Columbia Plateau Western Juniper Woodland

This woodland system is found along the northern and western margins of the Great Basin, from southwestern Idaho, along the eastern foothills of the Cascades, south to the Modoc Plateau of northeastern California. Elevations range from under 200 m along the Columbia River in central Washington to over 1500 m. Generally, soils are medium-textured, with abundant coarse fragments, and derived from volcanic parent materials. In central Oregon, the center of distribution, all aspects and slope positions occur. Where this system grades into relatively mesic forest or grassland habitats, these woodlands become restricted to rock outcrops or escarpments with excessively drained soils. The vegetation is characterized by an open stand of Juniperus occidentalis with an understory of open shrub-steppe (big sage, bitterbrush and/or rabbitbrush) with perennial bunchgrasses representing the dominant vegetation. Pinus monophylla is not present in this region, so Juniperus occidentalis is typically the only tree species, although Pinus ponderosa or Pinus jeffreyi may be present in some stands. Cercocarpus ledifolius may occasionally codominate. Artemisia tridentata is the most common shrub; others are Purshia tridentata, Ericameria nauseosa, Chrysothamnus viscidiflorus, Ribes cereum, and Tetradymia spp. Graminoids include Carex filifolia, Festuca idahoensis, Poa secunda, and Pseudoroegneria spicata. These woodlands are generally restricted to rocky areas where fire frequency is low. Throughout much of its range, fire exclusion and removal of fine fuels by grazing livestock have reduced fire frequencies and allowed Juniperus occidentalis seedlings to colonize adjacent alluvial soils and expand into the sagebrush shrub-steppe and grasslands. Juniperus occidentalis savanna may occur on the drier edges of the woodland where trees are intermingling with or invading the surrounding grasslands and where local edaphic or climatic conditions favor grasslands over shrublands.

Pinus monophylla is not present in this region, so Juniperus occidentalis is the only tree species, although Pinus ponderosa or Pinus jeffreyi may be present in some stands. Cercocarpus ledifolius may occasionally codominate. Artemisia tridentata is the most common shrub; others are Purshia tridentata, Ericameria nauseosa, Chrysothamnus viscidiflorus, Ribes cereum, and Tetradymia spp. Graminoids include Carex filifolia, Festuca idahoensis, Poa secunda, and Pseudoroegneria spicata.

This woodland system is found along the northern and western margins of the Great Basin, from southwestern Idaho, along the eastern foothills of the Cascades, south to the Modoc Plateau of northeastern California (Tirmenstein 1999h, Sawyer et al. 2009). Elevations range from under 200 m along the Columbia River in central Washington to over 1500 m. In northwestern California stands range from 700 to 2300 m elevation (Tirmenstein 1999h, Sawyer et al. 2009).

Climate: Throughout the range the climate is cool, semi-arid, continental with 200-360 mm of precipitation annually, with the majority falling in winter. The temperature regime is cool in summer, with a wide range in diurnal temperatures and night frosts occurring most of the year. Summer lightning storms and associated fire are common and are presumably important in structuring the vegetation. (Franklin and Dyrness 1973).

Physiography/landform: In central Oregon, the center of the woodland's range, stands are found on all aspects and slope positions. Where this type grades into relatively mesic forest or grassland habitats, the vegetation becomes restricted to rock outcrops or escarpments with excessively drained soils.

Soils/substrate/hydrology: Juniperus occidentalis stands occur on a wide variety of soil types. Generally, soils are well-drained, shallow and stony with rock outcrops common, but soils may be deeper. They are medium-textured, with abundant coarse fragments, and derived from volcanic parent materials such as basalt, andesite, rhyolite, pumice, volcanic ash, tuff, welded tuff, as well as colluvial, alluvial, or eolian material (Tirmenstein 1999h, LANDFIRE 2007a). Soils derived from pumice ash are the most common edaphic characteristic of this woodland (LANDFIRE 2007a). Origins of the pumice sands are Mount Mazama and Newberry Crater (Miller et al. 1999). In most other areas, it occurs on rimrock, shallow soil scablands and in other isolated pockets.

