Ecological dynamics
The historic reference community of this site supports a grassland-shrub community. The potential grass understory is dominated by Thurber's needlegrass (Achnatherum thurberianum) with bluebunch wheatgrass (Pseudoroegneria spicata) and minor amounts of Sandberg bluegrass (Poa secunda) also common. Basin big sagebrush (Artemisia tridentata ssp. tridentata) is the most common shrub. Broom snakeweed (Gutierrezia sarothrae) and rabbitbrush (Chrysothamnus spp.) may also occur and may be more common on disturbed sites. Forbs may include members of the buckwheat (Eriogonum spp.), yarrow (Achillea spp.) and astragalus (Astragalus spp.) genera.
Basin big sagebrush is a fire intolerant species that is readily killed by most fires and does not resprout, it will often be reduced on a site following fire and may be eliminated with frequent fires. Periodic insect outbreaks of Aroga moth (Aroga websteri) are an important component of the natural disturbance regime for big sagebrush, resulting in complete or partial mortality of sagebrush plants and potentially impacting thousands of acres.
Gutierrezia is typically associated with disturbed habitats and early seral conditions in the sagebrush biome (Tirmstein 1999). It can rapidly invade following improperly managed grazing. Broom snakeweed is highly drought tolerant and tends to rapidly increase in density following drought periods as well as fire. While broom snakeweed is typically killed by fire, seeds are often unharmed and rapid colonization from nearby sites is common. It has been found to increase following fire in Great Basin sagebrush communities where it often appears by the sixth year but may require up to 25 years to become dominant. While it effectively competes with many grasses (similar rooting depth and possible allelopathy) it is often out-competed by other shrubs overtime.
Thurber's needlegrass is a common bunchgrass in the sagebrush steppe of Eastern Oregon, especially within the 8 to 10 inch precipitation zone (Archer 2000). Considered to be the least fire-resistant needlegrass, Thurber's is often damaged by moderately severe fire. It recovers slowly following fire and regenerates primarily by seed rather than resprouting from crowns.
Bluebunch wheatgrass is considered to be a highly fire adapted grass species with low buds often protected from fire. Recovery following fire is rapid and it often increases relative to other plants post fire, especially after spring burning. While burning may improve the nutritional quality of bluebunch, defoliation during the regeneration period can be very detrimental to the stand and grazing should be avoided immediately after.
Western juniper (Juniperus occidentalis) is a native conifer species in western North America but its density and range have dramatically increased since the late 1800s likely due to a combination of factors, namely: reductions in fire frequency; heavy livestock grazing; and increased atmospheric carbon dioxide (Fryer and Tirmenstein 2019). Juniper is sensitive to fire and most young trees are killed by even low severity fire. As Juniper trees mature and bark thickens, however, they become resistant to low severity fire yet are still killed by crown fires or high severity surface fires.
Climate cycles would have been an important driver of ecological dynamics historically, with drought periods possibly leading to reductions in sagebrush cover and wet years increasing fire occurrence due to increased perennial grass production and fine fuels loads. Historically these communities would have likely encountered infrequent mixed and replacement severity fires with an average fire return interval of 50 years (yet with a high degree of variability depending on the site) (Landfire 2007). Livestock grazing has altered the plant community composition of much of the extent of this site. Increases in shrub and juniper cover and decreases in perennial grass cover may result from chronic improperly managed grazing. Given an altered disturbance regime and degraded site conditions, invasions of exotic forb species and annual grass species may occur on this site. Exotic annual grasses such as cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) may invade and become problematic. Exotic annual grass invasion may increase the frequency of fires and extend the season when fires are likely by augmenting early season fine fuel loads and fuel continuity. Sites may be particularly fire prone following years of above average precipitation during which invasive annual grass production can increase dramatically (Pilliod et al. 2017). In addition to exotic species, native shrub species such as broom snakeweed and rabbitbrush may also increase following disturbance and may be important members of early seral communities.
