Columbia Plateau Lava Rock Shrubland

This ecological system is found in the Columbia Plateau region and consists of extensive low shrublands. These xeric shrublands occur under relatively extreme soil-moisture conditions. Substrates are typically shallow lithic soils with limited water-holding capacity over fractured basalt. Because of poor drainage through basalt, these soils are often saturated from fall to spring by winter precipitation but typically dry out completely to bedrock by midsummer. Vegetation cover is typically low, generally less than 50% and often much less than that. Vegetation is characterized by an open dwarf-shrub canopy dominated by Artemisia rigida along with other shrub and dwarf-shrub species, particularly Eriogonum compositum, Eriogonum douglasii, Eriogonum microthecum, Eriogonum niveum, Eriogonum sphaerocephalum, Eriogonum strictum, Eriogonum thymoides, and/or Salvia dorrii. Other shrubs are uncommon in this system; mixes of Artemisia rigida and other Artemisia species typically belong to different ecological systems than this. Low cover of perennial bunchgrasses, such as Danthonia unispicata, Elymus elymoides, Festuca idahoensis, or primarily Poa secunda, as well as scattered forbs, including species of Allium, Antennaria, Balsamorhiza, Lomatium, Phlox, and Sedum, characterize these sites. Individual sites can be dominated by grasses and semi-woody forbs, such as Nestotus stenophyllus. Annuals may be seasonally abundant, and cover of moss and lichen is often high in undisturbed areas (1-60% cover).

Total vegetation cover is typically low, generally less than 50% and often much less than that. Vegetation is characterized by an open dwarf-shrub canopy dominated by Artemisia rigida along with other shrub and dwarf-shrub species, particularly Eriogonum spp. Other shrubs are uncommon in this system; mixes of Artemisia rigida and other Artemisia species typically belong to different ecological systems than this. Low cover of perennial bunchgrasses, such as Danthonia unispicata, Elymus elymoides, Festuca idahoensis, or primarily Poa secunda, as well as scattered forbs, including species of Allium, Antennaria, Balsamorhiza, Lomatium, Phlox, and Sedum, characterize these sites. Individual sites can be dominated by grasses and semi-woody forbs, such as Nestotus stenophyllus (= Stenotus stenophyllus). Annuals may be seasonally abundant, and cover of moss and lichen is often high in undisturbed areas (1-60% cover).

This open, low shrubland ecological system is characteristic of the scablands in the Columbia Basin and portions of the Snake River plain. Elevations range from 190-1830 m.

Climate: Climate is semi-arid and temperate with a winter precipitation peak. Mean annual precipitation ranges from 25-50 cm and occurs primarily in the winter as snow or rain.

Physiography/landform: Stands are found on flat to undulating to rolling plateaus, plains, ridgetops and brows. Sites are nearly level to moderately sloping (to 30%). It occurs on all aspects, but is more common on southern slopes, although given that most sites are flat, aspect is not very significant.

Soil/substrate/hydrology: These xeric shrublands occur under relatively extreme soil-moisture conditions. Substrates are typically shallow lithic soils (7-30 cm) with a high percentage of rock fragments (10-70%), limited water-holding capacity over fractured basalt. This moisture is stored in the soil profile and utilized during the typically dry summers. Because of poor drainage through basalt, these soils are often saturated from fall to spring by winter precipitation but typically dry out completely to bedrock by midsummer. The soils are non-calcareous, sandy to clay loams, with pH of 6.3-6.6. Parent material is restricted to colluvium and residuum derived from basalt and acidic lava. Soil surface is mostly rock, erosion pavement (pebble surface), bare ground, and moss. Litter accumulates under the scattered Artemisia rigida plants forming moss-covered mounds up to 20 cm deep. These hummocks persist several years after the death of the dwarf-shrub (Daubenmire 1970, 1992). Moss and lichen cover a significant amount of the ground surface, often with up to 50% cover.

