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A3500 Juniperus occidentalis Grassy Open Woodland Alliance

Type Concept Sentence: This open woodland and savanna alliance is characterized by diagnostic tree species Juniperus occidentalis that forms an open to moderately dense tree layer with an understory dominated by a moderately dense to dense layer of perennial grasses and lacking significant cover of shrubs (1-20% cover). It occurs on the Columbia Plateau extending from southwestern Idaho, along the eastern foothills of the Cascades, south to the Modoc Plateau of northeastern California.

Common (Translated Scientific) Name: Western Juniper Grassy Open Woodland Alliance

Colloquial Name: Western Juniper Grassy Open Woodland

Hierarchy Level:  Alliance

Type Concept: Vegetation within this woodland and savanna alliance is characterized by a sparse to open canopy of Juniperus occidentalis and a moderately dense to dense herbaceous layer. Juniperus occidentalis is typically the sole tree species; however, the tree form of Cercocarpus ledifolius may occasionally be present to codominant. Additionally, a sparse to open shrub layer (1-20% cover) may be present. The most common shrub is Artemisia tridentata, but others include Artemisia arbuscula, Artemisia rigida, Cercocarpus ledifolius (shrub form), Chrysothamnus viscidiflorus, Ericameria nauseosa, Purshia tridentata, Ribes cereum, and Symphoricarpos oreophilus. The herbaceous layer is typically dominated by graminoids such as Achnatherum occidentale, Achnatherum thurberianum, Carex geyeri, Carex filifolia, Festuca idahoensis, Koeleria macrantha, Leymus cinereus, Poa secunda, and Pseudoroegneria spicata. Common forbs include Achillea millefolium, Balsamorhiza spp., and Trifolium macrocephalum. This woodland and savanna alliance is found on the Columbia Plateau and extends to the northern and western margins of the Great Basin, from southwestern Idaho to the eastern foothills of the Cascades, and south to the Modoc Plateau of northeastern California. Stands range in elevation from under 200 m along the Columbia River in central Washington to over 1800 m in southwestern Idaho. Generally soils are medium-textured, with abundant coarse fragments derived from volcanic parent materials. In central Oregon, the center of distribution, all aspects and slope positions occur. Where this type grades into relatively mesic forest or grassland habitats, the vegetation becomes restricted to rock outcrops or escarpments with excessively drained soils.

Diagnostic Characteristics: This woodland and savanna alliance is characterized by a sparse to open canopy of Juniperus occidentalis and a moderately dense to dense herbaceous layer. Juniperus occidentalis is diagnostic and typically the sole tree species; however, the tree form of Cercocarpus ledifolius may occasionally be present to codominant. The herbaceous layer may be composed of several different characteristic graminoid species such as Achnatherum occidentale, Achnatherum thurberianum, Carex geyeri, Carex filifolia, Festuca idahoensis, Koeleria macrantha, Leymus cinereus, Poa secunda, and Pseudoroegneria spicata.

Rationale for Nominal Species or Physiognomic Features: No Data Available

Classification Comments: This alliance is composed of wooded herbaceous vegetation associations that have an open tree canopy of about 5-15% cover. Additionally, sometimes there is enough shrub cover (1-20%) to form a shrub layer. Some stands with near 20% shrub cover might be confused with ~Juniperus occidentalis Shrubby Woodland Alliance (A3499)$$; however, it is the strong herbaceous layer that characterizes this alliance.

Similar NVC Types: No Data Available
note: No Data Available
Physiognomy and Structure: Stands of this alliance are characterized by a sparse to open canopy (5-30% cover) of evergreen scale-leaved trees which vary widely in density due to edaphic patterns. A patchy layer of evergreen and winter-deciduous shrubs is often present, interspersed in a well-developed layer of cespitose perennial bunch grasses and forbs.

