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Community Field Guide

Scientific Name:
Abies lasiocarpa / Streptopus amplexifolius Forest

Common Name:
Subalpine fir / clasping twisted-stalk Forest

Community Description

The Abies lasiocarpa / Streptopus amplexifolius association occurs predominantly as a small patch community type with a broad geographic distribution, found in nearly all mountainous terrain from the inland Pacific Northwest to the Southern Rocky Mountains. This association is strongly associated with high water tables, being saturated for extensive periods during the growing season and in some years and for some landscape positions it is seasonally flooded. Not uncommonly small rivulets will course across stands early in the growing season only to desiccate completely by mid-summer. It occurs in the lower to middle portions of the subalpine zone, with actual elevation parameters being associated with geographical context. The canopy is usually relatively open and dominated by Picea engelmannii as a long-lived seral species with a subordinate layer of Abies lasiocarpa that is projected to be the dominant of long-term stable stands; the critical recognition feature is that Abies lasiocarpa be successfully reproducing. With the exception of Pinus contorta, a minor seral associate, other tree species are only incidentally present. The undergrowth is typified by a diverse assemblage of tall, moist- to wet-site forbs; depending on the region from which the type is cited it may be recognized by the mere presence of Streptopus amplexifolius or by a whole host of forbs considered ecological analogues.

In the northern part of its range this association is found from around 4,000 to 8,800 feet, while in the southern portion it ranges between 7,700 and 10,000 feet. This type is found on wetland sites of many different landscape positions from sub-irrigated toeslopes to stream and river terraces and swales within otherwise gently undulating to planar terrain. The moisture regime is characterized as saturated throughout the majority of the growing season in most years; in exceptionally wet years these sites may be subject to seasonal flooding. Soils of floodplain sites are typically derived from alluvium (predominantly silty clay loams to loams), while those of upland positions frequently have coarse fragment inclusions of granitics, sandstone, quartzite, and argillite. In northern Idaho and Montana, clay pans and compacted till are consistently found, supposedly causing perched water tables and ultimately reflected in relatively shallow (average only 16 inches) effective rooting depths. As excessive moisture is the primary driving factor in these systems there appears to be no distinctions according to parent material; this type occurs on soils derived from all major rock types.

Across the type’s range Picea engelmannii usually dominates the canopy as a long-lived seral species and persists into the “climax” stage. Abies lasiocarpa is the diagnostic species being the most shade-tolerant species capable of establishing on these sites. It is consistently the most abundant understory tree but mortality is apparently higher than for Picea, which dominates all successional stages. Pinus contorta is a minor and apparently short-lived seral component. Pseudotsuga menziesii, Larix occidentalis and other upland tree species are notably absent. In the northern portion of its range, where this association overlaps the distribution limits and moisture requirements of Thuja plicata and Tsuga heterophylla, these species are absent due to these sites ostensibly exceeding the cold limits of these species. Across the range of the type, the presence of Streptopus amplexifolius or Senecio triangularis with at least 5% canopy cover is diagnostic. Some authors (Hansen et al. 1995, Cooper et al. 1991) have relaxed the cover requirements to 1% and expanded the list of diagnostic species to include Mitella breweri, Mitella pentandra, and Gymnocarpium dryopteris to accommodate local situations where for inexplicable reasons Streptopus amplexifolius is absent but site parameters correspond to the type as recognized elsewhere. Generally these sites have a rich, tall forb layer that can include, at least in the type’s northern expression, Mertensia paniculata, Valeriana sitchensis, Actaea rubra, Ligusticum canbyi, Trautvetteria caroliniensis, and Veratrum viride. The southern expression also has high forb diversity but component species are shorter in stature, including Mertensia ciliata, Osmorhiza depauperata, Arnica latifolia, Saxifraga odontoloma, Saxifraga arguta, Polemonium pulcherrimum and Bromus ciliatus. With the notable exception of a Menziesii ferruginea phase recognized in western Montana and northern Idaho and Ribes lacustre being sporadically well represented, the shrub component is relatively depauperate.

This is a broadly distributed small patch association occurring east of the Cascades in Oregon and Washington and extending throughout virtually all the ranges of the intermountain West to Southern Rocky Mountains and high plateaus of southern Utah.

This is apparently an association where post-disturbance tree species colonizing the site (Picea engelmannii, Abies lasiocarpa persist to the “climax” stage.

Global Rank: G4 State Rank: S3

Community References


01-03-12 / S. V. Cooper

Cooper et al. 1987, Cooper et al. 1991, Hansen et al. 1991, Hansen et al. 1995, Johnson and Simon 1987, Kovalchik 1993, Mauk and Henderson 1984, Padgett et al. 1988, Steele et al. 1981, Steele et al. 1983, Youngblood and Mauk 1985

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This information is from the:
Montana Natural Heritage Program
Montana State Library--Natural Resource Information System
1515 East Sixth Ave., Helena, MT 59620-1800
406 444-5354