Ecological dynamics
Communities of this ecological site are dynamic in response to changes in disturbance regimes and weather patterns. Primary natural disturbances affecting this site include periodic wildfire, disease, insect attack and drought. Plant community structure is controlled in part by infrequent wildfire and in part by pulses of seedling recruitment. Species growing on this site are adapted to receiving limited annual precipitation and typically do not experience drought related mortality. However, prolonged drought will result in reduced perennial herbaceous components and increased shrub dominance.
Black sagebrush (Artemisia nova) is an evergreen, aromatic shrub that is low-growing and decumbent. Black sagebrush is highly drought tolerant; it is more likely to endure drought than most sagebrush taxa. Black sagebrush occurs throughout the successional process, it is highly light tolerant and shade intolerant (Fryer, 2009). Mature plants are slightly salt tolerant and flood intolerant. The root system of black sagebrush maximizes water uptake with a deep taproot and shallow branching roots. Roots can be deep on favorable sites, but shallow soils prevent deep root development on most sites where black sagebrush is dominant. The combination of deep and shallow roots provides excellent soil stabilization. Black sagebrush tends to have a larger number of fibrous roots than big sagebrush species. The breakdown of aging roots also contributes to organic matter and nutrient cycling in the sagebrush system. Carbon and nitrogen concentration are higher under sagebrush canopies when compared to interspaces (Chen and Stark, 2000).
Black sagebrush flowers from midsummer to mid-fall across its range. Seeds have no morphological adaptations for dispersal and commonly fall close to the parent plant (Fryer, 2009). Seeds typically overwinter and germinate the spring after production. Seeds may germinate as early as April on some sites. Survival of sagebrush seedlings is dependent on adequate moisture conditions. Young plants are susceptible to less than desirable condition for several years following germination (Fryer, 2009). Ephemeral leaves are shed during summer drought, while persistent leaves remain on the branches through winter.
Muttongrass (Poa fendleriana) is a cool-season perennial bunchgrass with occasional short rhizomes and considered to be drought resistant (Howard, 1997). Muttongrass prefers open, dry conditions, but can tolerate partial shade. It can persist throughout the successional process (Howard, 1997). Muttongrass and other perennial bunchgrasses growing on this site improve ecological resistance and resilience by increasing infiltration, decreasing runoff and increasing soil organic matter.
Sagebrush species are generally long-lived, therefore it is not necessary for new individuals to recruit every year for perpetuation of the stand. Infrequent large recruitment events and simultaneous low, continuous recruitment is the foundation of population maintenance (Noy-Meir, 1973). Mature properly functioning sagebrush communities have higher infiltration rates and lower sediment production, than degraded systems. Reoccurring disturbances, natural or anthropogenic, will result in decreased sagebrush cover and increased cover of disturbance tolerant shrubs and non-natives. Loss of structural and functional groups impacts ecosystem functioning and can result in soil loss. Improper grazing or recreation management can result in the reduction, or potential loss, of black sagebrush degrading ecosystem function.
Semiarid rangelands are characterized by spatially variable vegetative cover. Patches of vegetation or individual plants intercept runoff, increase infiltration and nutrient cycling, providing favorable sites for additional vegetation to establish. Infiltration of water in to the soil surface and possibly deeper into the soil profile, increases water stores. Water stored in the soil profile promotes biological activity by organisms, such as soil invertebrates, to form soil aggregate and macro-pores further enhancing infiltration (Ludwig et al., 2005). Interspaces or bare ground does not capture and store water in the same way as vegetated patches. Runoff is often discharged from the system and is therefore not available to plants. Runoff also redistributes soil and nutrients down slope. This results in less water and nutrients available for plant use, reducing plant production, vigor and seedling establishment.
Changes in species composition can occur in black sagebrush communities following repeated wildfire and abusive land management practices. Following wildfire, green ephedra (Ephedra viridis), Stansbury's cliffrose (Purshia stansburiana), and desert ceanthous (Ceanothus greggii) increase in canopy cover. Chronic excessive grazing by domestic livestock allows yellow rabbitbrush (Chrysothamnus viscidiflorus), Fremont’s mahonia (Mahonia fremonti), spiny greasebush (Glossopetalon spinescens) and black sagebrush to increase; while muttongrass, Indian ricegrass (Achnatherum hymenoides), and other perennial grasses and palatable shrubs decrease. Pinyon and juniper cover will increase on this site in the absence of natural occurring fire. Non-native species found on this site include cheatgrass (Bromus tectorum).