Floristics: This woodland and savanna alliance is characterized by a sparse to open canopy of Juniperus occidentalis and a moderately dense to dense herbaceous layer. Juniperus occidentalis is typically the sole tree species; however, the tree form of Cercocarpus ledifolius may occasionally be present to codominant. Additionally, a sparse to open shrub layer (1-20% cover) may be present. The most common shrub is Artemisia tridentata, but others include Artemisia arbuscula, Artemisia rigida, Cercocarpus ledifolius (shrub form), Chrysothamnus viscidiflorus, Ericameria nauseosa, Purshia tridentata, Ribes cereum, and Symphoricarpos oreophilus. The herbaceous layer is typically dominated by graminoids such as Achnatherum occidentale, Achnatherum thurberianum, Carex geyeri, Carex filifolia, Festuca idahoensis, Koeleria macrantha, Leymus cinereus, Poa secunda, and Pseudoroegneria spicata. Annual forbs (Blepharipappus scaber, Cleome platycarpa, Collinsia parviflora, Crocidium multicaule, Eriogonum vimineum, Mimulus nanus) and perennial forbs (Achillea millefolium, Astragalus filipes, Astragalus purshii, Balsamorhiza sagittata, Calochortus macrocarpus, Castilleja spp., Erigeron filifolius, Eriogonum umbellatum, Lomatium hendersonii, Trifolium macrocephalum) occur and make the association species-rich, but provide very little cover. At the mesic edge of the woodland''s range, occasional emergent Pinus ponderosa, Pseudotsuga menziesii, or other trees become increasingly common. At higher elevations, this alliance becomes surrounded by forests and occurs only on xeric microhabitats within the forest matrix.

Dynamics:  Juniperus occidentalis has been documented to germinate and grow preferentially under the canopy of Artemisia and other shrubs (Everett 1986), and is currently expanding its range into surrounding steppe vegetation types. Burkhardt and Tisdale (1969) noted that larger, older trees are often associated with rock outcrops, while younger trees are prevalent on adjacent alluvial soils. This pattern has also been observed in northeastern California (Barbour and Major 1977). This pattern has been interpreted to mean that Juniperus occidentalis colonizes from rocky refuges which offer shelter from fire, and that the recent expansion of Juniperus occidentalis woodlands can be linked to fire suppression.
Environmental Description:  This woodland and savanna alliance is found on the Columbia Plateau and extends to the northern and western margins of the Great Basin, from southwestern Idaho to the eastern foothills of the Cascades, and south to the Modoc Plateau of northeastern California. Throughout the range the climate is semi-arid, with 23-36 cm of precipitation annually, and a maximum in winter. The temperature regime is cool summer-continental, with a wide range in diurnal temperatures and night frosts through most of the year. Summer lightning storms and associated fire are common in the environments where this type occurs and are presumably important in structuring the vegetation. Elevations range from under 200 m along the Columbia River in central Washington to over 1800 m in southwestern Idaho. Generally soils are medium-textured, with abundant coarse fragments derived from volcanic parent materials or eolian deposits such as pumice over well-cracked Columbia basalt bedrock. In central Oregon, the center of distribution, all aspects and slope positions occur. Where this type grades into relatively mesic forest or grassland habitats, the vegetation becomes restricted to rock outcrops or escarpments with excessively drained soils.
Geographic Range: This woodland alliance is found largely in the Columbia Plateau and occurs along the northern and western edges of the Great Basin from southern California north to central Washington and east to southwestern Idaho. The areas of broadest distribution are central Oregon and the Modoc Plateau of northeastern California. Both areas lie just east of the Cascade Range and have extensive moderate-elevation terrain with gentle topography. This alliance is found west of the Cascade-Sierra axis in the upper Klamath Basin, where it occurs in the rainshadow of the Klamath Mountains.

