Ecological dynamics

The information in this ecological site description, including the state-and-transition model (STM), was developed based on historical data, current field data, professional experience, and a review of the scientific literature. As a result, all possible scenarios or plant species may not be included. Key indicator plant species, disturbances, and ecological processes are described to inform land management decisions.

The Swale provisional ecological site in MLRA 52 Dry Grassland consists of four states: The Reference State (1.0), the Invaded State (2.0), the Cropland State (3.0), and the Post-Cropland State (4.0). Plant communities associated with this ecological site evolved under the combined influences of climate, grazing, and fire. Extreme climatic variability results in frequent droughts, which have the greatest influence on the relative contribution of species cover and production (Coupland, 1958, 1961; Biondini et al., 1998). Due to the dominance of cool-season graminoids, annual production is highly dependent upon mid- to late-spring precipitation (Heitschmidt and Vermeire, 2005; Anderson, 2006).

Native grazers also shaped these plant communities. Bison (Bison bison) were the dominant historic grazer, but pronghorn (Antilocarpa americana), elk (Cervus canadensis), and deer (Odocoileus spp.) were also common. Additionally, small mammals such as prairie dogs (Cynomys spp.) and ground squirrels (Urocitellus spp.) influenced this plant community (Salo et al., 2004). Grasshoppers and periodic outbreaks of Rocky Mountain locusts (Melanoplus spretus; Lockwood, 2004) also played an important role in the ecology of these communities.

The historic ecosystem experienced relatively frequent lightning-caused fires with estimated fire return intervals of 6 to 25 years (Bragg, 1995). Historically, Native Americans also set periodic fires. The majority of lightning-caused fires occurred in July and August, whereas Native Americans typically set fires during spring and fall to correspond with the movement of bison (Higgins, 1986). Generally, the mixedgrass ecosystem is resilient to fire and the historic fire return interval had neutral or slightly positive effects on the plant community (Vermeire et al., 2011, 2014). However, studies have shown that shorter fire return intervals can have a negative effect, shifting species composition toward warm-season short-statured grasses (Shay et al., 2001; Smith and McDermid, 2014). Conversely, long-term fire suppression in the 20th century removed periodic fire from the ecosystem altogether. Lack of periodic fires can result in an increase in litter accumulation and, in some cases, provide ideal conditions for seed germination and seedling establishment of non-native annual brome species, such as field, or Japanese, brome (Bromus japonicus; Whisenant, 1990). These species have become naturalized in relatively undisturbed grasslands (Ogle et al., 2003; Harmoney, 2007) and can be present in any state within the scope of this ecological site. They typically do not have a significant ecological impact; however, their presence can reduce the production of cool-season perennial grasses in some cases (Haferkamp et al., 1997). Their abundance varies depending on precipitation and germination conditions.

Improper grazing of this site can result in a reduction in the cover of cool-season midgrasses (Smoliak et al., 1972; Smoliak, 1974). Improper grazing practices include any practices that do not allow sufficient opportunity for plants to physiologically recover from a grazing event or multiple grazing events within a given year and/or that do not provide adequate cover to prevent soil erosion over time. These practices may include, but are not limited to, overstocking, continuous grazing, and/or inadequate seasonal rotation moves over multiple years. Western wheatgrass appears to be relatively resistant to grazing on this site, presumably due to the increased moisture availability and the rhizomatous nature of the grass. Periods of drought can also reduce mid-statured cool-season grasses (Coupland, 1958, 1961). The warm-season species blue grama does not appear to be prevalent on this site; however, needleleaf sedge (Carex duriuscula) can be common and appears to increase under grazing pressure (Adams et al., 2013). Further degradation of the site due to improper grazing can result in reduced vigor of rhizomatous wheatgrasses and an increase in unpalatable forbs such as white sagebrush, more commonly known as cudweed sagewort (Artemisia ludoviciana). The cover of mid-statured bunchgrasses and sedges is severely reduced or absent. This site is quite resilient and has not been documented to cross a threshold into an altered state; however, in this condition the site is highly susceptible to invasion by non-native species. Introduced bluegrasses (Poa spp.) are the most common invasive species.

