Ecological dynamics

The information in this ecological site description, including the state-and-transition model (STM) (Figure 2), 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 Discharge Closed Depression provisional ecological site in MLRA 52 Dry Grassland consists of five vegetative states: The Reference State (1), the Invaded State (2), the Undrained Cropland State (3), the Impounded State (4) and the Drained Cropland State (5). Historically, plant communities associated with the Discharge Closed Depression ecological site evolved under the combined influences of climate, grazing, hydrology, and fire. Extreme climatic variability results in frequent droughts, which can have great influence on the relative contribution of species cover and production (Coupland, 1958, 1961; Biondini et al., 1998).

Hydrology is a crucial dynamic on this site. The site receives water from groundwater discharge from adjacent sites and surface runoff from spring snowmelt and from high intensity thunderstorms. The duration of ponding, or hydroperiod, dramatically influences the vegetation of the site. The hydroperiod varies depending by the catchment size and by annual precipitation patterns. The majority of sites in MLRA 52 contain water for only a few weeks in the spring. Larger catchment basins, above average precipitation cycles, or a combination of these factors may increase the hydroperiod to several months. Plant communities vary depending on the hydroperiod.

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

The historic ecosystem also 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, fires were less frequent on the Discharge Closed Depression ecological site than on adjacent drier sites, however, early reports indicate that fires did occur in wetlands (Higgins, 1986). The Discharge Closed Depression ecological site is resilient to fire and the historic fire return interval most likely had very little effect on the plant community except for removing excess litter accumulations. However, long term fire suppression in the 20th century removed periodic fire from the ecosystem altogether, which may result in an increase in litter accumulation. In some cases, this may provide ideal conditions for seed germination and seedling establishment of invasive species such as Canada thistle (Cirsium arvense).

Plant communities on the Discharge Closed Depression ecological site are very complex. Much of the dynamics of this site are still under investigation and are not fully understood. Frequently, sites contain multiple plant community zones that correspond to the hydroperiod for that portion of the site. In the Ephemeral Phase (1.1) the hydroperiod is only 7 to 10 days early in the spring and the site supports facultative upland species. The center of the depression is dominated by a wheatgrass plant community intermixed with scattered plants of foxtail barley. Periods of above average precipitation or depressions with a moderate catchment size will transition to the Temporarily Ponded Phase (1.2). In this phase the hydroperiod increases to 1.5 to 4 weeks and the site begins to support more hydrophytic vegetation. There are typically two distinct vegetation zones with a wheatgrass-foxtail barley plant community occupying the center of the depression. The rim of the depression transitions to wheatgrass plant community. The Seasonally Ponded Phase (1.3) occurs primarily in depressions that have large catchment basins, although it can occur in moderate-sized catchment basins during periods of above average precipitation. The hydroperiod for this phase is much longer, typically 1 to 4 months, and is sufficient for a significant amount of hydrophytic vegetation to establish. The center of the depression frequently supports a foxtail barley dominated plant community or an inland saltgrass-Nutall’s alkaligrass plant community, this is surrounded by a wheatgrass-foxtail barley plant community, and a rim of wheatgrass. Plant communities in all phases are often dynamic and diversity varies from site to site. Further study is needed to fully describe all major species and plant community dynamics.

Disturbances to the Discharge Closed Depression ecological site can have significant effects on hydrology and vegetation. Disturbances that directly affect the site include, but are not limited to, excavation, draining, impoundment, conversion to cropland, and grazing. The effects of improper grazing of this site have not been documented in detail, but improper grazing is known to cause a reduction in palatable forage species and an increase in unpalatable species such as curlycup gumweed (Grindelia squarrosa) and mountain rush (Juncus arcticus ssp. littoralis), also known as Baltic rush. 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 may not be providing adequate cover to prevent soil erosion over time. This may include, but is not limited to, overstocking, continuous grazing, and/or inadequate seasonal rotation moves over multiple years.

