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A3727 Tsuga mertensiana - Abies amabilis - Callitropsis nootkatensis Tree Island Alliance
Type Concept Sentence: This is a forested alliance with dominants of Abies lasiocarpa, Tsuga mertensiana, Abies amabilis, and/or Callitropsis nootkatensis. These tree islands occur near treeline where extremely deep snowpacks discourage tree growth in topographic depressions, and the forest interdigitates with herbaceous communities. This alliance occurs in the Washington and Oregon Cascades, on the Olympic Peninsula and Coastal Mountains of British Columbia.
Common (Translated Scientific) Name: Mountain Hemlock - Pacific Silver Fir - Alaska-cedar Tree Island Alliance
Colloquial Name: Mountain Hemlock - Pacific Silver Fir - Alaska-cedar Tree Island
Hierarchy Level: Alliance
Type Concept: This is a forested alliance with dominants of Abies lasiocarpa, Tsuga mertensiana, Abies amabilis, and/or Callitropsis nootkatensis. The structure of this alliance is limited to tree islands, which are stands of full-height trees surrounded by meadows. These tree islands occur near treeline where extremely deep snowpacks discourage tree growth in topographic depressions, and the forest interdigitates with herbaceous communities. The entire landscape of meadow with dotted tree islands is often referred to as "parkland." This alliance contains only the forested part of that landscape. Tree islands are typically individually quite small, consisting of 5-10 trees or more, and do not form a continuous canopy typical of forests or woodlands. These tree islands are not krummholz, but are full-statured trees. This alliance occurs in the Washington and Oregon Cascades, on the Olympic Peninsula and Coastal Mountains of British Columbia.
Diagnostic Characteristics: Full-statured forests in subalpine elevations dominated by Abies lasiocarpa, Tsuga mertensiana, Abies amabilis, and/or Callitropsis nootkatensis, with extensive mountain-heathers and other high-elevation species.
Rationale for Nominal Species or Physiognomic Features: No Data Available
Classification Comments: This alliance is easily distinguished as clumps of trees in the subalpine near treeline; it does not form continuous forest, but a distinct pattern of either individual tree islands, or a pattern of repeating "islands." Each "island" is very small, less than 10 m in diameter, surrounded by herbaceous meadow. These islands are full-height or near full-height. This is in contrast to krummholz which are less than 5 m in height, and look like shrub clumps, not tree clumps. Krummholz stands belong to Abies lasiocarpa, Tsuga mertensiana, Abies amabilis, Chamaecyparis nootkatensis Krummholz Alliance (A.NCCN-015).
Similar NVC Types: No Data Available
note: No Data Available
Physiognomy and Structure: This vegetation is characterized by a mosaic of forest patches and tree clumps surrounded by meadows. The tree groups have dense to open cover of needle-leaved evergreen trees less than 20 m in height; canopy 8-11 m tall and subcanopy 2-8 m tall. Where adequate light intensities reach the ground, a shrub layer may be present. The herbaceous layer is present and dominated by subalpine forbs and graminoids.
Floristics: The tree canopy has a sparse overstory and a moderately dense subcanopy that is dominated by Callitropsis nootkatensis (= Chamaecyparis nootkatensis), Abies amabilis, Abies lasiocarpa, and/or Tsuga mertensiana. There are also sparse shrub and dwarf-shrub layers, including Alnus viridis ssp. sinuata, Cassiope mertensiana, Elliottia pyroliflora, Menziesia ferruginea, Paxistima myrsinites, Phyllodoce empetriformis, Rhododendron albiflorum, Rubus lasiococcus, Rubus pedatus, Vaccinium deliciosum, Vaccinium membranaceum, Vaccinium ovalifolium, and Vaccinium scoparium, as well as juvenile trees. The herbaceous layer is also present under the tree canopy. It is dominated by forbs with a significant graminoid and bryophyte component. Characteristic species include Arnica latifolia, Blechnum spicant, Clintonia uniflora, Eucephalus ledophyllus (= Aster ledophyllus), Elymus glaucus, Erythronium montanum, Ligusticum grayi, Lupinus latifolius, Mitella breweri, Phlox diffusa, Streptopus lanceolatus var. curvipes (= Streptopus roseus), Tiarella trifoliata var. unifoliata, Valeriana sitchensis, Veratrum viride, and Xerophyllum tenax.