Due to the productivity of the soils, this ecological site has the potential to be productive cropland. However, conversion to cropland depends on the steepness of the site. Side slopes greater than 8 percent are generally inaccessible to farm equipment. Regardless, many acres have been cultivated and planted to cereal grain crops, such as winter wheat, spring wheat, and barley. Due to the concentration of runoff, this site is very susceptible to erosion when farmed and soil loss is common. Unfarmed swales are also susceptible to deposition when surrounding areas are farmed, and deposition impairs the ecological function of the site. When taken out of production, this site is most commonly seeded back to perennial grass. Introduced species such as pubescent wheatgrass are most common, but native species may also be seeded. Reseeding projects commonly involve extensive earthmoving and soil disturbance, therefore the site is unlikely to return to the Reference State even if seeded to native species. Sites seeded with non-native species may persist as this cover type indefinitely. Sites left to undergo natural plant succession after cultivation will most likely continue to erode and, as a result, have drastically altered soil properties, hydrology, and vegetation. Such a site may stabilize over time and support perennial vegetation, but is it likely that non-native or invasive species will be common. Under ideal circumstances, it may take over 75 years for soil organic matter to return to its pre-disturbed state (Dormaar et al., 1990); it is likely to take much longer in a swale if soil erosion is significant.

The STM diagram suggests possible pathways that plant communities on this site may follow as a result of a given set of ecological processes and management. The site may also support states not displayed in the STM diagram. Landowners and land managers should seek guidance from local professionals before prescribing a particular management or treatment scenario. Plant community responses vary across this MLRA due to variability in weather, soils, and aspect. The reference community phase may not necessarily be the management goal. The lists of plant species and species composition values are provisional and are not intended to cover the full range of conditions, species, and responses for the site. Species composition by dry weight is provided when available and is considered provisional based on the sources identified in the narratives associated with each community phase.

State 1: Reference State
The Reference State contains three community phases. This state evolved under the combined influences of climate, grazing, and fire with climatic variation having the greatest influence on cover and production. In general, this state was resilient to grazing and fire, although these factors could influence species composition in localized areas. Vegetation is characterized by mid-statured cool-season bunchgrasses, mid-statured cool-season rhizomatous grasses, and silver sagebrush. Dense spike-moss, also known as dense clubmoss (Selaginella densa), is commonly present and may constitute significant ground cover; however, it is absent in some areas. Its dynamics are not well understood, and its abundance appears to vary greatly from site to site without discernable reason.

Community Phase 1.1: Green Needlegrass – Rhizomatous Wheatgrass Phase
The Green Needlegrass – Rhizomatous Wheatgrass Phase is dominated by mid-statured cool-season bunchgrasses. Green needlegrass is the primary species although needle and thread is commonly present at low to moderate cover. Western porcupine grass also occurs and is most prevalent north of the Milk River. The mid-statured cool-season rhizomatous grasses are subdominant in this phase. Both western and thickspike wheatgrass may be present, with thickspike becoming more common in the northern extent of this site. Short-statured graminoids, such as blue grama and needleleaf sedge, are not abundant in this phase but are generally present at low cover. Common forbs are American vetch (Vicia americana), large Indian breadroot (Pediomelum esculentum), and common yarrow (Achillea millefolium). Silver sagebrush is the most common shrub and may comprise up to 10 percent of the canopy cover. Snowberry (Symphoricarpos spp.) is sometimes present at low cover. The approximate species composition of the reference plant community is as follows:

Percent composition by weight
Green Needlegrass 35%
Rhizomatous Wheatgrass 25%
Western Porcupine Grass 0-10%
Needle and Thread 5-10%
Other Native Grasses 5%
Perennial Forbs 15%
Shrubs/Subshrubs 5-10%

