Physiognomy and Structure: Vegetation included in this alliance has a closed tree canopy that is typically 2-20 m tall with cover ranging from very open to completely closed. The canopy is dominated by evergreen and deciduous needle-leaved trees. Sparse cover of cold-deciduous and evergreen, broad-leaved or scale-leaved shrub or dwarf-shrub species may be present. A sparse to moderately dense herbaceous layer dominated by perennial graminoids or forbs is also present.

Floristics: These forests and woodlands have a coniferous tree canopy 2-20 m tall that is dominated by Pinus albicaulis or codominated by Abies lasiocarpa. In some cases Pinus albicaulis is the only canopy tree species present. Other associated tree species vary by geography and elevation zones throughout the alliance''s range. In the lower subalpine, Pinus contorta and Pseudotsuga menziesii are present to codominant. In mesic stands in Canada and elsewhere, Picea engelmannii may be present to codominant. Tsuga mertensiana may be found in mesic stands in the Cascades. Near the southern range extent, Pinus flexilis is often present. The undergrowth is typically sparse because sites are often dry. There may be a sparse shrub layer consisting of juvenile trees and other woody species such as Arctostaphylos uva-ursi, Juniperus communis, Ribes montigenum, Ribes viscosissimum, Shepherdia canadensis, Symphoricarpos oreophilus, and Vaccinium scoparium. The herbaceous layer is typically sparse and ranges from nearly absent to moderately dense cover of graminoids with scattered forbs. Characteristic species include Achillea millefolium, Antennaria microphylla, Arnica spp., Carex geyeri, Carex rossii, Festuca idahoensis, Lupinus sierrae-blancae (= Lupinus laetus), Poa nervosa, Potentilla diversifolia, Solidago multiradiata, and Xerophyllum tenax.

Dynamics:  Pinus albicaulis is a slow-growing, long-lived conifer that is common at higher elevations in the upper subalpine zone. 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, and Tsuga mertensiana. Although crown fires and hot 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 pine seeds 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, Steele 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 beetles build up in lower elevation Pinus contorta stands, then move up into the Pinus albicaulis stands (Steele 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).