Intermountain Greasewood Flat

This ecological system occurs throughout much of the western U.S. in intermountain basins and extends onto the western Great Plains and into central Montana. It typically occurs near drainages on stream terraces and flats or may form rings around more sparsely vegetated playas. Sites typically have saline soils, a shallow water table and flood intermittently, but remain dry for most growing seasons. The water table remains high enough to maintain vegetation, despite salt accumulations. This system usually occurs as a mosaic of multiple communities, with open to moderately dense shrublands dominated or codominated by Sarcobatus vermiculatus. In high salinity areas, greasewood often grows in nearly pure stands, and on less saline sites, it commonly grows with other shrub species and typically has a grass understory. Other shrubs that may be present to codominant in some occurrences include Atriplex canescens, Atriplex confertifolia, Atriplex gardneri, Atriplex parryi, Artemisia tridentata ssp. wyomingensis, Artemisia tridentata ssp. tridentata, Artemisia cana ssp. cana, or Krascheninnikovia lanata. Occurrences are often surrounded by mixed salt desert scrub or big sagebrush shrublands. The herbaceous layer, if present, is usually dominated by graminoids. There may be inclusions of Sporobolus airoides, Pascopyrum smithii, Distichlis spicata (where water remains ponded the longest), Calamovilfa longifolia, Eleocharis palustris, Elymus elymoides, Hordeum jubatum, Leymus cinereus, Poa pratensis, Puccinellia nuttalliana, or herbaceous types. In more saline environments, Allenrolfea occidentalis, Nitrophila occidentalis, and Suaeda moquinii may be present.

This system is characterized by typically open shrublands dominated or codominated by the deciduous, facultative halophytic shrub Sarcobatus vermiculatus. Associated species vary with salinity, alkalinity, substrates, and depth to ground water and may create a mosaic of types or form rings surrounding a saline basin, depending on environmental variables. Stands are frequently surrounded by less saline mixed salt desert scrub or big sagebrush shrublands. In higher salinity areas, greasewood often grows in nearly pure stands or with halophytes Allenrolfea occidentalis, Atriplex gardneri, Nitrophila occidentalis, or Suaeda moquinii present to codominant, often with the salt-tolerant grass Distichlis spicata present in the understory. In less saline, often alkaline sites, it commonly grows mixed with upland shrub species Atriplex canescens, Atriplex confertifolia, Atriplex parryi, Grayia spinosa, Krascheninnikovia lanata, or Picrothamnus desertorum. In non-saline sites, Artemisia tridentata ssp. wyomingensis, Artemisia tridentata ssp. tridentata, and Artemisia cana ssp. cana may be present to codominant, and in highly disturbed areas, such as sand deposits over playas, disturbance-tolerant species such as Ericameria nauseosa, Chrysothamnus spp., or Gutierrezia sarothrae are abundant. Herbaceous layers range from absent to a moderately dense canopy of medium-tall to short bunchgrasses or sod grasses (0-25% cover). Species include Bouteloua gracilis, Distichlis spicata, Eleocharis palustris, Elymus elymoides, Hordeum jubatum, Juncus arcticus ssp. littoralis (= Juncus balticus), Leymus cinereus, Pascopyrum smithii, Poa secunda (= Poa juncifolia), Puccinellia nuttalliana, or Sporobolus airoides. Sand deposit sites may have Achnatherum hymenoides (= Oryzopsis hymenoides) or other psammophiles. Perennial forbs are typically sparse and often include Achillea millefolium, Artemisia ludoviciana, Astragalus spp., Chenopodium fremontii, Glycyrrhiza lepidota, Grindelia squarrosa, Iva axillaris, Opuntia polyacantha, and/or Sphaeralcea coccinea. Exotic species can be abundant on disturbed weedy sites and include such species as Bassia scoparia (= Kochia scoparia), Bromus arvensis (= Bromus japonicus), Bromus rubens, Bromus tectorum, Descurainia spp., Halogeton glomeratus, Helianthus annuus, Lactuca serriola, Lepidium perfoliatum, and/or Poa pratensis.

This ecological system occurs throughout much of the intermountain western U.S. from the Mojave Desert and extends onto the western Great Plains and into central Montana. Elevation ranges from 100 to 2400 m. Sarcobatus vermiculatus commonly occurs in areas with a seasonally high water table and is often the only green shrub in pluvial desert sites with available groundwater.