An understanding of the site specific ecological dynamics for this site are incomplete. Thresholds between states and phases have yet to be quantified and restoration pathways and outcomes are poorly understood. Current and anticipated effects of climate change are not included in this model, yet this site may experience significant impacts as climate continues to change. The model below represents an approximation of ecological dynamics informed by group 4 of (Stringham 2007) and is likely to undergo refinements and revisions as more data becomes available. The reference plant community described below has been determined by study of rangeland relic areas or areas protected from excess disturbance.
State 1
Historical Reference State
The Reference State is representative of the natural range of variability for the site under pristine conditions. The reference state is a bunchgrass shrubland. 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 are maintained by ecosystem processes and structural elements such as the presence of all structural and functional plant groups, the retention of organic matter and the maintenance of plant community cover. Plant community phase changes are primarily driven by infrequent fire and/or periodic drought.
Community 1.1
Reference Plant Community
Bunchgrasses and sagebrush dominate this site. The reference plant community consists of basin big sage and Thurber's needlegrass with bluebunch wheatgrass and some Sandberg bluegrass. Broom snakeweed and rabbitbrush occur in minor amounts. Young western juniper may be present in very small amounts. Ground cover is approximately 60 to 70 percent.
Table 6. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
572 |
762 |
953 |
| Forb |
73 |
101 |
123 |
| Shrub/Vine |
28 |
34 |
45 |
| Total |
673 |
897 |
1121 |
Community 1.2
Bunchgrass dominant community
This community represents an early seral phase following disturbance and is dominated by perennial grasses such as Thurber's needlegrass and bluebunch wheatgrass. Sprouting shrubs will decrease following fire but will soon increase in abundance while non-sprouting shrubs will take longer to recolonize. Western juniper will be reduced or eliminated following fire but may be returning as saplings in this community.
Community 1.3
Shrub dominant community
Following a lack of fire for an extended period, basin big sagebrush and western juniper will increase in abundance while perennial grasses will decrease.
Pathway P1.1a
Community 1.1 to 1.2
Fire occurs with enough severity to kill most of the shrub community.
Pathway P1.1b
Community 1.1 to 1.3
Time and lack of disturbance, such as fire, facilitates an increase in the shrub and tree overstory.
Pathway P1.2a
Community 1.2 to 1.1
Time and lack of disturbance, such as fire, allows for regeneration of the shrub community.
Pathway P1.3a
Community 1.3 to 1.2
Fire occurs with enough severity to kill most of the shrub and tree community.
State 2
Current Potential State
This state is similar to the Reference State. Ecological function has not changed fundamentally, however the resiliency of the site has been reduced by the presence of invasive plants. Additionally, livestock herbivory may be present as a disturbance process and changes in climate may be altering ecological dynamics. Non-native plant species may increase in abundance but will not become dominant or control ecological processes within this state. These species 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 are maintained by ecosystem processes and structural elements such as the presence of all structural and functional groups, and retention of organic matter and nutrients. Positive feedbacks driven by plant community invasion decrease ecosystem resilience and stability of the state. These include exotic plant species' high seed output, persistent seed bank, rapid growth rate, ability to cross pollinate, and adaptations for seed dispersal. Plant community phase changes are primarily driven by infrequent fire, periodic drought and ungulate herbivory.
Current potential plant communities mirror those of the above Reference State 1.0 yet with the addition of a low level of invasive exotic plant invasion and influences of livestock herbivory. Livestock herbivory may result in decreases in deep rooted perennial grasses, and related increases in shallow rooted perennial grasses (such as Sandberg's bluegrass) and shrubs, among other changes.
State 3
Shrub State
Within this state, site resources are primarily controlled by shrub species and western juniper. Native perennial grass composition has been reduced considerably. Sprouting and non-sprouting shrubs as well as western juniper are present. Exotic herbaceous species are often present.