Juniperus occidentalis is a long-lived tree that can exceed 3000 years in age in rocky, fire-protected areas such as along rimrock (Waigchler et al. 2001, Thorne et al. 2007). These fire sensitive trees do not sprout following fire and are typically killed by moderate to severe fires (Tirmenstein 1999h, Sawyer et al. 2009). Young junipers have thin bark and are readily killed by surface fires (Martin et al. 1978), whereas mature trees with thicker bark are described as "moderately resistant" (Fowells 1965). Reproductive age begins at about 20 years, peaks after 50 years and continues for many years (Miller and Rose 1995, Tirmenstein 1999h). Following stand-replacing fire, recovery time is relatively slow and depends on stand maturity, the size and season of burn, fire severity and juniper mortality, the persistence of the seeds in the seed bank, location of seed source, the presence of animal dispersers such as Clark's nutcrackers, competition from herbaceous species and shrubs, and the amount of post-fire precipitation (Burkhardt and Tisdale 1976, Tirmenstein 1999h). Large burns and long distances from seed sources slow recovery rates because seed dispersal is dependent on water and animals (Tirmenstein 1999h).

Juniperus occidentalis woodlands become "closed" at about 40% canopy cover when lateral tree roots fill interspaces between trees (Young et al. 1982, Thorne et al. 2007). At this stage cover of shrub and herbaceous layers begin to rapidly decline (Thorne et al. 2007).

Juniperus occidentalis savanna often occurs on the drier edges of the woodland where trees are intermingling with or invade the surrounding grasslands where local edaphic or climatic conditions favor grasslands over shrublands. Stands occur between the ponderosa zones and the sagebrush moisture zones and are expanding into big sagebrush steppe areas at a fairly rapid rate, creating extensive young stands, increasing the acreage of this type by more than five times (LANDFIRE 2007a, BpS 0910170). Western juniper woodlands and savannas experienced both large- and small-scale natural disturbances (LANDFIRE 2007a). Small-scale fires (less than 5 acres) and insects and disease kill single trees to small patches of trees throughout the stand on a frequent interval. Large-scale fires (>1000 acres) are less common, occurring once every 500 years or more (Miller et al. 1999). Drought can cause dieback and death of trees.

Areas where this system occurs contain some of the largest concentrations of ancient trees. Individuals may exceed 2000 years of age. These ancient western juniper woodlands provide important wildlife habitat. Cavities form in older trees and are important for many neotropical migrants. Western juniper cone-berries provide food for many animals, including elk, deer, coyotes, and small mammals such as mice, chipmunks, rabbits, squirrels, and woodrats; many such as coyotes serve as important dispersing agents of the junipers (Schupp et al. 1997, Tirmenstein 1999h). They are also used by wintering birds such as the American robin and Townsend solitaire (Burkhardt and Tisdale 1969, Eddleman 1984, Tirmenstein 1999h). This juniper is also an important food source for insects with 25 species of bark and wood boring beetles identified (Miller et al. 2005).

LANDFIRE developed a VDDT model for this system which has five classes (LANDFIRE 2007a, BpS 0910170):

A) Early Development (herbaceous-dominated with 0-60% cover - 2% of type in this stage): Herbaceous plants dominate this stage immediately following disturbance. Perennial bunchgrasses dominate the plant community. However, in the first few years following disturbance annual plants may dominate while perennial grasses and forbs recover. Succession to class B after 30 years. (Replacement and mixed fires).

B) Mid Development 1 Open (shrub-dominated with 0-30% cover - 5% of type in this stage): Shrubs dominate this stage. The composition of the shrub layer will be dependent on soil depth and climatic factors. Rabbitbrush will most likely be the dominant shrub following disturbance. However, big sagebrush, bitterbrush and wax current may also be found. Western juniper seedlings and saplings are present throughout the shrub layer. Western juniper has established below the canopy of the shrub layer. Shrub cover is approaching 20% on more productive sites but is most likely <15%. Herbaceous plants are being suppressed by the increase in woody plants. Succession to class C after 45 years. (Mixed and replacement fires).

