Ecological dynamics
The Reference Plant Community is dominated by low sagebrush and bluebunch wheatgrass. Idaho fescue, Thurber's needlegrass, and a variety of forbs are present. Sandberg bluegrass is the dominant shallow rooted perennial grass. Vegetative composition of the community is approximately 70 percent grasses, 10 percent forbs and 20 percent shrubs.
Ecological Dynamics and Disturbance Response:
Ecological dynamics of this site are primarily driven by interactions between climatic patterns and disturbance regimes. Infrequent and typically small area fires were the historical disturbance that maintained the reference state and drove plant community shifts within the state. Intensity and frequency of these fires is strongly influence by drought cycles and insect or disease attacks on the plant community. Introduction of exotic annual grasses compromises the resistance and resiliency of the site, putting it at higher risk of crossing a threshold into another state.
Periodic drought regularly influences sagebrush ecosystems and drought duration and severity as increased throughout the 20th century in much of the Intermountain West. Major shifts away from historical precipitation patterns have the greatest potential to alter ecosystem function and productivity. Species composition and productivity can be altered by the timing of precipitation and water availability with the soil profile (Bates et al. 2006).
The Great Basin sagebrush communities have high spatial and temporal variability in precipitation both among years and within growing seasons. Nutrient availability is typically low but increases with elevation and closely follows moisture availability. The invasibility of plant communities is often linked to resource availability. Disturbance can decrease resource uptake due to damage or mortality of the native species and depressed competition or can increase resource pools by the decomposition of dead plant material following disturbance. The invasion of sagebrush communities by cheatgrass (Bromus tectorum) has been linked to disturbances (fire, abusive grazing) that have resulted in fluctuations in resources (Chambers et al. 2007).
The range and density of western juniper has increased since the middle of the nineteenth century (Tausch 1999, Miller and Tausch 2000). Causes for expansion of western juniper into sagebrush ecosystems include wildfire suppression, historic livestock grazing, and climate change (Bunting 1994). Mean fire return intervals prior to European settlement in low sagebrush ecosystems were greater than 100 years, however frequent enough to inhibit the encroachment of western juniper into these low productive sagebrush cover types (Miller and Tausch 2000). Thus, trees were isolated to fire-safe areas such as rocky outcroppings and areas with low-productivity. An increase in crown density causes a decrease in understory perennial vegetation and an increase in bare ground. This allows for the invasion of non-native annual species such as cheatgrass. With annual species in the understory wildfire can become more frequent and increase in intensity. With frequent wildfires these plant communities can convert to annual species with a sprouting shrub and juvenile tree overstory.
The species most likely to invade these sites are cheatgrass and medusahead. Medusahead is a cool-season annual grass that germinates in the fall, overwinters as a seedling, and initiates growth in the spring (Miller et al. 1999a). Expansion of Medusahead creates seed reserves that can infest adjoining areas and cause changes to the fire regime. Medusahead has a high silica content which may contribute to its resistance to decomposition (Bovey et al. 1961), and the accumulation of the thatch layer. Mature medusahead is very flammable. Fire can remove the thatch layer, consume standing vegetation, and even reduce seed levels. Furbush (1953) reported that timing a burn while the seeds were in the milk stage effectively reduced medusahead the following year. He further reported that adjacent unburned areas became a seed source for reinvasion the following year. Medusahead can be successfully controlled with a combination of prescribed burning and application of pre-emergent herbicide. Revegetation of medusahead invaded rangelands has a higher likelihood of success when using introduced perennial bunchgrasses such as crested wheatgrass (Davies et al. 2015).
Fire Ecology:
Fire will remove aboveground biomass from bluebunch wheatgrass but plant mortality is generally low (Robberecht and Defossé 1995) because the buds are underground (Conrad and Poulton 1966) or protected by foliage. Uresk et al. (1976) reported burning increased vegetative and reproductive vigor of bluebunch wheatgrass. Thus, bluebunch wheatgrass is considered to experience slight damage to fire but is more susceptible in drought years (Young 1983). Plant response will vary depending on season, fire severity, fire intensity and post-fire soil moisture availability. Sandberg bluegrass (Poa secunda), a minor component of this ecological site, has been found to increase following fire likely due to its low stature and productivity (Daubenmire 1975). Sandberg bluegrass may retard reestablishment of deeper rooted bunchgrass.