Nations: US

States/Provinces:  CA, ID, NV?, OR, WA


Confidence Level: Low

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.Nc Western North American Pinyon - Juniper Woodland & Scrub Division D010 1.B.2.Nc
Macrogroup 1.B.2.Nc.1 Singleleaf Pinyon - Utah Juniper - Western Juniper Intermountain Woodland Macrogroup M896 1.B.2.Nc.1
Group 1.B.2.Nc.1.b Western Juniper Open Woodland Group G248 1.B.2.Nc.1.b
Alliance A3500 Western Juniper Grassy Open Woodland Alliance A3500 1.B.2.Nc.1.b
Association CEGL000724 Western Juniper / Curl-leaf Mountain-mahogany / Geyer''s Sedge Wooded Grassland CEGL000724 1.B.2.Nc.1.b
Association CEGL001715 Western Juniper / Little Sagebrush / Sandberg Bluegrass Wooded Grassland CEGL001715 1.B.2.Nc.1.b
Association CEGL001716 Western Juniper / Little Sagebrush / Idaho Fescue Wooded Grassland CEGL001716 1.B.2.Nc.1.b
Association CEGL001717 Western Juniper / Little Sagebrush / Bluebunch Wheatgrass Wooded Grassland CEGL001717 1.B.2.Nc.1.b
Association CEGL001718 Western Juniper / Scabland Sagebrush / Sandberg Bluegrass Wooded Grassland CEGL001718 1.B.2.Nc.1.b
Association CEGL001719 Western Juniper / Big Sagebrush / Threadleaf Sedge Wooded Grassland CEGL001719 1.B.2.Nc.1.b
Association CEGL001720 Western Juniper / Big Sagebrush / Idaho Fescue Wooded Grassland CEGL001720 1.B.2.Nc.1.b
Association CEGL001721 Western Juniper / Big Sagebrush / Bluebunch Wheatgrass Wooded Grassland CEGL001721 1.B.2.Nc.1.b
Association CEGL001722 Western Juniper / Big Sagebrush - Antelope Bitterbrush Wooded Grassland CEGL001722 1.B.2.Nc.1.b
Association CEGL001723 Western Juniper / Curl-leaf Mountain-mahogany / Basin Wildrye Wooded Grassland CEGL001723 1.B.2.Nc.1.b
Association CEGL001724 Western Juniper / Idaho Fescue Wooded Grassland CEGL001724 1.B.2.Nc.1.b
Association CEGL001727 Western Juniper / Sandberg Bluegrass - Western Needlegrass Wooded Grassland CEGL001727 1.B.2.Nc.1.b
Association CEGL001728 Western Juniper / Bluebunch Wheatgrass Wooded Grassland CEGL001728 1.B.2.Nc.1.b
Association CEGL002622 Western Juniper / Antelope Bitterbrush / Idaho Fescue - Bluebunch Wheatgrass Wooded Grassland CEGL002622 1.B.2.Nc.1.b
Association CEGL002635 Western Juniper / Thurber''s Needlegrass Open Woodland CEGL002635 1.B.2.Nc.1.b
Concept Lineage: This alliance is composed of 13 associations from Old Alliance: V.A.6.N.f. Juniperus occidentalis Wooded Herbaceous Alliance (A.1500); a single association from Old Alliance: V.A.6.N.b. Juniperus occidentalis Wooded Tall Herbaceous Alliance (A.1489), and a single herbaceous dominated association from Old Alliance II.A.4.N.a. Juniperus occidentalis Woodland Alliance (A.535). A.535 (in part).

Predecessors: No Data Available

Obsolete Names: No Data Available

Obsolete Parents: No Data Available
Synonomy: > Juniperus occidentalis - Agropyron - Festuca (Hall 1973)
> Juniperus occidentalis / Agropyron spicatum (Driscoll 1964a)
> Juniperus occidentalis / Festuca idahoensis - Agropyron spicatum (Johnson and Simon 1987)
Concept Author(s): R.S. Driscoll (1964a)

Author of Description: K.A. Schulz

Acknowledgements: We have incorporated significant descriptive information previously compiled by M. Jennings and D. Sarr.

Version Date: 03-14-14
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