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A3776 Thuja plicata - Tsuga heterophylla Rocky Mountain Swamp Forest Alliance

Type Concept Sentence: These are seasonally flooded forests are dominated by conifer species such as Thuja plicata and/or Tsuga heterophylla. Other tree species that may be present include Pseudotsuga menziesii, Abies grandis, and Abies lasiocarpa. They occur in riparian areas and toeslopes that remain saturated throughout the growing season. These stands occur in the marine-influenced interior mountains of northeastern Washington, northern Idaho, southeastern British Columbia and northwestern Montana east of the Continental Divide.

Common (Translated Scientific) Name: Western Red-cedar - Western Hemlock Rocky Mountain Swamp Forest Alliance

Colloquial Name: Rocky Mountain Western Red-cedar - Western Hemlock Swamp Forest

Hierarchy Level:  Alliance

Type Concept: These seasonally flooded forests are characterized by a dense to somewhat open coniferous canopy dominated by Thuja plicata. Tsuga heterophylla is a typical associate in these stands. Pseudotsuga menziesii, Abies grandis, and Abies lasiocarpa may also share the upper tree canopy. The herbaceous layer is diverse and dominated by wetland and moist forest species, including Lysichiton americanus, Athyrium filix-femina, Achlys triphylla, and Senecio triangularis. Mosses and lichens are common on trees, downed logs, and the forest floor. The alliance occurs in the marine-influenced coastal and interior mountains of northeastern Washington, northern Idaho, southeastern British Columbia and northwestern Montana east of the Continental Divide. This alliance is found from sea level in coastal areas to over 1800 m in the northern Rocky Mountains of Idaho and western Montana. Annual precipitation ranges from less than 100 cm in the Northern Rockies to 300 cm or more in the coastal ranges of Oregon and Washington. These forests occur on riparian, toeslope, or valley bottom sites that are flooded for a substantial portion of the growing season. The seasonal flooding originates mostly from precipitation and snowmelt collecting in basins, with a small amount of streamside flooding. The soils are organic and saturated for part of the growing season. These forests are often transitional to non-forested wetlands.

Diagnostic Characteristics: Saturated soil wetlands dominated by Thuja plicata and/or Tsuga heterophylla.

Rationale for Nominal Species or Physiognomic Features: No Data Available

Classification Comments: Maritime influence of interior Rocky Mountains creates pockets where Thuja plicata and Tsuga heterophylla thrive.

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: Stands are dominated or codominated by Thuja plicata. Tsuga heterophylla is a typical associate that may be confined to higher microsites such as buttress roots, stumps and nurse logs. Other important and occasionally codominant conifers may include Abies grandis, Abies lasiocarpa, Larix occidentalis, Pseudotsuga menziesii, or Taxus brevifolia. Broad-leaved cold-deciduous trees, including Alnus rubra, Acer macrophyllum, Betula papyrifera, and Populus balsamifera ssp. trichocarpa, may form a subcanopy. Understory shrubs include Acer glabrum, Alnus incana, Linnaea borealis, Paxistima myrsinites, Rubus parviflorus, and/or Vaccinium membranaceum. The herbaceous layer may have Adiantum pedatum, Aralia nudicaulis, Asarum caudatum, Athyrium filix-femina, Clintonia uniflora, Coptis occidentalis, Cornus canadensis, Geum macrophyllum, Gymnocarpium dryopteris, Lysichiton americanus, Tiarella trifoliata, and/or Trillium ovatum . Mosses and lichens are common on trees, downed logs, and the forest floor.

Dynamics:  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 support older stands with more Tsuga heterophylla and Thuja plicata.
Environmental Description:  Stands of the alliance generally occur on all slopes and aspects, but grow best on sites with high soil moisture, such as toeslopes and bottomlands (Cooper et al. 1987). These forests generally occur at moist, non-flooded or upland sites that are not saturated yearlong, such as riparian, toeslope, or valley bottom sites that are flooded for a substantial portion of the growing season. The seasonal flooding originates mostly from precipitation and snowmelt collecting in basins, with a small amount of streamside flooding. The soils are organic and saturated for part of the growing season.
Geographic Range: This alliance occurs in the marine-influenced coastal and interior mountains of northeastern Washington, northern Idaho, southeastern British Columbia and northwestern Montana east of the Continental Divide.

Nations: CA?,US

States/Provinces:  BC?, ID, MT, OR, WA, WY


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.3 Temperate Flooded & Swamp Forest Formation F026 1.B.3
Division 1.B.3.Nc Rocky Mountain-Great Basin Montane Flooded & Swamp Forest Division D195 1.B.3.Nc
Macrogroup 1.B.3.Nc.1 Engelmann Spruce - Narrowleaf Cottonwood / Red-osier Dogwood Riparian & Swamp Forest Macrogroup M034 1.B.3.Nc.1
Group 1.B.3.Nc.1.b Western Red-cedar - Engelmann Spruce / American Skunk-cabbage Swamp Forest Group G505 1.B.3.Nc.1.b
Alliance A3776 Western Red-cedar - Western Hemlock Rocky Mountain Swamp Forest Alliance A3776 1.B.3.Nc.1.b
Association CEGL000479 Western Red-cedar - Western Hemlock / Devil''s-club Rocky Mountain Swamp Forest CEGL000479 1.B.3.Nc.1.b
Association CEGL005931 Western Red-cedar / Softleaf Sedge Swamp Forest CEGL005931 1.B.3.Nc.1.b
Concept Lineage: No Data Available

Predecessors: No Data Available

Obsolete Names: No Data Available

Obsolete Parents: No Data Available
Synonomy: >< Eastside Tsuga heterophylla-Thuja plicata Forests (Chappell et al. 1997)
>< Tsuga heterophylla-Thuja plicata Coniferous Wetlands (Chappell et al. 1997)
>< Western Needleleaf Forests: 2: Cedar-Hemlock-Douglas Fir Forest (Thuja-Tsuga-Pseudotsuga) (Küchler 1964)
>< Western Redcedar - Western Hemlock: 227 (Eyre 1980)
>< Western Redcedar: 228 (Eyre 1980)
>< Westside Pseudotsuga menziesii-Tsuga heterophylla Forests (Chappell et al. 1997)
Concept Author(s): G. Kittel, in Faber-Langendoen et al. (2013)

Author of Description: G. Kittel

Acknowledgements: No Data Available

Version Date: 09-26-14
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