Estimated Total Annual Production (lbs/ac)*
Low - 900
Representative Value - 1250
High - 1600
*Estimated based on current data – subject to revision

Community Phase 1.2: Rhizomatous Wheatgrass - Needlegrass Phase
In the Rhizomatous Wheatgrass – Needlegrass Phase, mid-statured rhizomatous wheatgrasses have replaced green needlegrass as the dominant plant species. Green needlegrass has been significantly reduced and persists at low cover, typically less than 5 percent of species composition. The mid-statured bunchgrasses are dominated by other needlegrass species, such as needle and thread or western porcupine grass. Needleleaf sedge may also increase in this phase. Unpalatable forbs, such as cudweed sagewort and common yarrow, also increase in abundance. The species structure has begun to shift from dominantly bunchgrasses to dominantly rhizomatous grasses.

Community Phase 1.3: At-Risk Phase
Prolonged drought (approximately 3 years or more) or continued improper grazing may transition the Rhizomatous Wheatgrass – Needlegrass Phase to the At-Risk Phase. In the At-Risk Phase, mid-statured bunchgrasses have been eliminated or nearly so. The plant community is dominated by rhizomatous wheatgrasses, primarily western wheatgrass, but vigor is reduced and unpalatable forbs are common. Cover of blue grama increases, but information suggests that blue grama tends not to dominate this site as it does other sites. The decreased vigor of native species may make this phase more susceptible to invasion by non-native species such as Kentucky bluegrass (Poa pratensis) and Canada bluegrass (Poa compressa).

Community Phase Pathway 1.1a
Drought, improper grazing management, or a combination of these factors can shift the Green Needlegrass – Rhizomatous Wheatgrass Phase (1.1) to the Rhizomatous Wheatgrass - Needlegrass Phase (1.2). These factors favor a decrease in cool-season midgrasses (Coupland, 1961).

Community Phase Pathway 1.2a
The Rhizomatous Wheatgrass - Needlegrass Phase can return to the Green Needlegrass – Rhizomatous Wheatgrass Phase (1.1) with normal or above-normal spring precipitation and proper grazing management.

Community Phase Pathway 1.2b
Prolonged drought, continued improper grazing practices, or a combination of these factors can shift the Rhizomatous Wheatgrass - Needlegrass Phase to the At-Risk Phase. The Rhizomatous Wheatgrass - Needlegrass Phase transitions to the At-Risk Phase when mid-statured bunchgrasses become rare and contribute little to production. In addition, mid-statured rhizomatous grass cover is reduced and vigor is lowered.

Community Phase Pathway 1.3a
The At-Risk Phase (1.3) can return to the Rhizomatous Wheatgrass - Needlegrass Phase (1.2) with normal or above-normal spring precipitation and proper grazing management.

Transition T1A
The Reference State (1) transitions to the Invaded State (2) when aggressive perennial grasses or noxious weeds invade. Kentucky bluegrass is a widespread invasive species in the northern Great Plains (Toledo et al., 2014). Decreased vigor of native species may be one factor that increases susceptibility to grazing. Studies have also shown that exclusion of grazing and fire favors invasive bluegrass species (Dekayser et al., 2013).

Transition T1B
Tillage or application of herbicide followed by seeding of cultivated crops, such as winter wheat, spring wheat, and barley, transitions the Reference State (1) to the Cropland State (3).

