Semi-desert Silt Loam (Wyoming big sagebrush/ Bluebunch wheatgrass)
Scenario model
Current ecosystem state
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Management practices/drivers
Select a transition or restoration pathway
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Transition T1A
The simultaneous introduction of exotic species, both plants and animals, and possible extinctions of native flora and fauna, along with climate change
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Transition T2A
Hot wildfire
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Transition T2B
Seeding
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Transition T3A
Seeding
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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Description
The Reference State is a description of this ecological site just prior to Euro-American settlement but long after the arrival of Native Americans. The description of the Reference State was determined by NRCS Soil Survey Type Site Location information and familiarity with rangeland relict areas where they exist. The major influences on vegetation in the Reference State would have been time since the last fire and where extra wet periods were followed by Aroga moth or snowmold outbreaks. Thus the three phases within State 1 reflect these combined influences on re-setting plant succession. Phase 1.1 is a plant community with mature Wyoming big sagebrush (Artemisia tridentate ssp. wyomingensis) which dominates both cover and production but has many perennial grass and forb companions. Wildfire (1.1a, 1.3a) or an unusually wet period followed by either Aroga moth or snowmold outbreak (1.1b) would have created a grass-dominated phase (1.2) prevailing for approximately one decade. Wyoming big sagebrush would have re-established approximately 10 years from the last fire event and a return to normal climate conditions. As the fire interval and/or periods without Aroga moth or snowmold outbreaks increased (i.e. 40 plus years) (1.1c), the sagebrush would have eventually become over-mature and super-dominant with a diminished understory (1.3). The most abundant grass would have been the caespitose form of bluebunch wheatgrass (Pseudoroegneria spicata). Needle-and-thread (Hesperostipa comata), bottlebrush squirreltail (Elymus elymoides), Indian ricegrass (Achnatherum hymenoides), and small caespitose bluegrasses (Poa spp.) would have also co-occurred. Dominant forbs would have included buckwheat (Eriogonum spp.), biscuitroot (Lomatium spp.), fleabane (Erigeron spp.), milkvetch (Astragalus spp.), and beardtongue (Penstemon spp.), particularly in the mid-seral condition. The patchiness of these natural disturbances would have produced a mosaic of all three phases across the landscape where this ESD occurs. A more complete list of species by lifeform for the Reference State is available in the accompanying tables in the “Plant Community Composition by Weight and Percentage” section of this document.
Submodel
Description
State 2 is similar to Phase 1.3 of State 1 except that some exotic plants, cheatgrass (Bromus tectorum), and mustards (Brassica spp.) and animals (i.e. livestock) are now present. Additionally, some previously influential forces have now since been removed, namely Native American horses and Native American burning practices. The climate has become warmer and the atmosphere holds more carbon dioxide, nitrogen, and sulphur. State 2 is thus a description of the vegetation on this site shortly following Euro-American settlement. Reduction of the native graminaceous understory on these fine-textured soils created a positive feedback including the baring of the soil surface between the shrubs, allowing the wind to move the silt into mounds (coppice dunes) around the larger shrubs (2.1) (West, 1983). Soil organic matter and nutrient reserves concentrated in these coppice dunes, which further contributed to the patchiness of the vegetation. Decades of pronounced removal of the herbaceous component also resulted in a lengthening of the fire-free interval, allowing the height and density of the less palatable sagebrush to increase over that of State 1. Later, both voluntary and mandatory (after 1934 on BLM lands) reductions in numbers of livestock and duration of their grazing (2.1a), brought light recovery of the understory because of the super-dominance of sagebrush and its longevity. In fact, rest from livestock grazing allows the buildup of fire continuous fuels (2.2), especially following wet winters and springs (2.1a). The vegetation is characterized by Wyoming big sagebrush dominance and a caespitose form of bluebunch wheatgrass in the understory.
Submodel
Description
State 3 is characterized by the dominance of several introduced fire-prone species. When fire return intervals are frequent (3.2a) annual species such as cheatgrass and Russian thistle (Salsola spp.) will predominate (3.1). Longer intervals between fire events (3.1a) will result in a plant community dominated by biennial forbs such as knapweeds (Centaurea spp.) and tumble mustard (Sisymbrium altissimum) (3.2). There is little evidence for the return of the native grasses or forbs in State 3, even if all livestock grazing is removed for decades.
Submodel
Description
When livestock grazing is heavy, particularly during the growing season of grasses (4.1a), State 4 will eventually be re-invaded by woody species, first by rabbitbrush (Chrysothamnus spp.) and later followed by which ever sagebrush species has the highest seed rain. Thus, if the maintenance of grass production is desired in State 4, it will require re-treatment of the brush either mechanically, chemically, or with fire (4.2a).
There is little evidence for the unassisted return of native grasses or forbs in State 4, even if all livestock grazing is removed for decades. Diversification of the plant mix in State 4 will probably require scalping of some of the crested wheatgrass and inter-seeding.
Submodel
Mechanism
Transition T1a: from State 1 to State 2 (Reference State to Super-dominant Wyoming Big Sagebrush State)
The simultaneous introduction of exotic species, both plants and animals, and possible extinctions of native flora and fauna, along with climate change, has caused State 1 to transition to State 2. The unrestricted year around grazing on these sites, first by cattle and later by sheep (Parson 1996), was the major cause of the changes between State 1 and State 2. Reversal of such historic changes (i.e. a return pathway) back to State 1 is not practical.
Mechanism
Transition T2a: from State 2 to State 3 (Super-dominant Wyoming Big Sagebrush State to Introduced Annuals & Biennials State)
A hotter, larger fire usually follows a build up of fine fuels in the understory in State 2 leading to a transition from State 2 to State 3. Rest from livestock use may facilitate this fuel loading by allowing for the accumulation of more continuous fine fuels, especially following wet winters and springs.
Reductions in the intensity of livestock grazing or changes in seasons of use have had little effect changing the proportions of shrubs and herbs in these communities due to the super-dominance of sagebrush.
Mechanism
Transition T2b: from State 2 to State 4 (Super-dominant Wyoming Big Sagebrush State to Crested Wheatgrass State)
Because of national demands for red meat production following World War II, many of the areas in State 2 were tilled and seeded to crested wheatgrass to increase forage for livestock.
Mechanism
Transition T3a: from State 3 to State 4 (Introduced Annuals & Biennials State to Crested Wheatgrass State)
Similarly to the impoverished sites in State 2, because of national demands for red meat production following World War II, many of the areas in State 3 were also tilled and seeded to crested wheatgrass (T3a) to increase forage for livestock, especially in areas which re-burned frequently.
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