Ecological dynamics
The Saline Lowland (SL) Ecological Site is characterized by the production and composition of the Reference Plant Community, which is defined by soils, precipitation, and the temperature regime influencing the site. The Saline Lowland ecological site receives
additional effective moisture from offsite runoff from precipitation events or stream overflow. The site has saline or saline-sodic conditions within 4 inches of the soil surface which is expressed as an Electrical Conductivity of 4-30 mmhos/cm. This site may also have a seasonal water table within 40 inches of the soil surface.
In the Rocky Mountain Foothills of Central Montana, MLRA 46X LRU B is found where an Ustic soil moisture regime occurs. This area is typified by a frigid soil temperature phase which receives a yearly representative value of 15 to 19 inches of relative effective annual precipitation and between 70 and 110 consecutive frost-free days annually.
Much of the precipitation comes in May and June. Primary plant growth typically occurs between May and early July however due to the increased soil moisture the growing season is extended longer into the summer than other sites in this MLRA. Dominant plants are those that have adapted to these conditions. A period of fall “green-up” can occur amongst this cool season dominated plant community if adequate precipitation is present.
The reference plant community is dominated by alkali sacaton (Sporobolus airoides), basin wildrye (Leymus cinereus), western wheatgrass (Pascopyrum smithii), alkali cordgrass (Spartina gracilis), and Sandberg bluegrass (Poa secunda). Subdominant species may include black greasewood (Sarcobatus vermiculatus), big sagebrush (Artemisia tridentata), winterfat (Krascheninnikovia lanata) and Gardner's saltbush (Atriplex gardnerii).
As the community changes away from reference, cool season shortgrasses tend to increase. If allowed to continue non-native grasses (cheatgrass, field brome, and ventenata) tend to take over the site. Throughout this time bare ground increases exponentially. The short rooted nature of the sodforming grasses erosion can occur rapidly.
Historical records indicate, prior to the introduction of livestock (cattle and sheep) during the late 1800’s, elk and bison grazed this ecological site. Due to the nomadic nature and herd structure of bison, areas that were grazed received periodic high intensity short duration grazing pressure.
Livestock grazing has occurred on most of this ecological site for more than 150 years. The gold boom in the 1860s brought the first herds of livestock overland from Texas, and homesteaders began settling the area. During this time cattle were the primary domestic grazers in the area.
Natural fire was a frequent ecological driver of this Ecological Site however due to the relatively low plant density and fire resistant plants (saltbush and greasewood) stand replacement was rare. The reference community with a high amount of herbaceous growth as a result of favorable growing conditions has the highest susceptibility to extreme fire. A herbaceous invaded community that contains high amounts of exotic annual grasses can greatly increase risk of fire frequency and intensity resulting in potential removal of native species.
Some of the major invasive species that can occur on this site include (but not limited to) spotted knapweed (Centaurea stoebe), leafy spurge (Euphorbia esula), cheatgrass (Bromus techtorum), Canada thistle (Cirsium arvense), dandelion (Taraxicum spp), and Kentucky
bluegrass (Poa pratensis). Invasive weeds have a high impact on this Ecological Site.
Plant Communities and Transitions
A State and Transition Model for this ecological site is depicted below. Thorough descriptions of each state, transition, plant community, and pathway follow the model. This model is based on available experimental research, field data, field observations, and interpretations by experts. It is likely to change as knowledge increases.
The plant communities within the same ecological site will differ across the MLRA due to the naturally occurring variability in weather, soils, and aspect. The biological processes on this site are complex; therefore, representative values are presented in a land management context. The species lists are representative and are not botanical descriptions of all species occurring, or potentially occurring, on this site. They are intended to cover the core species and known range of conditions and responses. Both percent species composition by weight and percent canopy cover are referenced in this document. Canopy cover drives the transitions between communities and states because of the influence of shade, interception of rainfall and competition for available water. Species composition by dry weight remains an important descriptor of the herbaceous community and of the community as a whole. Woody species are included in species composition for the site. Calculating similarity index requires use of species composition by dry weight.
