Ecological dynamics

As ecological condition deteriorates inland saltgrass, black greasewood, rubber rabbitbrush and big saltbush increase. Plants which are tolerant of strongly saline-alkali conditions and the fluctuating water table are dominant on this site. Species most likely to invade this site are annuals.

Fire Ecology:
Black greasewood communities have been historically subject to stand-replacing fire regimes with intervals of <100 years. Black greasewood may be killed by severe fires, but it commonly sprouts soon after low to moderate-severity fire. Torrey’s quailbush has been shown to have reduced flammability due to high moisture and ash contents. Torrey’s quailbush can survive at least some fires. The limited information available suggests that the most likely postfire regeneration strategy of Torrey’s quailbush is seed production.
Alkali sacaton is classified as tolerant of, but not resistant to, fire. Top-killing by fire is probably frequent, and the plants can be killed by severe fire. Saltgrass rhizomes occur deep in the soil where they are insulated from the heat of most fires. Saltgrass survives fire by sending up new growth from rhizomes. Basin wildrye is top-killed by fire. Older basin wildrye plants with large proportions of dead material within the perennial crown can be expected to show higher mortality due to fire than younger plants having little debris. Basin wildrye is generally tolerant of fire but may be damaged by early season fire combined with dry soil conditions.

State and Transition Model Narrative – Group 14

This is a text description of the states, phases, transitions, and community pathways possible in the State and Transition model for MLRA 26 Disturbance Response Group 14. Sites included in this group are R026XY021NV, R026XY013NV, and R026XY004NV.

Reference State 1.0:

The Reference State 1.0 represents the natural range of variability under pristine conditions. The Reference State has three 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 long-term drought and/or insect or disease attack.

Community Phase 1.1:
This community is dominated by black greasewood. Shadscale and rubber rabbitbrush are also common. The herbaceous understory is dominated by basin wildrye and inland saltgrass. Squirreltail and alkali sacaton make up minor components.

Community Phase Pathway 1.1a, from phase 1.1 to 1.2:
A low severity fire would decrease the overstory of black greasewood and allow the understory perennial grasses to increase. Fires are typically low severity and rare due to low fuel loadsbut would result in a mosaic pattern of shrubs and grasses. A fire following an unusually wet spring facilitating an increase in fine fuels may be more severe and reduce black greasewood cover to trace amounts.

Community Phase Pathway 1.1b, from phase 1.1 to 1.3:
Absence of disturbance over time, significant herbivory, long term drought, or combinations of these would allow the black greasewood overstory to increase and dominate the site. This will generally cause a reduction in basin wildrye. Inland saltgrass may increase in the understory depending on the timing and intensity of herbivory. Heavy spring utilization will favor an increase in black greasewood.

Community Phase 1.2:
This community phase is characteristic of a post-disturbance, early-seral community phase. Basin wildrye dominates the community. Black greasewood will decrease but will likely sprout and return to pre-burn levels within a few years. Early colonizers such as rabbitbrush and inland saltgrass may increase.

Community Phase Pathway 1.2a, from phase 1.2 to 1.1:
Time and lack of disturbance will allow shrubs to increase.

Community Phase 1.3:
Black greasewood and shadscale increase in the absence of disturbance. Decadent shrubs dominate the overstory and basin wildrye is reduced either from competition with shrubs, herbivory, drought, or combinations of these.

Community Phase Pathway 1.3a, from phase 1.3 to 1.1:
Fire will decrease the overstory of black greasewood and allow for the perennial bunchgrasses to dominate the site. Fires will typically be high intensity in this phase due to the dominance of greasewood, resulting in removal of the overstory shrub community.

T1A: Transition from Reference State 1.0 to Current Potential State 2.0:

Trigger: This transition is caused by the introduction of non-native annual plants, such as cheatgrass, mustards, halogeton, or Russian thistle.

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. 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.

Current Potential State 2.0:

This state is similar to the Reference State 1.0 with three similar community phases. Ecological function has not changed; however, the resilience 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 Phase 2.1:
This community phase is similar to the Reference State Community Phase 1.1. This community is dominated by black greasewood. Shadscale and rubber rabbitbrush are also common. The herbaceous understory is dominated by basin wildrye and inland saltgrass. Squirreltail and alkali sacaton make up minor components. Non-native annual species such as halogeton, Russian thistle and cheatgrass are present.

Community Phase Pathway 2.1a, from Phase 2.1 to 2.2:
A low severity fire would decrease the overstory of black greasewood and allow the understory perennial grasses to increase. Fires are typically low severity and rare due to low fuel loads but would result in a mosaic pattern of shrubs and grasses. A fire following an unusually wet spring facilitating an increase in fine fuels may be more severe and reduce black greasewood cover to trace amounts. Brush treatments with minimal soil disturbance may also reduce black greasewood and allow for perennial bunchgrasses to increase. Annual non-native species are likely to increase after fire.

Community Phase Pathway 2.1b, from Phase 2.1 to 2.3:
Absence of disturbance over time, significant herbivory, long term drought, or combinations of these would allow the black greasewood overstory to increase and dominate the site. Inappropriate grazing management reduces basin wildrye, and inland saltgrass may increase in the understory.