The most common disturbance on this site is most likely conversion to cropland. Smaller depressions, particularly those with ephemeral hydroperiods, are frequently farmed through with no further alteration. Seasonally ponded depressions are typically too wet to farm without artificial drainage. Typically, water is drained by ditching, then the site is converted to cropland. In these cases, the natural hydrology is severely altered and the site is unlikely to return to the Reference State (1) without significant restoration. Another common alteration of hydrology is impoundment of water for livestock or wildlife. Impoundment increases the hydroperiod and effectively converts the site from a temporarily or seasonally ponded wetland to a semi-permanent wetland.

Invasive species are a common concern on this ecological site. Non-native species such as curly dock (Rumex crispus) and salt lover (Halogeton glomeratus) commonly invade this site. Sites that are in the Ephemeral Phase (1.1) or the Temporarily Ponded Phase (1.2) are particularly prone to invasion by these species. Invasive species on seasonally ponded sites have not been studied, however invasive species such as saltcedar (Tamarix ramosissima) may be a concern in some areas. Noxious weeds such as Canada thistle and leafy spurge (Euphorbia esula L.) have also been documented on this site.

The state-and-transition model (STM) diagram and legend (Figure 2) 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 vegetative 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 State (1) 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 (1) contains three community phases. Ecological dynamics of this state are still under investigation, therefore, this model may not cover the full range of conditions, species, and responses for the state. Seasonal ponding is a key dynamic on this site and varies depending on annual precipitation patterns and catchment size. Vegetation is characterized concentric rings, or zones, within the depression that correspond to the hydroperiod of that particular zone. The Ephemeral Phase (1.1) may only exhibit one vegetation zone dominated by facultative upland species. Other phases may exhibit two or more zones with the most hydrophytic vegetation in the center of the depression and subsequent, drier plant communities toward the edges. Diagnostic plant associations that occur on the Discharge Closed Depression ecological site are western wheatgrass, foxtail barley, western wheatgrass-foxtail barley, and inland saltgrass-Nutall’s alkaligrass (Crowe and Kudray, 2003). Plant communities can be diverse and may contain numerous other species in addition to the diagnostic species. Table 5 contains a list of species that have been observed in the Reference State (1).

Community Phase 1.1: Ephemeral Phase
The Ephemeral Phase (1.1) typically occurs in depressions with small catchment areas, but can also occur in depressions with moderate catchment areas during drought years. The hydroperiod in this phase is very short, typically 10 days or less. The western wheatgrass plant community is the only one present, frequently with scattered plants of foxtail barley intermixed.

Community Phase Pathway 1.1a
In small catchment areas, 2 to 3 years of above-average precipitation will transition the Ephemeral Phase (1.1) to the Temporarily Ponded Phase (1.2). Moderate-sized catchment areas will transition to this phase during average precipitation years.

Community Phase 1.2: Temporarily Ponded Phase
The Temporarily Ponded Phase (1.2) typically occurs in depressions with moderate-sized catchment areas during normal years, but may also occur in small catchment areas during wet cycles or large catchment areas during drought. In this phase the hydroperiod ranges from 1.5 to 4 weeks. Vegetation begins to exhibit zonation in this phase. At the center of the depression, ponding is longest and a wheatgrass-foxtail barley plant community appears. A western wheatgrass plant community is usually present around the rim of the depression. A number of minor graminoid species such as American sloughgrass and Baltic rush (Juncus arcticus ssp. littoralis) may also be present.

Community Phase Pathway 1.2a
In moderate-sized catchment areas, drought will transition the Temporarily Ponded Phase (1.2) to the Ephemeral Phase (1.1). Small catchment areas will transition to this phase during average precipitation years.




Community Phase Pathway 1.2b
In moderate-sized catchment areas, 2 to 3 years of above-average precipitation will transition the Temporarily Ponded Phase (1.2) to the Seasonally Ponded Phase (1.3).