Although there is considerable qualitative experience supporting the pathways and transitions within the State and Transition Model (STM), no quantitative information exists that specifically identifies threshold parameters between grassland types and invaded types in this ecological site. For information on STMs, see the following citations: Bestelmeyer el al. 2003, Bestelmeyer et al. 2004, Bestelmeyer and Brown 2005, Stringham et al. 2003.
State 1
Reference State
The Reference State of this ecological site consists of 2 known potential plant communities 1.1 Reference Bunchgrass Community and 1.2 Wheatgrass Community. These are described below but are generally characterized by a mid-statured, cool season grass community with limited shrub production. Community 1.1 is dominated by a mix of western wheatgrass, winterfat, basin wildrye, alkali sacaton, alkali cordgrass. Community 1.1 is considered the reference while Community 1.2 has a codominance of western wheatgrass, alkali sacaton, saltbush, Sandberg bluegrass, and other shrubs including big sagebrush and black greasewood.
Community 1.1
Reference Bunchgrass Community
In the Reference Bunchgrass Community, basin wildrye, alkali sacaton, and western wheatgrass are dominant. On the wettest of sites, alkali cordgrass will increase. Alkalie bluegrass (now known as Sandberg bluegrass), Inland saltgrass, and prairie Junegrass are also component of the reference state. Big sagebrush, black greasewood, Gardner's saltbush, and winter fat are dominant shrubs. Forbs tend to be restricted to salt tolerant succuents such as seepweed (Suaeda) and poverty sumpweed (Iva axillaris) however several goosefoot species (Chenopodium ssp) may also be present.
About 90 percent of the annual production is from grasses and sedges, most of which is produced during the cool season. Forbs and shrubs each contribute about 5 percent to total annual production. Total vegetative production averages 3000 lbs/ac in normal years, 4500 lbs/ac in favorable years, and 1750 lbs/ac in “unfavorable” years.
Community 1.2
Mixed Grass Community
The Mixed Grass Community is defined by a plant community formed primarily of a codominance of midstatured bunchgrasses and rhizomatous grasses with an increase in forbs and shrubs. This is typically a result of non-prescribed grazing removing some of the basin wildrye and alkali sacaton. If present, basin wildrye may be in such low density it no long contributes to the structural integrity of the community.
This community is extremely susceptible to invasive non-native species due to an increase in bare ground is expected due to a reduction of basal area occupied by the larger bunchgrasses. In this community, cheatgrass possibly exists as a trace amount which poses a risk to the hydrologic function, biotic integrity, and site stability due to its shallow rooting structure and ability to overtake areas.
Pathway 1.1a
Community 1.1 to 1.2
The community pathway from the Reference Bunchgrass Community (1.1) to the Mixed Grass Community (1.2) is primarily driven by improper grazing. When vigor declines enough for plants to die or become smaller, species with higher grazing tolerance, such as western wheatgrass and Sandberg bluegrass, increase in vigor and production as they access the resources previously used by green needlegrass and other tall grasses. Decrease of species composition by weight of the tall grasses to be equal to that of rhizomatous grasses specifically western wheatgrass and thickspike wheatgrass indicates that the reference plant community has shifted to the Mixed Grass Community (1.2). The driver for community shift 1.1A is improper grazing management or prolonged drought. This shift is triggered by the loss of vigor of tall grasses, soil erosion, or prolonged drought coupled with improper grazing. Blaisdell (1958) stated that drought and warmer than normal temperatures are known to advance plant phenology by as much as one month. During drought years, plants may be especially sensitive or in a critical stage of development earlier than expected.
