Common (Translated Scientific) Name: Western Larch Central Rocky Mountain Forest Alliance

Colloquial Name: Central Rocky Mountain Western Larch Forest

Hierarchy Level:  Alliance

Type Concept: These seral forests are reported from the northern Rocky Mountains in northwestern Montana, and occur in Idaho, Washington, Oregon and possibly British Columbia, Canada. Stands dominated by Larix occidentalis grow best on mesic sites, but also may occur on relatively dry sites. Sites include valley bottoms, benches and lower mountain slopes often on the more mesic north and east aspects, but it is found on all aspects in its northern extent. Substrates are variable, but may include glacial till or colluvium derived from limestone, argillite and quartzite. Soils range from gravelly to non-gravelly loams and silt loams, occasionally with finer texture subsoils. Stands are dominated by the shade-intolerant, fire-tolerant, cold-deciduous conifer Larix occidentalis. They are typically even-aged, developing after catastrophic disturbances such as crownfires or clearcuts. Stand height varies from 30-55 m but can be much less if trees are stunted by high stocking densities of seedlings that may form "doghair" stands similar to Pinus contorta. Young stands may appear to be pure stands of Larix occidentalis, but with time develop into mixed-species stands as slower growing, shade-tolerant species become codominant, including Pseudotsuga menziesii or Abies lasiocarpa. Unless there are frequent fires to reduce the shade-tolerant subcanopy, Larix occidentalis may be over overtopped and quickly decline. Other than juvenile shade-tolerant trees, the understory may have shrub and herbaceous layers depending on tree canopy closure. Shrubs may include Acer glabrum, Alnus viridis ssp. sinuata, Amelanchier alnifolia, Paxistima myrsinites, Physocarpus malvaceus, Rubus parviflorus, Salix scouleriana, Symphoricarpos albus, and Vaccinium spp. The herbaceous layer is often conspicuous. Adjacent stands include other montane and subalpine forests dominated by Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii, Pinus contorta, Pinus ponderosa, Thuja plicata, and Tsuga heterophylla. This alliance ranges from 500-1600 m in elevation. Stands in Montana are mostly above 1000 m. Climate is temperate, continental with influences from the Pacific maritime. Mean annual precipitation ranges from 71-81 cm, occurring mostly in the winter with about 20% of the annual precipitation falling in the growing season in May-June. July and August are typically dry. Snow is common in the middle to upper elevation and higher latitude sites. These stands are restricted by cold, short growing seasons and drought at lower elevations.

Diagnostic Characteristics: Forests occupying valley bottoms, benches and lower mountain slopes of the northern Rocky Mountains dominated by the needle-leaved evergreen tree species Larix occidentalis in young stands. In older stands, Larix occidentalis becomes codominant with Pseudotsuga menziesii or Abies lasiocarpa.

Rationale for Nominal Species or Physiognomic Features: No Data Available

Classification Comments: This seral forest type has not been well described, but is mentioned as a component of seral stands in Pseudotsuga menziesii-, Abies lasiocarpa-, Thuja plicata- and Tsuga heterophylla-dominated habitat types (Burns and Schmidt et al. 1976, Pfister et al. 1977, Honkala 1990a). Eyre (1980) reported Larix occidentalis as a seral species in 11 forest cover types. This alliance only has a provisional association from Montana and needs more investigation to develop association-level classification and description of these stands throughout its range.

Similar NVC Types: No Data Available

note: No Data Available