Climate: This system is tolerant of a wide range of climatic conditions: warm or cool, temperate, semi-arid and continental, but is most abundant in areas with hot, dry summers. Average annual precipitation ranges from 12.7 to 25.4 cm (5-10 inches).

Physiography/landform: Stands occur on dry, sunny, flat valley bottoms, on lowland floodplains, in ephemeral stream channels, at playa margins, on slopes and in sand dune complexes. Greasewood communities generally occur at lower elevations than moister sagebrush or shadscale zones. In high saline areas, greasewood often grows in nearly pure stands, although on less saline sites, it commonly grows with other shrub species and typically has a grass understory. It typically occurs near drainages on stream terraces and flats or may form rings around more sparsely vegetated playas. Some Sarcobatus vermiculatus stands occur on sandsheets when associated with a shallow water table such as near the Great Sand Dunes National Park and Preserve in Colorado.

Soil/substrate/hydrology: Sites typically have saline/alkaline soils, with a shallow or perched water table and flood intermittently, seasonally to semipermanently (West 1983b). The water table is usually within 5 m of surface, generally well within the root zone of greasewood and saltbush (Donovan et al. 1996). Sites can become dry for much of the growing season, or remain saturated due to poor drainage; however, the water table generally remains high enough to maintain vegetation, which can thrive despite salt accumulations (West 1983b, Knight 1994). Stands occur on floodplains, along the margins of perennial lakes, and in alkaline closed basins with low-gradient shorelines. Substrates are fine-textured saline or alkaline soils, or occasionally coarse-textured non-saline soils (USU 2002). Greasewood flats are typically subirrigated and rarely have open water except when associated with playas. As the water evaporates, salinity increases, affecting the biota.

Greasewood flats are tightly associated with saline soils and groundwater that is near the surface. The primary ecological process that maintains greasewood flats is groundwater recharge, rather than surface water. Sarcobatus vermiculatus is a wetland obligate phreatophyte that taps into groundwater generally at less than 5 m, but taproots may reach great depth (>10 m). Hansen et al. (1995) reported that it can tolerate saturated soil conditions for up to 40 days. Like many facultative halophytes, greasewood is tolerant of alkaline and saline soil conditions that allow the species to occur in sites with less interspecific competition (Ungar et al. 1969, Branson et al. 1976).

Floristic variation within Sarcobatus vermiculatus-dominated vegetation varies with depth to water table, salinity and alkalinity, soil texture, and past land use or disturbance. Hanson (1929) described stands in south-central Colorado and found that pure stands of Sarcobatus vermiculatus and Distichlis spicata are more common on strongly saline/alkaline sites with fine-textured soil and shallow water tables, whereas stands with mixed shrubs such as Chrysothamnus or Artemisia are more common on drier, coarser-textured, low-alkaline sites. Understory dominated by Sporobolus airoides is found on dry, strongly alkaline sites, while stands dominated by Pascopyrum smithii are more common on less alkaline, moist sites in low-lying areas. The degree of salinity can vary seasonally as well as from year to year. During exceptionally wet years, the salt concentration drops, allowing less salt-tolerant species to appear, such as cattails (Typha spp.) or bulrushes (Scirpus and/or Schoenoplectus spp.) (Knight 1994). Some areas only flood during wet years, sometimes only once or twice in a decade. Others will have standing water every spring, except in the driest of years. As stands dry out, strong evaporation concentrates salt in the soils.

Fires are uncommon in this system because many stands are open and lack a continuous fuel layer (Sawyer et al. 2009). Severe hot fires can kill Sarcobatus vermiculatus, while after low- to moderate-severity fire it commonly sprouts after being top-killed (Anderson 2004b). Vigorously sprouting following fire can increase growth and stem density, growing up to 0.76 m (2.5 feet) in height within three years, with 90% of the plants surviving one year after burning (Daubenmire 1970, Anderson 2004b, Sawyer et al. 2009). Fire regime for greasewood communities is reported as generally less than a 100-year return interval (Anderson 2004b) although LANDFIRE (2007a) applied fire regime V (200+ years) and treated fire as a minor ecological driver within this system.