Low sagebrush is killed by fire and does not sprout (Young 1983). Establishment after fire is from seed, generally blown in and not from the seed bank (Bradley et al. 1992). Fire risk is greatest following a wet, productive year when there is greater production of fine fuels (Beardall and Sylvester 1976). Fire return intervals have been estimated at 100 to 200 years in black sagebrush dominated sites (Kitchen and McArthur 2007) and likely is similar in the low sagebrush ecosystems; however, historically fires were probably patchy due to the low productivity of these sites. Fine fuel loads generally average 100 to 400 pounds per acre (110 to 450 kg/ha) but are occasionally as high as 600 pounds per acre (680 kg/ha) in low sagebrush habitat types (Bradley et al. 1992). Recovery time of low sagebrush following fire is variable (Young 1983). After fire, if regeneration conditions are favorable, low sagebrush recovers in 2 to 5 years, however on harsh sites where cover is low to begin with and/or erosion occurs after fire, recovery may require more than 10 years (Young 1983). Slow regeneration may subsequently worsen erosion (Blaisdell et al. 1982).
Western juniper is intolerant of fire and historically was kept in restricted sites by natural fires. With the increased suppression of wildfire and livestock grazing which reduces ground fuels and understory competition, regeneration and establishment of western juniper have expanded into suitable sites previously dominated by sagebrush (Burns and Honkala 1990). Fire resistance depends on age of tree. Seedlings, saplings and poles are highly vulnerable to fire. Mature trees, because they have foliage further from the ground, less fine fuels near the trunk and thick bark have some fire resistance (Burns and Honkala 1990). With the low production of the understory vegetation, high severity fires within this plant community were not likely and rarely became crown fires (Bradley et al. 1992, Miller and Tausch 2000). With an increase of cheatgrass in the understory, fire severity is likely to increase. Western juniper reestablishes by seed from nearby seed source or surviving seeds.
Adapted from: Stringham, T.K., D. Snyder, and A. Wartgow. 2016. State-and-Transition Models for USFS Crooked River National Grassland Major Land Resource Area B10 Oregon. DRAFT Report. University of Nevada Reno.
State 1
Historical Reference State
The Reference State 1.0 is a representative of the natural range of variability under pristine conditions. The reference state has 3 general community phases; a shrub-grass dominant phase, a perennial grass dominant phase and a shrub dominant phase. State dynamics are maintained by interactions between climatic patterns and disturbance regimes. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These include the presence of all structural and functional groups, low fine fuel loads, and retention of organic matter and nutrients. Plant community phase changes are primarily driven by fire, periodic drought and/or insect or disease attack.
Community 1.1
Reference Plant Community
The reference plant community is dominated by low sagebrush and bluebunch wheatgrass. Idaho fescue, Thurbers needlegrass, and a variety of forbs are present. Sandberg bluegrass is the dominant shallow rooted perennial grass. Vegetative composition of the community is approximately 70 percent grasses, 10 percent forbs and 20 percent shrubs.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
275 |
410 |
550 |
| Forb |
60 |
90 |
120 |
| Shrub/Vine |
55 |
85 |
110 |
| Tree |
10 |
15 |
20 |
| Total |
400 |
600 |
800 |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Community 1.2
Perennial Bunchgrasses
Deep-rooted perennial bunchgrasses are dominant; forbs may increase.
Community 1.3
Low Sagebrush/Perennial Bunchgrasses
Low sagebrush increases. Deep-rooted perennial bunchgrasses decrease. Young juniper increasing.
Pathway 1.1A
Community 1.1 to 1.2
Low severity fire creates a grass and sagebrush mosaic; high severity fire significantly reduces sagebrush cover and leads to an early- to mid-seral community , dominated by grasses and forbs.
Pathway 1.1B
Community 1.1 to 1.3
Time and lack of disturbance. Excessive herbivory and long-term drought may also reduce perennial understory.
Pathway 1.2A
Community 1.2 to 1.1
Time and lack of disturbance allows for shrub regeneration.
Pathway 1.3A
Community 1.3 to 1.2
Low severity fire creates a sagebrush and grass mosaic.
State 2
Current Potential
This state is similar to the Reference State 1.0 with three similar community phases. Ecological function has not changed, however the resiliency of the state has been reduced by the presence of invasive weeds. Non-natives may increase in abundance but will not become dominant within this State. These non-natives can be highly flammable and can promote fire where historically fire had been infrequent. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These feedbacks include the presence of all structural and functional groups, low fine fuel loads, and retention of organic matter and nutrients. Positive feedbacks decrease ecosystem resilience and stability of the state. These include the non-natives’ high seed output, persistent seed bank, rapid growth rate, ability to cross pollinate, and adaptations for seed dispersal.
Community 2.1
Perennial Bunchgrasses/Low Sagebrush/Annuals
Deep-rooted perennial bunchgrasses and low sagebrush are dominant, with a diverse forb component present. Annual non-native species are present.
Community 2.2
Perennial Bunchgrasses/Annuals
Deep-rooted perennial bunchgrasses are dominant. Annual non-native species are present to increasing. Forbs may increase.