Community Phase 1.3: Seasonally Ponded Phase
The Seasonally Ponded Phase (1.3) typically occurs in depressions with large catchment areas, although it can occur in moderate-sized catchment areas during wet cycles. The hydroperiod in this phase is 1 to 4 months and the vegetation typically exhibits three or more zones. The wettest zone in this phase is frequently a foxtail barley-dominated plant community, but some areas may support an inland saltgrass-Nutall’s alkaligrass plant community. This transitions into a middle zone of wheatgrass-foxtail barley or wheatgrass-inland saltgrass. The rim of the depression in this phase is typically a western wheatgrass dominated plant community. A number of minor species such as common threesquare, Baltic rush, and native forbs may also be present.

Community Phase Pathway 1.3a
On large catchment areas, drought will transition the Seasonally Ponded Phase (1.3) to the Temporarily Ponded Phase (1.2). Moderate-sized catchment areas will transition to this phase during average-precipitation years.

Transition T1A
Tillage or application of herbicide and seeding of cultivated crops such as wheat, barley, or introduced hay transitions the Reference State (1) to the Undrained Cropland State (3). This transition occurs primarily in the Ephemeral Phase (1.1) or the Temporarily Ponded Phase (1.2).

Transition T1B
Artificial impoundment of water by damming or excavation transitions the Reference State (1) to the Impounded State (4).

Transition T1C
Introduction of invasive species transitions the Reference State (1) to the Invaded State (2).

Transition T1D
The combination of artificial drainage, tillage or herbicide application, and seeding of annual crops transitions the Reference State (1) to the Drained Cropland State (4). This transition occurs primarily in the Seasonally Ponded Phase (1.3).

State 2: Invaded State
The Invaded State (2) occurs when invasive plant species invade adjacent native plant communities. Introduced species such as curly dock and salt lover are the most common concerns. Effects of these species on site dynamics are not well documented at the time of this writing, but reduced plant species diversity, simplified structural complexity, and altered biologic processes are likely concerns.

Noxious weeds are not widespread on the Discharge Closed Depression ecological site, however, leafy spurge and Canada thistle have both been documented as a concern on this site (Cooper and Jones 2003; Crowe and Kudray 2003; Jones 2004). Saltcedar has not been documented on this site, but is present on nearby riverine sites. It could potentially expand into the Seasonally Ponded Phase (1.3) of this site (Vance et al., 2013). These species are very aggressive perennials that typically displace native species and dominate ecological function when they invade a site. Sometimes, these species can be suppressed through intensive management (herbicide, 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.

Community Phase 2.1: Invaded Community Phase
The Invaded Community Phase (2.1) occurs when non-native species or noxious weeds invade the site. In the Ephemeral Phase (1.1) or the Temporarily Ponded Phase (1.2), curly dock and salt lover are the primary invasive species. Noxious weeds such as Canada thistle may also be a concern in some phases. Because invasive species have not been well documented in the Seasonally Ponded Phase (1.3), conclusive determinations are not available.

Transition T2A
Tillage or application of herbicide, and seeding of cultivated crops such as wheat or barley transitions the Invaded State (2) to the Undrained Cropland State (3). This transition occurs primarily in the Ephemeral Phase (1.1) or the Temporarily Ponded Phase (1.2).

Transition T2B
Artificial impoundment of water by damming or excavation transitions the Reference State (1) to the Impounded State (4).

State 3: Undrained Cropland State
The Undrained Cropland State (3) occurs when native vegetation is killed out, either by tillage or by herbicide application, and the site is seeded to annual crops. No other alterations are made to the natural hydrology or soils. This state typically only occurs in the Ephemeral Phase (1.1) and the Temporarily Ponded Phase (1.2). In this state the site is suitable for spring seeded crops such as spring wheat and barley, although seeding of crops may be delayed 2 to 3 weeks due to wet soil conditions. In wet years, this state may be too wet to farm and will transition to the Wet/Foxtail Barley Phase (3.2).

Community Phase 3.1: Annual Crops Phase
The Annual Crops Phase (3.1) occurs when land is put into cultivation. Major crops include spring wheat, barley, and peas.