LANDFIRE (2007a) VDDT model for this system (BpS 2311530) has three classes:
A) Early Development 1 All Structures (5% of type in this stage): Shrub cover is 10-20%. Some grasses, with greasewood sprouts present. Some representation of other sprouting species may be present (creosotebush, rabbitbrush). Grass species vary geographically but include the following for Utah and Nevada: inland saltgrass, bottlebrush squirreltail, Sandberg bluegrass and alkali sacaton. Succession to class B after two years.

B) Mid Development 1 Open (30% of type in this stage): Shrub cover (21-60%): Greasewood shrubs are maturing, with a good mix of perennial grasses. Other shrub species that may be found with greasewood include creosotebush and rabbitbrush, and in transition zones to Mojave Desert, it may occur with various sagebrush species and salt desert shrub vegetation (shadscale, saltbushes, winterfat, budsage and spiny hopsage). Greasewood communities would stay in this class for 3-20 years, then succeed to class C. Vegetation will revert to class A with flooding (mean return interval of 75 years) or replacement fire (mean FRI of 200 years).

C) Late Development 1 Closed (65% of type in this stage): Shrubs (41-70%): Greasewood shrubs have reached maturity and will increase canopy closure. Perennial grasses will still be in the understory. Vegetation will revert to class A with replacement fire (mean FRI of 200 years). Flooding (mean return interval of 75 years) causes two transitions: to class A (50% of the time) or to class B (50% of the time).

There was some question in the model about whether flooding in class C (late-development) would send the entire system back to class A (early-development), or Class B (mid-development). As a compromise, flooding was attributed to take both pathways with equal probability.

The major land uses that alter the natural processes of this system are associated with alteration of hydrology, livestock practices, annual exotic species invasion, fire regime alteration, and fragmentation (WNHP 2011). Any activity resulting in hydrological alterations, sedimentation, nutrient inputs, and/or physical disturbance may negatively shift species composition and allow for non-native species establishment. Declining water tables create perennially dry soils, stop surface salt accumulation, and allow salts to leach deeper and create a drier, less saline soil resulting in a change in vegetation composition and pattern (Cooper et al. 2006).

Although Sarcobatus vermiculatus is not ordinarily browsed by livestock, Daubenmire (1970) found that under heavy stocking rates, the shrubs will develop a compact canopy. Hansen et al. (1995) also reported browsing damage with heavy spring and summer grazing. Sarcobatus vermiculatus is noted to be important winter browse for domestic sheep, cattle, big game animals, as well as jackrabbits (Hanson 1929, Anderson 2004b). The shrub provides quality forage throughout the growing season although it contains soluble sodium and potassium oxalates that may cause poisoning and death in domestic sheep and cattle when it makes up too much of their diet (Anderson 2004b). Livestock grazing is reported to decrease small mammal numbers in Sarcobatus vermiculatus / Distichlis spicata (= Distichlis stricta) vegetation in Nevada and adjacent California (Page et al. 1978). Distichlis spicata is considered a grazing increaser. Grazing early when the upper part of the soil may be wet can sometimes cause compaction (WNHP 2011). Grazing and other disturbances can lead to biomass increases in the spring associated with an increase in Bromus tectorum and other fine fuel annuals which influence fire regime (Brown and Smith 2000).

The presence of invasive, exotic plant species such as Acroptilon repens, Cardaria draba, Centaurea diffusa, Centaurea stoebe, Euphorbia esula, Lepidium latifolium, Linaria vulgaris, and Tamarix spp. reduces habitat quality for numerous wildlife species, decreases forage for livestock, reduces ecosystem native species richness, increases soil erosion potential and decreases ecosystem resiliency and resistance to damage from impacts, including climate change. These non-native invasive species decrease the abundance of shorter native grasses and forbs and have the potential to alter structure and composition if they become dominant. The introduction of Bromus tectorum and other annual exotic species into these shrublands has altered fuel loads and fuel distribution allowing for increased fire frequency and severity (Anderson 2004b). Fire drastically alters the community composition because salt-desert shrubs are not adapted to periodic fire (WNHP 2011).

Human development has impacted many locations throughout the range of this system resulting in altered hydrologic regimes, fragmentation, altered fire regime, increased non-native plant species which reduces habitat quality for numerous wildlife species, decreases forage for livestock, reduces ecosystem native species richness, increases soil erosion potential and decreases ecosystem resiliency and resistance to damage from impacts, including climate change.

This system occurs throughout much of the western U.S. in Intermountain basins and extends onto the western Great Plains.