Community 2.3
Low Sagebrush/Annuals
Low sagebrush increases. Deep-rooted perennial bunchgrasses decrease. Young juniper increasing. Non-native annual species present to increasing.
Pathway 2.1A
Community 2.1 to 2.2
Low severity fire creates a grass and shrub mosaic; high severity fire significantly reduces sagebrush cover and leads to an early- to mid-seral community, dominated by grasses and forbs.
Pathway 2.1B
Community 2.1 to 2.3
Time and lack of disturbance. Inappropriate grazing management and long-term drought may also reduce perennial understory.
Pathway 2.2A
Community 2.2 to 2.1
Time and lack of disturbance allows for shrub regeneration.
Pathway 2.3A
Community 2.3 to 2.2
Low severity fire creates a sagebrush and grass mosaic. Brush treatments with minimal soil disturbance; late-fall and winter grazing caused mechanical damage to sagebrush would reduce the shrub overstory.
State 3
Shrub State
This state is a product of many years of heavy grazing during time periods harmful to perennial bunchgrasses. Sandberg bluegrass will increase with a reduction in deep rooted perennial bunchgrass competition and become the dominant grasses. Sagebrush dominates the overstory and rabbitbrush may be a significant component. Sagebrush cover exceeds site concept and may be decadent, reflecting stand maturity and lack of seedling establishment due to competition with mature plants. The shrub overstory and bluegrass understory dominate site resources such that soil water, nutrient capture, nutrient cycling and soil organic matter are temporally and spatially redistributed.
Community 3.1
Sagebrush/Bluegrass/Annuals
Low sagebrush is dominant. Sandberg bluegrass increases. Deep-rooted perennial grasses are a minor component or missing. Non-native annual species present to increasing. Western juniper present to increasing.
Community 3.2
Bluegrass/Sagebrush/Annuals
Sandberg bluegrass is dominant and Low sagebrush decreased. Deep-rooted perennial grasses are a minor component or missing. Non-native annual species present to increasing. Western juniper present to increasing.
Pathway 3.1A
Community 3.1 to 3.2
Fire
Pathway 3.2A
Community 3.2 to 3.1
Time without disturbance
State 4
Annual State
This community is characterized by the dominance of annual non-native species such as medusa head, cheatgrass and tansy mustard in the understory. Sagebrush and/or rabbitbrush may dominate the overstory.
Community 4.1
Low Sagebrush-Rabbitbrush/Annuals
Low sagebrush and rabbitbrush are dominant. Annual non-native species are dominant in the understory. Mat forming forbs increase and western juniper is present or increasing.
Community 4.2
Annuals
Annual non-native species are dominant. Perennial grasses decrease. Western juniper may be present. Rabbitbrush and other sprouting shrubs may increase.
Pathway 4.1A
Community 4.1 to 4.2
Fire
Pathway 4.2A
Community 4.2 to 4.1
Time without disturbance
State 5
Tree State
This state is characterized by a dominance of western juniper in the overstory. Low sagebrush and perennial bunchgrasses may still be present, but they are no longer controlling site resources. Soil moisture, soil nutrients and soil organic matter distribution and cycling have been spatially and temporally altered.
Community 5.1
Western Juniper/Low Sagebrush/Annuals
Western juniper is dominant. Low sagebrush decreases. Deep-rooted perennial grasses decrease. Sandberg bluegrass may increase. Non-native annual species increase.
Community 5.2
Juniper Woodland
Western juniper is dominant. Low sagebrush is a minor component. Deep-rooted perennial grasses are a minor component or missing. Non-native annual species increase and bare ground may be increasing.
Pathway 5.1A
Community 5.1 to 5.2
Time and lack of disturbance allows for maturation of the tree community.
Transition T1A
State 1 to 2
Introduction of non-native species
Transition T2A
State 2 to 3
Grazing management favoring shrubs and/or severe drought will reduce the perennial bunchgrasses in the understory
Transition T2B
State 2 to 4
Catastrophic fire and soil disturbing treatments such as drill seeding, roller chopper, Lawson aerator etc. Inappropriate grazing management in the presence of non-native annual species, may be combined with higher than normal spring precipitation
Constraints to recovery. Probability of success of seeding on this site is low.
Transition T3A
State 3 to 4
Catastrophic fire, multiple fires, and/or soil disturbing treatments, Inappropriate grazing management in the presence of non-native species, may be combined with higher than normal spring precipitation.
Context dependence. Bare ground levels depend on variations in annual precipitation
Transition T3B
State 3 to 5
Time and lack of disturbance allows for maturation of the tree community
Restoration pathway R5A
State 5 to 2
Mechanical treatment of trees coupled with seeding of desired species success of seeding on this site is low.
Restoration pathway R5B
State 5 to 3
Mechanical treatment of trees
Transition T5A
State 5 to 4
Catastrophic fire