Dynamics: Trees are slow-growing, long-lived conifers with flexible crowns and branches that resist damage from heavy snows and avalanche. These tree are sensitive to fire and are readily killed by cool or hot surface fires, but will colonize burned sites, especially at lower elevations (Burns and Honkala 1990a). This alliance typically occurs near timberline in an ecotone between subalpine forest and meadow. The dynamics of meadow invasion by trees, and tree mortality, are largely controlled by short-term climatic changes such as a series of above-average precipitation years, or by changes in microenvironment caused by plants, such as more rapid snowmelt around trees in the spring (Franklin and Dyrness 1973).
Environmental Description: This woodland alliance occur as a mosaic of forest islands, tree clumps surrounded by meadows in the subalpine parkland zone of the Cascade Range. These stands are included in the upper parkland subzone within the Tsuga mertensiana Zone (Franklin and Dyrness 1973). They occur near treeline in a narrow 300- to 400-m elevational band above the closed-canopy subalpine forests and below the alpine meadows. Elevations range from 1700-2300 m depending on aspect and latitude. Climate is temperate, with a strong maritime influence, deep winter snowpacks and moderately cold winter temperatures. Precipitation ranges from 150-300 cm annually, occurring mostly in winter and largely as snow. The growing season is short with snowpacks often present into midsummer. Summers are cool with little rain. Tree groups within this parkland mosaic vary in size from a few trees to 0.1 ha or larger, and often occur on ridges and hummocks where the winter snowpack melts sooner. Landscapes are typically gently to moderately sloping with steep rocky ridges. Sites are relatively mesic and may occur on any aspect. Substrates are typically shallow, rocky, coarse-textured soils derived from volcanic rocks such as deposits of pumice, cinder and ash, and lava flows.
Geographic Range: This alliance is found in the Washington and Oregon Cascades, on the Olympic Peninsula and Coastal Mountains of British Columbia.
Nations: CA,US
States/Provinces: BC, OR, WA
Plot Analysis Summary:
http://vegbank.org/natureserve/ELEMENT_GLOBAL.2.899779
Confidence Level: Low
Confidence Level Comments: No Data Available
Grank: GNR
Greasons: No Data Available
Concept Lineage: A.NCCN-047
Predecessors: No Data Available
Obsolete Names: No Data Available
Obsolete Parents: No Data Available
Synonomy: >< Tsuga mertensiana-Abies amabilis Forest (Chappell et al. 1997)
? Group 8 (Antos and Zobel 1986)
>< Mountain Hemlock Series (Henderson et al. 1992)
>< Mountain Hemlock: 205 (Eyre 1980)
>< Upper Parkland Subzone of the Tsuga mertensiana Zone (Franklin and Dyrness 1973)
? Group 8 (Antos and Zobel 1986)
>< Mountain Hemlock Series (Henderson et al. 1992)
>< Mountain Hemlock: 205 (Eyre 1980)
>< Upper Parkland Subzone of the Tsuga mertensiana Zone (Franklin and Dyrness 1973)
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- Brooke, R. C., E. B. Peterson, and V. J. Krajina. 1970. The subalpine mountain hemlock zone. Volume 2, pages 148-439 in: V. J. Krajina, editor. Ecology of Western North America. University of British Columbia, Vancouver.
- Burns, R. M., and B. H. Honkala, technical coordinators. 1990a. Silvics of North America: Volume 1. Conifers. Agriculture Handbook 654. USDA Forest Service, Washington, DC. 675 pp.
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