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A3613 Tsuga heterophylla - Thuja plicata Warm-Mesic Central Rocky Mountain Forest & Woodland Alliance

Type Concept Sentence: Forests and woodlands dominated by Tsuga heterophylla and/or Thuja plicata and occupying warm-mesic topographic positions occurring in the Pacific Northwest of northeastern Washington, east to Idaho and Montana west of the Continental Divide and north into British Columbia.


Common (Translated Scientific) Name: Western Hemlock - Western Red-cedar Warm-Mesic Central Rocky Mountain Forest & Woodland Alliance

Colloquial Name: Central Rocky Mountain Western Hemlock - Western Red-cedar Warm-Mesic Forest & Woodland

Hierarchy Level:  Alliance

Type Concept: This alliance occurs in all the maritime-influenced regions of northeastern Washington, northern Idaho, British Columbia and northeastern Montana west of the Continental Divide. Communities occupy positions that represent the warmer part of the environmental gradient that supports these forests. These forests have a closed tree canopy less than 50 m tall that is dominated or codominated by Thuja plicata and/or Tsuga heterophylla. Common canopy associates include Abies grandis, Betula papyrifera, Larix occidentalis, Picea engelmannii, Pinus monticola (particularly in northern Idaho), and Pseudotsuga menziesii. The shrub layer is often species-rich, but without substantial cover and clearly subordinate to the herbaceous cover. Species with high constancy include Acer glabrum, Linnaea borealis, Lonicera utahensis, Menziesia ferruginea, Paxistima myrsinites, Rosa gymnocarpa, Rubus parviflorus, Taxus brevifolia, and Vaccinium membranaceum. Communities of this alliance typically feature a lush herbaceous layer indicative of abundant moisture. Herbaceous species of high constancy include Adenocaulon bicolor, Adiantum pedatum, Anemone piperi, Aralia nudicaulis, Asarum caudatum, Athyrium filix-femina, Clintonia uniflora, Galium triflorum, Gymnocarpium dryopteris, Maianthemum stellatum, Polystichum munitum, Prosartes hookeri, Rubus pedatus, Senecio triangularis, Tiarella trifoliata, Trautvetteria caroliniensis, Trillium ovatum, and Viola orbiculata. These forests are generally restricted to conditions of abundant moisture, including sheltered slopes and toeslopes, alluvial bottoms and benches. It shows no affinity for particular aspects due to sheltered positions and is perhaps subirrigated, at least until early summer. The elevation range is from 555 to 1525 m.

Diagnostic Characteristics: This alliance is a needle-leaved evergreen forest dominated by Tsuga heterophylla and/or Thuja plicata in association with species with affinities to warmer climatic topographic positions and abundant moisture availability. The mesic conditions also promote a diverse but sparse shrub layer and a lush herbaceous layer.

Rationale for Nominal Species or Physiognomic Features: No Data Available

Classification Comments: This alliance contains associations from former Tsuga heterophylla and Thuja plicata alliances and reflects climatic affinities among their common floristic elements.

Similar NVC Types: No Data Available
note: No Data Available

Physiognomy and Structure: Stands of this alliance typically contain a dense canopy of tall conifers approaching 50 m or more in height. There is often a sparse to dense layer of cold-deciduous or evergreen shrubs. The herbaceous layer is usually a dense layer of shade-tolerant forbs and ferns, but in some cases can be sparse.

Floristics: These forests have a closed tree canopy less than 50 m tall that is dominated or codominated by Thuja plicata and/or Tsuga heterophylla and can include all successional stages. Early-seral stands can be dominated by species other than Tsuga heterophylla or Thuja plicata; however, in late-seral to climax stands, these two conifers invariably dominate the canopy. Common canopy associates include Abies grandis, Betula papyrifera, Larix occidentalis, Picea engelmannii, Pinus monticola (particularly in northern Idaho), and Pseudotsuga menziesii. Sites often have considerable microsite variation due to hummocking, and this can be reflected in the within-stand vegetation patterning. The shrub layer is often species-rich, but without substantial cover and clearly subordinate to the herbaceous cover. Species with high constancy include Acer glabrum, Linnaea borealis, Lonicera utahensis, Menziesia ferruginea, Paxistima myrsinites, Rosa gymnocarpa, Rubus parviflorus, Taxus brevifolia, and Vaccinium membranaceum. Communities of this alliance typically feature a lush herbaceous layer indicative of abundant moisture. Herbaceous species of high constancy include Adenocaulon bicolor, Adiantum pedatum, Anemone piperi, Aralia nudicaulis, Asarum caudatum, Athyrium filix-femina, Clintonia uniflora, Galium triflorum, Gymnocarpium dryopteris, Maianthemum stellatum, Polystichum munitum, Prosartes hookeri (= Disporum hookeri), Rubus pedatus, Senecio triangularis, Tiarella trifoliata, Trautvetteria caroliniensis, Trillium ovatum, and Viola orbiculata.