Community Phase 2.2:
This community phase is characteristic of a post-disturbance, early-seral community phase. Basin wildrye dominates the community. Black greasewood will decrease but will likely sprout and return to pre-burn levels within a few years. Early colonizers such as rabbitbrush and inland saltgrass may increase. Annual non-native species are stable to increasing in the community.

Community Phase Pathway 2.2a, form Phase 2.2 to 2.1:
Absence of disturbance over time and/or grazing management that favors the establishment and growth of black greasewood allows the shrub component to recover.

Community Phase 2.3:
Black greasewood and shadscale increase in the absence of disturbance. Decadent shrubs dominate the overstory and basin wildrye is reduced either from competition with shrubs, herbivory, drought, or combinations of these. Annual non-native species are stable or increasing. This community is at risk of crossing a threshold to the Shrub State.

Community Phase Pathway 2.3a, from Phase 2.3 to 2.2:
Grazing management that reduces shrubs will allow for the perennial bunchgrasses in the understory to increase. Heavy late-fall/winter grazing may cause mechanical damage to black greasewood promoting the perennial bunchgrass understory. Brush treatments with minimal soil disturbance will also decrease black greasewood and release the perennial understory. Annual non-native species are present and may increase in the community. Fire will decrease the overstory of black greasewood and allow for the perennial bunchgrasses to dominate the site. Fires will typically be high intensity in this phase due to the dominance of greasewood, resulting in removal of the overstory shrub community.

T2A: Transition from Current Potential State 2.0 to Shrub State 3.0:
Trigger: To Community Phase 3.1: Inappropriate cattle/horse grazing will decrease or eliminate deep rooted perennial bunchgrasses and favor shrub growth and establishment. To Community Phase 3.2: Severe fire will reduce and/or eliminate the black greasewood overstory. Soil disturbing brush treatments will reduce black greasewood and possibly increase non-native annual species. Lowering of the water table due to groundwater pumping will also decrease basin wildrye and black greasewood, and will allow rabbitbrush and other shrubs to increase.
Slow variables: Long term decline in perennial grass density.
Threshold: Loss of perennial grasses alters nutrient cycling, nutrient redistribution, and reduces soil organic matter. Loss of long-lived black greasewood changes the temporal and the spatial distribution of nutrient cycling depending on the replacement shrub.
Shrub State 3.0:
This state has two community phases, one that is characterized by a dominance of a black greasewood overstory and the other with a rabbitbrush overstory. This site has crossed a biotic and abiotic threshold and site processes are being controlled by shrubs. Bare ground has increased and pedestalling of grasses may be excessive.

Community Phase 3.1:
Black greasewood dominates the overstory. Perennial grasses have significantly declined and may be missing. Annual non-native species increase. Bare ground is significant; however, there may be occasional flushes of annual forbs with certain moisture conditions in winter and spring.

Community Phase Pathway 3.1a, from Phase 3.1 to 3.2:
Long term drought and/or lowering of water table by groundwater pumping would reduce black greasewood and allow rabbitbrush and other shrubs on the site to dominate. Severe fire would also reduce black greasewood overstory and allow for an increase in rabbitbrush.

Community Phase 3.2:
Rabbitbrush is a significant component. Native and non-native annual forbs (primarily mustards) present. Perennial bunchgrasses may be present but are a minor component. Bare ground may be significant in years with little moisture to support an annual community.

Community Phase Pathway 3.2a, from Phase 3.2 to 3.1:
Release from drought conditions may allow black greasewood to increase.

T3A: Transition from Shrub State 3.0 to Annual State 4.0:
Trigger: Fire, or moisture conditions that cause a sudden increase in density and production of annual plants. May be coupled with a lowering of the water table that reduces vitality of perennial species.
Slow variable: Increasing non-native annuals causes an increase of fine fuel loads over time. These fuel loads cause frequent fires, or build up over time until it causes a catastrophic fire.
Threshold: Annual forbs and/or grasses dominate the site. Loss of perennial grasses changes spatial and temporal nutrient cycling and nutrient redistribution, and reduces soil organic matter. Non-native annual species increase in the seedbank and respond positively to fire.

Annual State 4.0:
This state has one community phase characterized by the dominance of annual native and non-native species such as western tansymustard and cheatgrass in the understory. Time since fire may facilitate the maturation of sprouting shrubs. Ecological dynamics are significantly altered in this state. Annual non-native species create a highly combustible fuel bed that shortens the fire return interval. Nutrient cycling is spatially and temporally truncated as annual plants contribute significantly less to deep soil carbon. Some perennial grasses may remain but they are a minor component. Without management, it is unlikely these plants will be able to recruit in the presence of dominant annual plants.

Community Phase 4.1:
Annual non-native species dominate. Black greasewood, other shrubs, and perennial bunchgrasses are a minor component or missing. Soil redistribution and erosion may be significant.

Community Phase Pathway 4.1a, from Phase 4.1 to 4.2:
Fire reduces shrub community and allows annuals to dominate.

Community Phase 4.2:
This is a post-fire community phase. Native and non-native annual forbs and grasses dominate. Black greasewood may be sprouting.