Fire Ecology:
There are limited records of historic fire frequencies or fire regimes for black sagebrush communities. Black sagebrush tends to occupy unproductive sites that have little fuel build-up, so they likely experienced extended fire return intervals. Black sagebrush plants are readily killed by all fire intensities. Following wildfire, black sagebrush is initially replaced by sprouting species. Reestablishment of sagebrush occurs through wind-dispersed seed sources. Black sagebrush requires 15 to 60 years post fire to regain dominance (Fryer, 2009). Therefore, frequent and repeated fires can easily eliminate it from a site. Modern invasion of non-native annual grasses provide a continuous fuel layer, changing the wildfire dynamics of this community.
Fire effects on Stansbury cliffrose are variable. Fire may kill or severely damage plants, at best it is a weak sprouter. Late-season fire increases the risk of mortality. Mojave ceanothus, spiny greasebush and green ephedra are favored by fire and may increase is cover following fire. Muttongrass is unharmed to slightly harmed by light-severity fall fires. It appears to be harmed by and slow to recover from severe fire. Indian ricegrass is killed by fire, but readily reestablishes on burned sites through seed.
State 1
Reference State
The reference state is representative of the natural range of variability under naturally stable, pre-European settlement conditions. Vegetation is dominated by black sagebrush, Stansbury cliffrose, desert ceanothus and muttongrass with scattered perennial forbs. Primary natural disturbances affecting this ecological site include periodic wildfire, insect attack, disease and drought. Community phases are dynamic in response to changes in disturbance regimes and weather patterns.
Community 1.1
Reference community
The reference community is representative of this ecological site under naturally stable, pre-European settlement conditions. The reference plant community is dominated by black sagebrush and muttongrass. Desert ceanothus, Stansbury cliffrose and Indian ricegrass are other important species associated with this site. Annual and perennial forbs are present, but remain inconspicuous or dormant until adequate precipitation is available. Approximate ground cover (basal and foliar) is 20 to 30 percent. Potential vegetative composition is 25 percent grasses, 15 percent forbs, 60 percent shrubs and less than or equal to 1 percent trees.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Shrub/Vine |
235 |
300 |
425 |
| Grass/Grasslike |
95 |
125 |
175 |
| Forb |
19 |
70 |
90 |
| Tree |
1 |
5 |
10 |
| Total |
350 |
500 |
700 |
Community 1.2
Bunchgrasses/shrubs
Black sagebrush and other shrubs decrease. Perennial bunchgrasses increase.
Community 1.3
Black sagebrush
Black sagebrush increases. Perennial bunchgrasses decrease. Scattered pinyon and juniper trees are present or increasing.
Pathway 1.1A
Community 1.1 to 1.2
Fire or other disturbance removes the sagebrush canopy.
Pathway 1.1B
Community 1.1 to 1.3
Absence of disturbance and natural regeneration of shrubs over time.
Pathway 1.2A
Community 1.2 to 1.1
Absence of disturbance and natural regeneration of shrubs over time.
Pathway 1.3A
Community 1.3 to 1.2
Fire or other disturbance that removes the sagebrush canopy.
State 2
Invaded State
The invaded state is characterized by the presence of non-native species. Compositionally, State 2 is similar to the reference state with a trace of non-native species in the understory. However, ecological resilience is reduced by the presence of non-natives, making it more difficult for this state to recover following disturbance. Low frequency, low intensity disturbances maintains the grass, shrub-grass and shrub community phases. However, non-natives are favored if disturbance regimes vary from the range of historic variation. Black sagebrush is characterized by a pronounced delay in establishment following disturbance. Therefore, frequent, repeated fire fueled by non-native species can eliminate black sagebrush from a site.
Community 2.1
Black sagebrush/perennial bunchgrasses/non-native species
Black sagebrush is dominant. Perennial native bunchgrasses are sub-dominant. Perennial native forbs and other native shrubs are present. Pinyon and juniper trees are scattered, if present. Annual non-native species are present.
Community 2.2
Perennial bunchgrasses
Black sagebrush and other shrubs decrease. Perennial bunchgrasses increase. Annual non-native species are stable or increasing.
Community 2.3
Black Sagebrush
Black sagebrush increases. Perennial bunchgrasses decrease. Annual non-native species are stable or increasing. Pinyon and juniper trees are scattered and may be increasing.