This xeric shrubland ecological system is driven by its tolerance of extreme low soil-moisture conditions and very thin soils that can be easily disturbed or eroded. Stands in this system are generally considered to be late-seral with species composition controlled by the harsh edaphic conditions of the site (Daubenmire 1970, Johnson and Simon 1987). While these soils are often saturated from fall to spring by the winter precipitation, they typically dry out completely to bedrock by midsummer (Daubenmire 1970, 1992, Johnson and Simon 1987). Poa secunda, a typical dominant graminoid, is well-adapted to these conditions because it starts growing early in the spring and completes its reproductive cycle early while there is still moisture in the soil (Daubenmire 1970, 1992, Johnson and Simon 1987). Also, if there is late summer or early fall precipitation, dormant Poa secunda can respond quickly and green up. Daubenmire (1970) and Johnson and Simon (1987) suggest that the basalt bedrock present under these dwarf-shrub/grassland stands is fractured enough to support deeper-rooted dwarf-shrubs. Moss does well in this habitat because of seasonally moist conditions. Artemisia rigida is favored winter browse for elk and deer, and moderately palatable to livestock (Johnson and Clausnitzer 1992).

Frost heaving may be severe, causing local soil disturbance in the winter when these thin, saturated soils freeze and push soil and plants up out of the ground. Pedestalled Artemisia rigida plants and bunchgrasses are common (Daubenmire 1970, Hironaka et al. 1983).

Fire is thought to be unimportant because it is unlikely that the sparse vegetation in these stands could carry a fire. However, if it does occur the Artemisia rigida plants are not tolerant and would be killed (Johnson and Simon 1987, Daubenmire 1992, Johnson and Clausnitzer 1992).

LANDFIRE developed a VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 0810650). This model includes sites where there is potential for pinyon (Pinus monophylla) and/or juniper (Juniperus osteosperma) establishment in classes C and D.

A) Early Development 1 All Structures (5% of type in this stage): Shrub cover is 0-10%. This class is dominated by sprouting buckwheats and other hemi-shrubs, surviving perennial grasses and forbs and annual forbs. Plant cover is typically extremely low. Sagebrush will be absent and patch size is very small in this class. Rock dominates the visual appearance and may dominate satellite imagery. Succession to class B after 10 years.

B) Mid Development 1 Open (5% of type in this stage): Shrub cover is 0-10%. Young stiff sagebrush appears while the other species reach their more-or-less mature sizes. Plant cover remains low but denser patches are now present, composed mostly of the hemi-shrubs and perennial grasses and forbs. Rock is less dominant visually but may still dominate satellite imagery. Succession to class C after 20 years.

C) Late Development 1 Open (90% of type in this stage): Shrub cover is 0-10%. Stiff sagebrush is fully mature and visually dominates the scene, particularly after spring leaf out and flowering. Total vegetation cover rarely exceeds 25% and is often <15%. Plant height rarely exceeds 0.5 m.

Replacement fire was modeled as mean fire-return interval = 250 years in all three classes, with no other disturbances modeled. Severe droughts can temporarily reduce herbaceous vegetation; however, all the species that occupy this BpS are very drought-tolerant (LANDFIRE 2007a).

The biggest threat is exotic invasive plants (Tisdale 1986, Daubenmire 1992). Common exotics include annual grasses, especially Bromus tectorum, and other annual exotic graminoids such as Bromus arvensis, Bromus briziformis, and Taeniatherum caput-medusae; annual forbs such as Epilobium brachycarpum, Erodium cicutarium, Holosteum umbellatum, Lactuca serriola, and Tragopogon dubius; and the perennial forb Hypericum perforatum. Bromus tectorum is moderately dense on some stands and may become abundant during wet years and possibly be dense enough to carry a fire, which would kill fire-sensitive shrubs Artemisia rigida (Bunting et al. 1987, Daubenmire 1992, McWilliams 2003b).

Disturbance from heavy use by livestock or vehicles, particularly on dry soils, disrupts the moss/lichen layer and increases exposed rock and bare ground, increasing the threat of invasion by exotic plants (WNHP 2011). The saturated spring soils are vulnerable to trampling, but the rocky soils discourage livestock (Daubenmire 1992). In areas excluded from grazing entirely, Pseudoroegneria spicata and Festuca idahoensis may dominate with Artemisia rigida in some areas, also growing in rock fractures. In addition to drought tolerance, Poa secunda is also tolerant of grazing and trampling by livestock (Daubenmire 1970, Ganskopp 1979). With disturbance, such as livestock impacts, comes an increase in erosion pavement and bare ground, and a decrease in moss and lichen cover (Daubenmire 1970, Johnson and Simon 1987).

In addition, large-scale wind and solar power development is becoming more common in the region of the system, potentially increasing fragmentation and facilitating establishment of invasive species.

This system occurs in the Columbia Plateau region of southern Idaho, eastern Oregon and eastern Washington, and extreme northern Nevada.