Multiple plant community phases are likely within this state, influenced by livestock herbivory and fire. Sagebrush cover exceeds site concept and may be decadent, reflecting stand maturity and lack of seedling establishment due to competition with mature plants. Sprouting species such as rabbitbrush and broom snakeweed as well as non-sprouting species such as big sagebrush may be dominant. The dominance of site resources by the shrub overstory and Sandberg bluegrass understory leads to a temporal redistribution of soil water, nutrient capture, nutrient cycling and soil organic matter. Western juniper may increase and begin to influence the understory vegetation.
State 4
Invaded State
Within this state, site resources are primarily controlled by exotic annual and perennial herbaceous species. Native perennial grass composition has been greatly diminished. Shrub species and western juniper may also be present. Overtime, with increasing invasion, soil stabilizing perennial root biomass will be decreased. Bare ground will increase potentially increasing erosion even on this low slope site during extreme weather events.
Multiple plant communities are possible within this state, all of which are dominated by invasive annual grasses such as cheatgrass and medusahead as well as invasive annual and perennial forbs. Invader shrub species such as broom snakeweed may also be common
State 5
Tree State
This state is characterized by a dominance of young juniper (less than 100 years old) in the overstory. Big 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 due to competition from Juniper as well as altered community structure. Juniper encroachment can decrease cover of grasses and shrubs by reducing light availability and altering site hydrology through increased interception of precipitation, reduced infiltration and increased erosion. Bare ground will increase and erosion may be increased even on this low slope site during extreme weather events.
Multiple plant community phases may occur within this state, defined in part by the degree of juniper encroachment and the relative composition of other structural and functional groups. Non-native annual plants are likely present in all phases.
State 6
Seeded State
Within this state, site resources are primarily controlled by introduced range grasses such as crested wheatgrass (Agropyron cristatum). Multiple community phases may occur within this state and will include different compositions of native and invasive shrub and grass species and western juniper. Similar to ecological dynamics within other states of this site, shrub species and western juniper will increase with greater time since fire and herbivory pressure. While introduced range grasses may provide some analogous ecological functions to native perennial grasses, they may outcompete native grasses in the long-term.
Transition T1A
State 1 to 2
Trigger(s): This transition is caused by the introduction of non-native annual plants, such as
cheatgrass, medusahead, and mustards.
Slow variables: Over time the annual non-native species will increase within the community.
Threshold: Any amount of introduced non-native species causes an immediate decrease in the resilience of the site.
Constraints to recovery. Altered Processes/Feedback Mechanisms: Annual non-native species cannot be easily removed from the system and have the potential to significantly alter disturbance regimes from their historic range of variation.
Transition T2A
State 2 to 3
Trigger(s): Chronic improperly managed grazing or prolonged time without fire sufficient enough for the deep rooted perennial grass seedbank to diminish, and grazing resistant and/or fire intolerant shrubs to increase.
Slow variables: Long term decrease in deep-rooted perennial grass density.
Altered Processes/Feedback Mechanisms: Loss of deep-rooted perennial bunchgrasses changes nutrient cycling, nutrient redistribution, and reduces soil organic matter.
Transition T2B
State 2 to 4
Trigger(s): Catastrophic fire, soil disturbing treatments or prolonged improperly managed grazing in the presence of non-native annual species; competitive increase in medusahead through lack of active management.
Slow variables: Increased production and cover of non-native annual species; increased
production and cover of medusahead as a percentage of the non-native total annual grass
production and total cover.
Altered Processes/Feedback Mechanisms: Loss of deep-rooted perennial bunchgrasses and shrubs truncates, spatially and temporally, nutrient capture and cycling within the community.
Constraints to recovery. Increased, continuous fine fuels from annual non-native plants modify the fire regime by changing intensity, size and spatial variability of fires.
Transition T2C
State 2 to 5
Trigger(s): Time and lack of disturbance or management action allows for western juniper to dominate.
Slow variables: Over time the abundance and size of trees will increase.