Dynamics:  Forest associations of this alliance are dominated by moderately fast-growing, shade-tolerant conifers, which require moist conditions, and a moderate temperature regime for establishment and growth. Following disturbance, a variety of other conifer species can become established and dominate sites previously supporting stands of this forest alliance. Pseudotsuga menziesii, in particular, can become established and dominate sites for many years following disturbance. In the northern Rocky Mountains, stand-replacing disturbance can result in conversion to communities dominated by either Larix occidentalis or Pinus monticola. Typically, stand-replacement fire-return intervals are 150-500 years with moderate-severity fire intervals of 50-100 years. Specific fire influences vary with site characteristics. Generally, wetter sites burn less frequently and are older stands with more Tsuga heterophylla and Thuja plicata. Drier sites are younger and have more diverse canopies. These are the moistest mid-montane environments on the east side of the Cascades and in northeastern Washington.

Environmental Description:  Vegetation of this alliance occurs within the maritime-influenced region of the Pacific Northwest, extending east into the northern Rocky Mountains. These forest associations represent major forest types in both regions. Throughout the range of this alliance, much of the annual precipitation occurs as rain. Where snow does occur, it can generally be melted by rain during warm winter storms. These forests are generally restricted to conditions of abundant moisture, including sheltered slopes and toeslopes, alluvial bottoms and benches. Slopes are moderate to steep. It shows no affinity for particular aspects due to sheltered positions and is perhaps subirrigated, at least until early summer. The elevation range is from 555 to 1525 m. Soils are acidic and derived of alluvium, colluvium or both. Textures may include silt, silt loam, loam and sandy loam.

Geographic Range: This alliance occurs in the interior regions of the Pacific Northwest, east of the Cascades, from interior British Columbia south to eastern Washington, Oregon, northern Idaho and western Montana east to the Continental Divide.

Nations: CA,US

States/Provinces:  BC, ID, MT, OR, WA




Confidence Level: Moderate

Confidence Level Comments: No Data Available

Grank: GNR

Greasons: No Data Available

Type Name Database Code Classification Code
Class 1 Forest & Woodland Class C01 1
Subclass 1.B Temperate & Boreal Forest & Woodland Subclass S15 1.B
Formation 1.B.2 Cool Temperate Forest & Woodland Formation F008 1.B.2
Division 1.B.2.Nb Rocky Mountain Forest & Woodland Division D194 1.B.2.Nb
Macrogroup 1.B.2.Nb.3 Western Hemlock - Grand Fir - Western Larch Mesic Lower Montane Forest Macrogroup M500 1.B.2.Nb.3
Group 1.B.2.Nb.3.b Western Red-cedar - Western Hemlock Central Rocky Mountain Forest Group G217 1.B.2.Nb.3.b
Alliance A3613 Western Hemlock - Western Red-cedar Warm-Mesic Central Rocky Mountain Forest & Woodland Alliance A3613 1.B.2.Nb.3.b
Association CEGL000113 Western Hemlock / Strawberry-leaf Raspberry Forest CEGL000113 1.B.2.Nb.3.b
Association CEGL000470 Western Red-cedar / Northern Maidenhair Forest CEGL000470 1.B.2.Nb.3.b
Association CEGL000472 Western Red-cedar / British Columbia Wild Ginger Forest CEGL000472 1.B.2.Nb.3.b
Association CEGL000473 Western Red-cedar / Common Ladyfern Forest CEGL000473 1.B.2.Nb.3.b
Association CEGL000476 Western Red-cedar / Western Oakfern Forest CEGL000476 1.B.2.Nb.3.b
Association CEGL000480 Western Red-cedar / Pacific Yew / British Columbia Wild Ginger Forest CEGL000480 1.B.2.Nb.3.b
Association CEGL000488 Western Hemlock / Wild Sarsaparilla Forest CEGL000488 1.B.2.Nb.3.b
Association CEGL000490 Western Hemlock / British Columbia Wild Ginger Forest CEGL000490 1.B.2.Nb.3.b
Association CEGL000491 Western Hemlock / Common Ladyfern Forest CEGL000491 1.B.2.Nb.3.b
Association CEGL000494 Western Hemlock / Western Oakfern Forest CEGL000494 1.B.2.Nb.3.b

Concept Lineage: A.145, A.166.

Predecessors: No Data Available

Obsolete Names: No Data Available

Obsolete Parents: No Data Available

Synonomy: No Data Available

Concept Author(s): C. Chappell, R. Crawford, J. Kagan, and P.J. Doran (1997)

Author of Description: M.E. Hall

Acknowledgements: No Data Available

Version Date: 01-08-14

  • Chappell, C., R. Crawford, J. Kagan, and P. J. Doran. 1997. A vegetation, land use, and habitat classification system for the terrestrial and aquatic ecosystems of Oregon and Washington. Unpublished report prepared for Wildlife habitat and species associations within Oregon and Washington landscapes: Building a common understanding for management. Prepared by Washington and Oregon Natural Heritage Programs, Olympia, WA, and Portland, OR. 177 pp.
  • Faber-Langendoen, D., J. Drake, M. Hall, G. Kittel, S. Menard, C. Nordman, M. Pyne, M. Reid, M. Russo, K. Schulz, L. Sneddon, K. Snow, and J. Teague. 2013-2019b. Screening alliances for induction into the U.S. National Vegetation Classification: Part 1 - Alliance concept review. NatureServe, Arlington, VA.