Pathway 2.1A
Community 2.1 to 2.2
Fire or other disturbance removes the shrub canopy.
Pathway 2.1B
Community 2.1 to 2.3
Absence of disturbance and natural regeneration of shrubs over time. This may be coupled with inadequate rest of recovery from defoliation.
Pathway 2.2A
Community 2.2 to 2.1
Absence of disturbance that removes sagebrush canopy.
Pathway 2.3A
Community 2.3 to 2.2
Fire or other disturbance that removes the sagebrush canopy.
State 3
Tree State
The Tree State is characterized by an over dominance of shrubs and reduced perennial herbaceous understory. Traces of deep-rooted perennial bunchgrasses remain in the community. Non-native annual grasses and forbs are abundant in the understory. A biotic threshold has been crossed with the loss of the deep-rooted perennial bunchgrasses, reducing ecological resilience of the site. The loss of structural and functional groups (deep-rooted perennial bunchgrasses and shrub seedlings), result in decreased herbaceous production, soil moisture and organic matter inputs. Changes in infiltration and runoff rates contribute to reduced soil moisture availability thereby reducing reproductive potential and recruitment of native species.
Community 3.1
Tree/Sagebrush
Pinyon and juniper trees are increasing and becoming dominant. Black sagebrush is decreasing. Annual species dominate the understory. Native perennial bunchgrasses are found in trace amounts. Bare ground is increasing.
Community 3.2
Tree Dominated
Pinyon and juniper trees dominate the overstory. Black sagebrush is decadent or dead. Annual species dominate the understory. Bare ground is significant. Rill erosion is possible in this phase.
Pathway 3.1A
Community 3.1 to 3.2
Absence of disturbance and natural regeneration over time.
State 4
Tree State
The tree state is characterized by infilling of pinyon and juniper, with tree cover greater than 20 percent. Lack of natural disturbances including fire, disease or insect attack allows pinyon and juniper to increase in cover, dominate and change site dynamics. Understory vegetation is reduced through shading, duff accumulation and competition for water and nutrients. Feedbacks contributing to the stability of this alternative state include, reduced infiltration resulting from reduced vegetative cover, lower soil moisture preventing the establishment of vegetation, as well as, the density, the rate of spread and dominance of trees all contributing to an abiotic threshold being crossed. The ability of a site to capture, transport and store water is directly related to vegetative cover. Prolonged drought results in an overall decline of the plant community and reduced annual growth of pinyon and juniper.
Community 4.1
Annual Non-Native Species
Cheatgrass, Russian thistle, halogeton, or annual mustards dominate the plant community.
Community 4.2
Annual Non-Native Species/Sprouting Shrubs
Sprouting shrubs increase. Black sagebrush and perennial native bunchgrasses are a minor component, if present. Annual species dominate the understory.
Pathway 4.1A
Community 4.1 to 4.2
Absence of disturbance and natural regeneration of shrubs over time.
Pathway 4.2A
Community 4.2 to 4.1
Fire or other disturbance that removes the shrub canopy.
Transition T1A
State 1 to 2
Trigger: Introduction of non-native species.
Slow variables: Surface disturbances, changes in the kinds of animals and their grazing patterns, drought and/or changes in fire history that altered the recruitment rates of native species.
Threshold: Non-native invasive species cannot be easily removed from the system and have the potential to significantly alter disturbance regimes from their historic range of variation. Changes in the herbaceous understory, associated with the introduction of non-native annuals, reduce ecological resistance and resilience following a disturbance.
Transition T2A
State 2 to 3
Trigger: Chronic inadequate rest and recovery from defoliation and/or prolonged drought.
Slow variables: Changes in historic wildfire patterns and frequency. Repeated, heavy growing season grazing changes natural recruitment and recovery rates of perennial herbaceous species.
Threshold: Reduction of deep and shallow-rooted perennial bunchgrasses and increased bare ground/interspaces changes soil hydrology, decreasing infiltration and increasing runoff.
Transition T2B
State 2 to 4
Re-occurring severe fire.
Transition T3A
State 3 to 4
Trigger: Infiling by pinyon and juniper. Possibly coupled with chronic inadequate rest and recovery from defoliation and/or prolonged drought.
Slow variables: Changes in historic wildfire patterns and frequency, as well ass, natural recruitment of native species over time.
Threshold: Reduction of understory (deep-rooted perennial bunchgrass and woody perennials) and increased bare ground changed soil hydrology, including infiltration and runoff.
