Common (Translated Scientific) Name: Western Red-cedar / Bride''s Bonnet - Common Beargrass Forest

Colloquial Name: No Data Available

Hierarchy Level:  Association

Type Concept: This is a small- to large-patch forested community found in the northern Rocky Mountains of western Montana west into northeastern Washington. This type generally occupies the highest elevation Thuja plicata communities, representing relatively cold and dry environments. Its known elevational range is 790 to 1675 m (2600-5500 feet). This type is very heterogeneous, occurring across a broad range of habitat conditions; it occurs from toeslope positions to the tops of ridges and on all degrees of slope and all aspects. Parent materials are variable, with loess and ash caps deposited over glacial outwash and till in some areas. The upper soil horizons are well- to excessively drained and coarse-textured (in some areas sandy soils predominate). Glacial compression is invoked as the reason many of the soil profiles have a compacted subsoil, which results in shallow rooting and accounts in part for the more xeric nature of these sites. The canopy cover of this forest type is usually in excess of 60%, with Thuja plicata comprising at least 25% of the total. Because seral tree species occupy these sites readily following disturbance, Pseudotsuga menziesii, Larix occidentalis, Pinus contorta, and Picea engelmannii commonly occur across the range of this type, but in northern Idaho Abies grandis often shares dominance with Thuja. Sites transitional to subalpine occasionally have appreciable cover of Abies lasiocarpa. The understory, in which Thuja is 100% constant, gives every indication that these stands will be Thuja-dominated late in the sere. The tall-shrub component is mostly dispersed clumps, and no one species has high constancy, though Acer glabrum, Alnus viridis ssp. sinuata, Amelanchier alnifolia, and Sorbus scopulina may have 5-10 % cover, singly or in the aggregate. The short-shrub layer is dominant with the indicator Vaccinium membranaceum nearly 100% constant and generally exhibiting greater than 15% cover. Other regularly occurring short shrubs include Paxistima myrsinites, Spiraea betulifolia, Rosa gymnocarpa, Rubus parviflorus, and Lonicera utahensis. The dwarf-shrub Linnaea borealis is always present and its cover can approach 20%. Bromus vulgaris is often the only graminoid represented. In the forb layer Clintonia uniflora and Tiarella trifoliata are reflective of relative mesic conditions, whereas Xerophyllum tenax (considered an indicator when having 5% or greater cover) is indicative of Thuja at its cold, dry extremes, transitional to subalpine habitats. In the northwestern portion of this type''s distribution, Xerophyllum appears to be sporadically distributed and Vaccinium is relied on as the alternative indicator.

Diagnostic Characteristics: No Data Available

Rationale for Nominal Species or Physiognomic Features: No Data Available

Classification Comments: This association is the result of recognizing at the plant association level what was known as the Thuja plicata / Clintonia uniflora Habitat Type, Xerophyllum tenax Phase (Cooper et al. 1987). There are clearly stands within the Thuja plicata / Vaccinium membranaceum Plant Association (Williams et al. 1995) and Thuja plicata / Clintonia uniflora Habitat Type, Clintonia uniflora Phase (Pfister et al. 1977) that would also key to the type under consideration. These existing classifications have all treated Thuja as an indicator species, its mere presence and successful reproduction (Pfister et al. 1977, Cooper et al. 1987) or minor cover (10%) (Williams et al. 1995) conveying the notion that these sites would ultimately be dominated by Thuja; in other words, this was a potential vegetation type. With recent classification work related to vegetation mapping in Waterton Lakes-Glacier International Peace Park (IPP), many of the stands that would have been assigned to Thuja plicata, Tsuga heterophylla or Abies lasiocarpa associations were instead allocated to seral tree-dominated associations. A rather arbitrary decision was made regarding the IPP classification to recognize Thuja associations based on this species having 25% relative cover in the upper (mature) canopy. This cover value was deemed sufficient to indicate a Thuja type but begs the question: why not 50% or having greater cover than any other canopy species? Forthcoming debate on appropriate cover values for type recognition when multiple species occupy the canopy is a certainty. Recognizing Clintonia uniflora and Tiarella trifoliata as indicators when they are merely present and not confined to microsites is also a contentious point for some.

Similar NVC Types: No Data Available

note: No Data Available