Honkala 1990a, Schmidt and McDonald 1990). The exotic pathogen white pine blister rust (Cronartium ribicola) is attacking and killing Pinus albicaulis trees in many parts of the interior northwestern U.S. It is especially destructive in more mesic habitats that favor infection of its alternate host Ribes spp. Pinus albicaulis is very susceptible to this disease, and the only real hope is propagating individuals that have high genetic resistance to blister rust (Steel et al. 1983, Burns and Honkala 1990a, Schmidt and McDonald 1990, Tomback et al. 2001).

Larix lyallii is a very slow-growing, long-lived tree, with individuals attaining up to 1000 years in age (Richards 1981). It is generally intolerant of shade from other trees, but extreme environmental conditions limit competition. Reproduction is typically by seed and is most favorable on moist mineral soil. Seedling growth is initially very slow and accelerates after an extensive root system is established. Major disturbances to stands of this group are windthrow and snow avalanches. Lightning damage to individual trees is common, but sparse canopies and rocky terrain serve to limit the spread of fire.

Populus tremuloides stands are the result of a stand-replacing disturbance in the mainly conifer forests of this macrogroup, primarily in southern regions. The stems of these thin-barked, clonal trees are easily killed by surface fires, but they can quickly and vigorously resprout in densities of up to 30,000 stems per hectare (Knight 1994). The stems are relatively short-lived (100-150 years), and the occurrences often succeed to longer-lived conifer forest if undisturbed. Occurrences are favored by fire in the conifer zone (Mueggler 1988). With adequate disturbance, a clone may live many centuries. Although Populus tremuloides produces abundant seeds, seedling survival is rare because the long moist conditions required to establish them are rare in the habitats where they occur. Superficial soil drying will kill seedlings (Knight 1994).