Pathway 1.2a
Community 1.2 to 1.1
The Mixed Grass Community (1.2) will return to the Reference Bunchgrass Community (1.1) with proper grazing management and appropriate grazing intensity. Favorable moisture conditions will facilitate or accelerate this transition. It may take several years of favorable conditions for the community to transition back to a basin wildrye and alkali sacaton dominated state. The driver for this community shift (1.2a) is increased vigor of the tall grasses resulting in increase biomass production and dominance of plant community. The trigger for this shift is the change in grazing management favoring basin wildrye. In general, conservative grazing management styles such as deferred or rest rotations utilizing moderate grazing (less than 50% use) coupled with favorable growing conditions like cool, wet springs are these triggers. These systems tend to promote increases in soil organic matter which promotes microfauna and can increase infiltration rates. Inversely, long periods of rest at a time when this state is considered to be stable may not result in an increase in native bunchgrasses and it has been suggested (Noy-Meir 1975) that these long periods of rest or underutilization may actually drive the system to a lower level of stability by creating large amounts of standing biomass, dead plant caudex centers, and gaps in the plant canopy.
State 2
Shortgrass State
State 2, Shortgrass State, has been altered by long term unmanaged, heavy grazing. In this State, drought conditions may speed the departure from reference.
Community 2.1
Shortgrass Community
The Shortgrass Community (2.1) receives its name by the overall dominance of shortgrasses such as inland saltgrass, sand dropseed, sandberg bluegrass, bottlebrush squirreltail, foxtail barley, and sedges. The mid and tall warm season perennial grasses (alkali sacaton, western/thickspike wheatgrasses, alkali cordgrass, etc.) are either significantly reduced or absent. Poverty weed, knotweed, seepweed, curlycup gumweed and other forbs account for about 10% of the annual production. Winterfat is removed from this plant community and replaced by a dominance of black greasewood, saltbush, and plains pricklypear cactus. This is a result of a combination of long term drought and overgrazing (grazing that exceeds 50 percent utilization without proper rest period and/or repeated seasonal use).
Amount of bare ground is moderately higher than expected. Surface salts are quite extensive.
State 3
Invaded State
Site is invaded by nonnative herbaceous species primarily cheatgrass, field brome, ventenata, and halogeton. This is often a result of reduced vigor in States 1 and 2. The reduced vigor may be attributed to overgrazing however longterm drought may also trigger reduced vigor of these communities
Community 3.1
Invaded Community
The Invaded Community consists primarily of non-native grasses and forbs. The primary species is cheatgrass however ventenata is known to be present in this MLRA allowing for rapid invasion. There tends to be an increase in sagebrush and greasewood cover. Native grasses are often limited to short bunchgrasses although some taller grasses may exist in the protective bases of shrubs. The increase of annual grasses can increase the intensity and severity of wildfire.
The transition to this community is driven by 2 likely disturbances. The first being repeated heavy, unmanaged grazing and the other is intense fire. These often occur in combination which creates bare ground, depletes organic matter, and increases evapotranspiration.
Extensive restoration practices are needed to make this community resemble the reference state however it will never return to reference due to the severe departure and often loss of soil resources needed to maintain reference. Restoration to a community that resembles reference will require extensive and expensive inputs such as pest management, brush management and range seeding however removal of existing species may actually accelerate erosion of the soil surface if not properly managed.
Transition T1
State 1 to 2
This transition is often triggered by long term overgrazing or other human disturbances. Long term drought or altered hydrologic function is also a factor. This system is seasonally wet and if a drying cycle lasts long enough health and vigor of the species present is reduce which allows short grasses to take advantage of limited water resources on the potentially reduced salinity of the soil surface.
Transition T3
State 1 to 3
Invasion of the community by nonnative herbaceous species including cheatgrass, field brome, and halogeton (Halogeton glomerata). Often as a result of long term overgrazing and/or intense fire which reduces vigor of native herbaceous species. This transition is often irreversible to due the high salinity as well as seasonally wet soil conditions.
Restoration pathway R1
State 2 to 1
Improved grazing management with favorable growing conditions may allow deep rooted bunchgrasses to reestablish dominance. Limited prescribed fire may reduce vigor of shortgrasses and shrubs temporarily. Due to the potentially wet soil conditions associated with this site mechanical alteration of the soil and vegetation is not recommended.
Transition T2
State 2 to 3
Invasion of the community by nonnative herbaceous species including cheatgrass, field brome, and halogeton (Halogeton glomerata). Often as a result of long term overgrazing and/or intense fire which reduces vigor of native herbaceous species. This transition is often irreversible to due the high salinity.