C) Mid Development 2 Open (shrub/tree mix, tree cover 0-20% - 15% of type in this stage): Western juniper forms an even-aged woodland. Trees are characterized by regular conical shapes. Shrubs are being suppressed by the emerging woodland. Herbaceous vegetation is also being suppressed by the competition from woody plants. Succession to class E (late closed) after 45 years. (Mixed and replacement fires. Certain sites are edaphically constrained and thus transition to class D - late-open).

D) Late Development 2 Open (shrub/tree mix, tree cover 0-20% - 35% of type in this stage): Ancient western juniper savanna or open woodland composed of multiple structural layers. Some western juniper trees have dead portions in their canopies. Canopies are irregular in shape. Young trees can be found in open areas where recent small-scale disturbances occurred. Edaphic factors often maintain wide spacing between junipers. Understory grasses remain dominant and variable. (Maintains in class D. Many disturbances cause transitions to younger or more open conditions).

E) Late Development 1 Open (tree-dominated 20-40% cover - 43% of type in this stage): Ancient western juniper woodland composed of multiple structural layers. Some western juniper trees have dead portions in their canopies. Canopies are irregular in shape. Young trees can be found in open areas where recent small-scale disturbances occurred. Understory grasses are variable, based on slope, aspect and soil depth. (Maintains in class E. Many disturbances cause transitions to younger or more open conditions) (LANDFIRE 2007a).

Conversion of this type has commonly come from catastrophic crown fires and "chaining" or mechanical removal of trees by land management agencies to convert woodlands to grasslands for livestock (Stevens 1999a, 199b, Stevens and Monsen 2004). Common stressors and threats include heavy grazing by livestock which removes the fine fuel layer that carries low-intensity fire. This results in an unnatural build-up of woody fuels, so when fires occur, they are large, high-intensity, severe fires that remove juniper from the system. If exotic species are present, post-crown fire and post-treatment outcomes may result in conversion to exotic species. Exotic annual grasses such as Bromus tectorum can replace the community creating an annual grassland which will be maintained by frequent fires (Mack 1981b, D'Antonio and Vitousek 1992, D'Antonio et al. 2009).

Some stands of this system contain ancient trees over 2000 years old. These ancient western juniper woodlands provide important wildlife habitat such as nesting cavities for neotropical migrants and berries for food (LANDFIRE 2007a). Uncharacteristic stand-replacing fire threatens these ancient stands.

Throughout much of the range of this system, Juniperus occidentalis populations are expanding into contiguous Artemisia shrub-steppe (Burkhardt and Tisdale 1976, Miller and Rose 1995, Bates et al. 2014). The reasons for this are not entirely clear, but Juniperus occidentalis has been documented to germinate and grow preferentially under the canopy of Artemisia and other shrubs (Everett 1986). 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 1988). This pattern has been interpreted to mean that Juniperus occidentalis is colonizing out from rocky refuges which offer shelter from fire, and that the recent expansion of Juniperus occidentalis woodlands can be linked to fire suppression (Bates et al. 2014). Active fire suppression and removal of fine fuels by grazing livestock have reduced fire frequency and allowed Juniperus occidentalis seedlings to colonize adjacent alluvial soils and expand into the shrub-steppe and grasslands (Tirmenstein 1999h, Bates et al. 2014).

Human development has impacted many locations throughout the ecoregion. High- and low-density urban and industrial developments also have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirect through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species. Management actions such as chaining juniper stands creates a large food source of injured junipers for insects such as western juniper bark beetle (Miller et al. 2005). However, insect attacks usually do not result in the killing of live trees, unless combined with drought such as in the 1920s and 1930s when western junipers were killed by insects in central Oregon (Furniss and Carolin 1977).

This woodland and savanna system is found along the northern and western margins of the Great Basin, from southwestern Idaho, along the eastern foothills of the Cascades, south to the Modoc Plateau of northeastern California (Tirmenstein 1999h, Sawyer et al. 2009). It also occurs in scattered localities of northern Nevada and south-central Washington. This system is most abundant in central and south-central Oregon (Franklin and Dyrness 1973, Tirmenstein 1999h, Sawyer et al. 2009).