Community Phase Pathway 3.1a
One or more years of above average precipitation transitions the Annual Crops Phase (3.1) to the Wet/Foxtail Barley Phase (3.2)

Community Phase 3.2: Wet/Foxtail Barley Phase
The Wet/Foxtail Barley Phase (3.2) occurs when precipitation is above average and the site is too wet to seed crop. Foxtail barley and annual weeds such as Kochia (Bassia scoparia) may colonize the site.
Community Phase Pathway 3.2a
Average or below-average precipitation transitions the Wet/Foxtail Barley Phase (3.2) to the Annual Crops Phase (3.1)

Transition T3A
Cessation of annual cropping combined with the introduction of invasive species transitions the site from the Undrained Cropland State (3) to the Invaded State (2). Non-native species such as curly dock and Canada thistle are common.

Restoration Pathway R3A
Cessation of annual cropping combined with the reestablishment of native species transitions the site from the Undrained Cropland State (3) to the Reference State (1). Specialized seeding techniques may be necessary, depending on site conditions, as well as intensive weed control to prevent invasion of non-native species, such as curly dock and Canada thistle. These restoration methods are labor intensive, costly and may not be a practical in all situations.

State 4: Impounded State
The Impounded State (4) occurs when water is artificially impounded on the site by damming or excavation. Water may be impounded for livestock water or for wildlife. Impoundment of water typically transitions the site to a semi-permanent wetland with open water, deep marsh, and shallow marsh vegetation zones. In many cases impoundment creates a larger wetland with high quality habitat for wildlife. In other cases, such as when an excavated pond is constructed in the center of the depression, a small semi-permanent wetland is created in the center while the remainder of the depression is drained, thus reducing total wetland area. In either case the natural hydrology of the site is significantly altered, resulting in a new state with different plant communities and ecological dynamics.

Community Phase 4.1: Bulrush Phase
The Bulrush Phase (4.1) occurs when water is impounded and the site is semi-permanently ponded. The hydroperiod in this phase is typically 6 to 9 months. Vegetation zonation has reached its maximum. Typically the center of the site is open water, which is surrounded by a deep marsh zone dominated by hardstem bulrush (Schoenoplectus acutus), and common threesquare (Schoenoplectus pungens). The rim of the depression is characterized by a shallow marsh zone supporting foxtail barley or inland saltgrass-Nutall’s alkaligrass. A wet meadow or low prairie zone of western wheatgrass-foxtail barley may form at the outermost edges where ponding duration is shortest.

Restoration Pathway R4A
Restoration of natural hydrology and reestablishment of native species transitions the site from the Impounded State (4) to the Reference State (1). Specialized seeding techniques may be necessary, depending on site conditions, as well as intensive weed control to prevent invasion of non-native grasses and noxious weeds. Restoration of natural hydrology may require removal of dams or refilling of excavated pits. These restoration methods are labor intensive, very costly, and may be impractical, perhaps even detrimental, in some situations.


State 5: Drained Cropland State
The Drained Cropland State (5) occurs when the site is drained, tilled or sprayed, and seeded to annual crops. Surface water is typically drained by means of surface ditches, diversions, or both. Following drainage, remaining native vegetation is killed out either by tillage or herbicide application, then the site is seeded to annual crops. The hydrology of the site is significantly altered and no longer functions in its natural condition.

Community Phase 5.1: Drained Cropland Phase
The Drained Cropland Phase (5.1) occurs when land is drained and put into cultivation. Major crops include spring wheat, winter wheat, and barley.

Restoration Pathway R5A
Cessation of annual cropping combined with restoration of natural hydrology and reestablishment of native species transitions the site from the Drained Cropland State (5) to the Reference State (1). Specialized seeding techniques may be necessary, depending on site conditions, as well as intensive weed control to prevent invasion of non-native grasses and noxious weeds. Restoration of natural hydrology may require removal of diversions, plugging drainage ditches, or both. These restoration methods are labor intensive, very costly, and may be impractical in some situations.