State 2: Invaded State
The Invaded State (2) occurs when invasive plant species invade adjacent native grassland communities. Introduced bluegrasses such as Kentucky and Canada bluegrass are the most common concerns. Kentucky bluegrass, in particular, is widespread throughout the Northern Great Plains (Toledo et al., 2014). It is very competitive and displaces native species by forming dense root mats, altering nitrogen cycling, and creating allelopathic effects on germination (DeKeyser et al., 2013). Plant communities dominated by Kentucky bluegrass have significantly less cover of native grass and forb species (Toledo et al., 2014; Dekeyser et al., 2009). Kentucky bluegrass can persist in either grazed or non-grazed sites and can substantially increase under long-term grazing exclusion (DeKeyser et al., 2009; Grant et al., 2009). Effects on soil quality are still unknown at this time, but possible concerns are alteration of surface hydrology and modification of soil surface structure (Toledo et al., 2014). Reduced plant species diversity, simplified structural complexity, and altered biologic processes result in a state that is substantially departed from the Reference State (1).

Noxious weeds are not widespread in MLRA 52, but leafy spurge and Canada thistle have both been documented on this site. These species are very aggressive perennials. They typically displace native species and dominate ecological function when they invade a site. In some cases, these species can be suppressed through intensive management (herbicide application, biological control, or intensive grazing management). Control efforts are unlikely to eliminate noxious weeds, but their density can be sufficiently suppressed so that species composition and structural complexity are similar to that of the Reference State (1). However, cessation of control methods will most likely result in recolonization of the site by the noxious species.

Transition T2A
Tillage or application of herbicide followed by seeding of cultivated crops, such as winter wheat, spring wheat, and barley, transitions the Invaded State (2) to the Cropland State (3).

State 3: Cropland State
The Cropland State (3) occurs when land is put into cultivation. Major crops in MLRA 52 include winter wheat, spring wheat, and barley. The Swale ecological site is highly susceptible to erosion due to the lack of perennial species in this state. Wheat/fallow rotations are also very inefficient at removing moisture from the soil in comparison to native vegetation. In some cases, the Swale ecological site may receive ground-water discharge from surrounding sites. In areas where soils or underlying parent materials have a high amount of soluble salts, a saline seep may develop.

Transition T3A
The transition from the Cropland State (3) to the Post-Cropland State (4) occurs with the cessation of cultivation. The site may also be seeded to perennial forage species, such as pubescent wheatgrass and alfalfa, or a mix of native species. This transition frequently involves extensive earthmoving, grading, and shaping of the site.
 
State 4: Post-Cropland State
The Post-Cropland State (4) occurs when cultivated cropland is abandoned and allowed to either re-vegetate naturally or is seeded back to perennial species for grazing or wildlife use. This state can transition back to the Cropland State (3) if the site is put back into cultivation.

Phase 4.1: Abandoned Cropland Phase
In the absence of active management, the site is susceptible to erosion that is likely to result in downcutting and gullying. Eventually, the site will probably stabilize and re-vegetate to a perennial grassland community, but not before a significant amount of soil has been lost. Due to significant changes in soil structure, organic matter content, and, possibly, hydrology, the site is unlikely to return to the Reference State. Invasion of the site by exotic species, such as Kentucky bluegrass, and annual bromes will depend upon the site’s proximity to a seed source.

Phase 4.2: Perennial Grass Phase
When the site is seeded to perennial forage species, particularly introduced perennial grasses, this community phase can persist for several decades. Smooth brome (Bromus inermis), in particular, is very aggressive. It frequently forms a monoculture and can invade adjacent sites if conditions are favorable. Typically, extensive earthmoving is employed to grade and shape the site prior to seeding. In most cases, the site is seeded to introduced rhizomatous grasses, such as pubescent wheatgrass, to control erosion. A mixture of native species may also be seeded to provide species composition and structural complexity similar to that of the Reference State (1). However, soil properties have been substantially altered and are unlikely return to pre-cultivation conditions within a reasonable timeframe. After reseeding, the surrounding area commonly remains in cropland and the Swale ecological site is managed as a grassed waterway. Under these conditions, the site is subject to deposition from surrounding slopes and requires continual management to prevent sedimentation.

Transition 5A
The Post-Cropland State (4) transitions back to the Cropland State (3) when the site is converted to cropland.