Altered Processes/Feedbacks: Trees dominate ecological processes.
Context dependence. This may be coupled with grazing management that favors tree establishment by reducing understory herbaceous competition for site resources.
Restoration pathway R3A
State 3 to 2
Shrub management and seeding of native species coupled with minimal soil disturbance, potentially requiring the use of herbicide.
Context dependence. Risk of failure may be high due to droughty nature of the site, shallow soils and low resilience.
Transition T3A
State 3 to 4
Trigger(s): Catastrophic fire, soil disturbing treatments or prolonged improperly managed grazing in the presence of non-native annual species; competitive increase in medusahead through lack of active management.
Slow variables: Increased production and cover of non-native annual species; increased
production and cover of medusahead as a percentage of the non-native total annual grass
production and total cover.
Altered Processes/Feedback Mechanisms: Increased, continuous fine fuels modify the fire regime by changing intensity, size and spatial variability of fires. Changes in plant community composition and spatial variability of vegetation due to the loss of perennial bunchgrasses and sagebrush truncate energy capture spatially and temporally thus impacting nutrient cycling and distribution.
Constraints to recovery. Increased, continuous fine fuels modify the fire regime by changing intensity, size and spatial variability of fires
Transition T3B
State 3 to 5
Trigger(s): Time and a lack of disturbance or management action allows for western juniper to dominate site. This may be coupled with grazing management that favors tree establishment by reducing understory herbaceous competition for site resources.
Slow variables: Over time the abundance and size of trees will increase.
Altered Processes/Feedback Mechanisms: Trees dominate ecological processes.
Restoration pathway R3B
State 3 to 6
Brush management such as mowing, coupled with seeding of deep rooted non-native wheatgrasses. Targeted herbicide treatments may be necessary to facilitate this restoration pathway.
Context dependence. If non-native annual grasses are present, restoration attempts causing soil disturbance will likely initiate a transition to an annual state
Restoration pathway R4A
State 4 to 5
Reduction of annual grasses through management actions such as herbicide, prescribed grazing, or mowing, and seeding of desired species.
Context dependence. Risk of failure may be high due to droughty nature of the site and low resilience.
Restoration pathway R4A
State 4 to 6
Seeding of deep-rooted bunchgrasses. Targeted herbicide treatments are often necessary to facilitate this restoration pathway.
Context dependence. The probability of success is extremely low due to the low resilience of the site.
Restoration pathway R5B
State 5 to 3
Tree removal practices with minimal soil disturbance.
Transition T5A
State 5 to 4
Trigger(s): Catastrophic, stand replacing fire or inappropriate tree removal practices with soil disturbance will cause a transition to the Invaded State.
Slow variables: Increased production and cover of non-native annual species under tree canopies.
Altered Processes/Feedback Mechanisms: Changes in plant community composition and spatial variability of vegetation due to the loss of perennial bunchgrasses and sagebrush truncate energy capture and impact nutrient cycling and distribution.
Constraints to recovery. Closed tree canopy with non-native annual species dominant in the understory changes the intensity, size and spatial variability of fires.
Restoration pathway R5A
State 5 to 6
Tree removal and seeding of desired non-native wheatgrass species. Tree removal practices with minimal soil disturbance are recommended. Herbicide treatment may be necessary.
Context dependence. Probability of success declines with increased presence of non-native annual species.
Transition T6A
State 6 to 3
Trigger(s): Repeated, heavy, growing season grazing will decrease or eliminate deep rooted perennial bunchgrasses, increase Sandberg bluegrass and favor shrub growth and establishment. Severe fire will remove sagebrush overstory, decrease perennial bunchgrasses and enhance Sandberg bluegrass.
Slow variables: Long term decrease in deep-rooted perennial grass density.
Altered Processes/Feedbacks: Loss of deep-rooted perennial bunchgrasses changes nutrient cycling, nutrient redistribution, and reduces soil organic matter.
