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 Recharge 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 Recharge 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 surface runoff primarily 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 on catchment size and 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. In addition, small mammals such as ground squirrels (Urocitellus spp.) also utilized this plant community (Salo et al., 2004). Grasshoppers and periodic outbreaks of Rocky Mountain locusts (Melanoplus spretus) may have also 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 Recharge Closed Depression ecological site than on adjacent drier sites; however, early reports indicate that fires did occur in wetlands (Higgins, 1986). The Recharge 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. This practice lead to an increase in litter accumulation, which in some cases, may provide ideal conditions for seed germination and seedling establishment of invasive species, such as Canada thistle (Cirsium arvense).

Plant communities on the Recharge Closed Depression ecological site are very complex. Much of the dynamics of this site are still under investigation and are not fully understood. Sites commonly 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 that transitions to a wheatgrass-sedge community toward the rim. 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 wetland vegetation. There are typically two distinct vegetation zones with a wheatgrass-spikerush plant community occupying the center of the depression. The rim of the depression transitions to wheatgrass or a wheatgrass-sedge 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 the establishment of a significant amount of hydrophytic vegetation. The center of the depression commonly supports a spikerush-dominated plant community, which is surrounded by a wheatgrass-spikerush plant community, and the rim supports wheatgrass or wheatgrass-sedge species. 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 Recharge 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 on 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 white sagebrush (Artemisia ludoviciana), and mountain rush (Juncus arcticus ssp. littoralis), also known as or 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 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.

The most common disturbance on this site is conversion to cropland. Smaller depressions, particularly those with ephemeral hydroperiods, are frequently farmed with no further alteration. Seasonally ponded depressions are typically too wet to farm without artificial drainage. Typically, water is drained by ditching and 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 due to 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 perennial grasses, such as Kentucky bluegrass (Poa pratensis) and smooth brome (Bromus inermis), are widespread throughout the Great Plains (Toledo et al., 2014). Sites that are in the Ephemeral Phase (1.1) or the Temporarily Ponded Phase (1.2) are particularly prone to invasion by these species. Wetter, seasonally ponded sites appear to have fewer non-native species but may still be affected by more ponding-tolerant species, such as reed canarygrass (Phalaris arundinacea) and creeping meadow foxtail (Alopecurus arundinaceus). Noxious weeds, such Canada thistle and leafy spurge (Euphorbia esula L.), have also been documented on this site.

The state-and-transition (STM) diagram (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 by 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 where the most hydrophytic vegetation is in the center of the depression and subsequent, drier plant communities are toward the edges. Diagnostic plant associations that occur on the Recharge Closed Depression ecological site are western wheatgrass, western wheatgrass-needleleaf sedge, western wheatgrass-needle spikerush, and common spikerush (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) occurs in depressions with small catchment areas but can also occur in depressions with moderate-sized catchment areas during drought years. The hydroperiod in this phase is very short, typically 10 days or less. The western wheatgrass community frequently is the only one present, although a wheatgrass-needleleaf sedge association may occur along the depression rim.

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) occurs in depressions with moderate-sized catchment areas during normal years, but it may also occur in small catchment areas during wet cycles or in 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-spikerush plant community appears. The most common species in this zone are western wheatgrass and needle spikerush, although common spikerush may also be present. A wheatgrass-needleleaf sedge community is commonly present around the rim of the depression. A number of minor graminoid species, such as American sloughgrass (Beckmannia syzigachne) and rough bentgrass (Agrostis scabra), 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) 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 typically a spikerush-dominated community. This transitions into a middle zone with a wheatgrass-spikerush community. The rim of the depression in this phase is typically a wheatgrass-needleleaf sedge association, although a western wheatgrass-dominated community may be present in some cases. A number of minor species, such as sedges, rushes, and smartweed, may also be present.

Community Phase Pathway 1.3a
In 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, particularly Kentucky bluegrass and smooth brome, 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 on 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 perennial grasses, such as Kentucky bluegrass and smooth brome, are the most common concerns. These species are widespread throughout the northern Great Plains (Toledo et al., 2014). They are very competitive and displace native species by forming dense root mats, altering nitrogen cycling, and having allelopathic effects on germination (DeKeyser et al., 2013). Plant communities dominated by Kentucky bluegrass and smooth brome have significantly less cover of native grass and forb species (Toledo et al., 2014; Dekeyser et al., 2009). Effects on soil quality and hydrology are still unknown at the time of this writing, 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 vegetative state that is substantially departed from the Reference State (1).

Noxious weeds are not widespread on the Recharge 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). These species are very aggressive perennials that typically displace native species and dominate ecological function when they invade a site. These species can sometimes 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.
 
Community Phase 2.1: Invaded Community Phase
The Invaded Community Phase (2.1) occurs when non-native perennial grasses or noxious weeds invade the site. In the Ephemeral Phase (1.1) or the Temporarily Ponded Phase (1.2), Kentucky bluegrass and smooth brome are the primary invasive species. Noxious weeds such as Canada thistle may also be a concern in some areas. Because invasive species have not been well documented in the Seasonally Ponded Phase (1.3), conclusive determinations are not available. Possible invasive species in these phases are reed canarygrass and creeping meadow foxtail, but this observation is considered hypothetical until further investigation.

Transition T2A
Tillage or herbicide application and seeding of cultivated crops (wheat, barley, or introduced hay) 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) or 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 Kentucky bluegrass and smooth brome, 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 Kentucky bluegrass and smooth brome. These restoration methods are labor intensive and costly and may not be 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 or 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: Cattail-Bulrush Phase
The Cattail-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 broadleaf cattail (Typha latifolia), hardstem bulrush (Schoenoplectus acutus), and common threesquare (Schoenoplectus pungens). The rim of the depression is characterized by a shallow marsh zone supporting sedge and spikerush species. A wet meadow or low prairie zone of western wheatgrass or wheatgrass-spikerush may form at the outermost edges where ponding duration is shortest.

Restoration Pathway R4A
Restoration of natural hydrology and reestablishment of native species transition 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 and 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 the 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 the restoration of natural hydrology and the 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 removing diversions, plugging drainage ditches, or both. These restoration methods are labor intensive and very costly and may be impractical in some situations.