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 site which is located in the Till Plains Region developed under Northern Great Plains climatic conditions and included natural influence of large herding herbivores and occasional fire. Changes will occur in the plant communities due to weather fluctuations and management actions. Under adverse impacts, a relatively rapid decline in vegetative vigor and composition can occur. Under favorable conditions the site has the potential to resemble the Reference State. Interpretations for this site are based primarily on the 1.1 Reference Plant Community Phase. This community phase and the Reference State have been determined by study of rangeland relic areas, areas protected from excessive disturbance, and areas under long-term rotational grazing regimes. Trends in plant community dynamics ranging from heavily grazed to lightly grazed areas, seasonal use pastures, and historical accounts also have been considered. Plant community phases, states, transitional pathways, and thresholds have been determined through similar studies and experience.
This site is very sensitive to precipitation fluctuations from year to year. With above average precipitation the site becomes very wet, leading to a much different plant community than what would be present with average to below average precipitation. In dry years, plant density becomes very low. The two plant communities influenced strongly by precipitation alone, Western Wheatgrass-Common Spikerush Plant Community Subphase and Common Spikerush-Pale Dock Plant Community Subphase make up the natural fluctuation of what could be considered the 1.1 Reference Plant Community Phase.
Following the state-and-transition diagram are narratives for each of the described states and community phases. These may not represent every possibility, but they are the most prevalent and repeatable states and community phases. The associated plant composition tables have been developed from the best available knowledge at the time of this revision. As more data are collected, some of these community phases and states may be revised or removed, and new ones may be added. The main purpose for including the descriptions here is to capture the current knowledge and experience at the time of this revision.
The following is a diagram that illustrates the common plant community phases that can occur on the site and the transition and community pathways between them. The ecological processes will be discussed in more detail in the plant community descriptions following the diagram.
State 1
Reference State
The Reference State represents the natural range of variability that dominated the dynamics of this ecological site (ES). This state was dominated by cool-season grasses, with warm-season grasses being subdominant. Before European settlement in North America, the primary disturbance mechanisms for this site in the Reference condition included periods of below and above average precipitation, periodic fire, and herbivory by insects and large ungulates. Timing of fires and herbivory coupled with weather events dictated the dynamics that occurred within the natural range of variability. Wheatgrass species can decline and a corresponding increase in foxtail barley (Hordeum jubatum), short, warm-season grasses and forbs would will occur. Under extended periods of disturbance, the main change is a reduction in vigor and production and an increase in bare ground and forb composition.
Interpretations are based primarily on the 1.1 Reference Plant Community Phase, which are also considered to be climax. This plant community evolved with grazing by large herbivores and occasional fire, as well as, periodic flooding and drying, and can be maintained with prescribed grazing, prescribed burning, or areas receiving occasional short periods of rest or deferment. This plant community phase has two sub-phases, just referred to as plant communities here. These sub-phases are mainly driven by precipitation and flooding and drying sequences.
Community 1.1
Reference Community
Community Sub-phase 1.1 Western Wheatgrass-Common Spikerush – Interpretations are based primarily on the 1.1 Western Wheatgrass-Common Spikerush Plant Community Sub-phase (this is also considered to be Reference). Following several years of above average precipitation, the plant community stabilizes and becomes dominated with perennial grasses such as western wheatgrass and common spikerush. Other grasses and grass-likes present include Nuttall’s alkaligrass (Puccinellia nuttalliana), sedge (Carex), rush (Juncus), slender wheatgrass (Elymus trachcaulus). The occurrence of forbs will be considerably lower, including some species such as American licorice (Glycyrrhiza lepidota), curlytop knotweed (Polygonum lapathifolium), Pennsylvania smartweed (Polygonum pensylvanicum), Pursh seepweed (Suaeda calceoliformis), and western dock (Rumex aquaticus). The plant community is made up of about 80 to 90 percent grasses and grass-likes, and about 10 to 20 percent forbs.
Community Subphase 1.1 Common Spikerush-Pale Dock – Interpretations are based primarily on the 1.1 Common Spikerush-Pale Dock Plant Community Subphase (this is also considered to be climax). This plant community often occurs after a period of higher precipitation that follows an extended dry cycle. Grasses and grass-likes commonly occurring include common spikerush, sedge, rush, foxtail barley, western wheatgrass, and bluegrasses. The forbs commonly found include pale dock, western dock, mint (Mentha), Pursh seepweed, lambsquarters (Chenopodium album), knotweed (Polygonum), evening-primrose (Oenothera), buttercup (Ranunculus), and New England aster (Symphyotrichum novae-angliae). The plant community is made up of about five to 10 percent grasses, 30 to 40 percent grass-likes, and about 50 to 60 percent forbs.
Precipitation cycles will shift this community between the 1.1 Western Wheatgrass-Common Spikerush Plant Communtity Subphase and the 1.1 Common Spikerush-Pale Dock Plant Communtity Subphase. After several years of above average precipitation, the plant community stabilizes and perennial grasses and western wheatgrass will dominate the site with few grass-likes and forbs; and in the instance of higher precipitation received after extended years of drought, there will be an increase in the grass-likes and forbs components.
Community 1.2
Western Wheatgrass-Inland Saltgrass
This plant community is the result of heavy, continuous grazing. Repeated defoliation depletes stored carbohydrates resulting in weakening and eventual death of the most palatable grasses. Lack of litter and reduced plant vigor result in higher soil temperatures, poor water infiltration rates, high evapotranspiration, and increased percolation of the high water table, which increases salt concentrations on the surface. This gives inland saltgrass (Distichlis spicata) and other salt tolerant species a competitive advantage over less tolerant species.
Inland saltgrass drastically increases and competes with western wheatgrass as the dominant species. Other grass and grass-like species present include Nuttall’s alkaligrass, plains bluegrass (Poa arida), common spikerush, needle Spikerush (Eleocharis acicularis), and other sedges (Cyperaceae) and rushes (Juncaceae). Early cool-season grasses including foxtail barley, fowl bluegrass (Poa palustris), and Kentucky bluegrass (Poa Pratensis) begin to invade. Forbs that will invade are curly dock (Rumex crispus) and cocklebur (Xanthium). Common forbs to the site include lambsquarters, Pennsylvania smartweed, curlytop knotweed, plantain (Plantago), and povertyweed (Iva axillaris).
This plant community is relatively stable and well adapted to increased salinity. Plant vigor, litter, frequency, and production have decreased. The biological integrity, water, and nutrient cycles of this plant community are becoming impaired. This plant community is less productive than the 1.1 Reference Plant Community Phase.
Pathway 1.1A
Community 1.1 to 1.2
Heavy, continuous grazing (grazing at full to heavy levels for extended portions of the growing season without adequate recovery periods) will shift this community to the 1.2 Western Wheatgrass-Inland Saltgrass Plant Community.
Pathway 1.2A
Community 1.2 to 1.1
Prescribed grazing, and/or prescribed burning returned to normal disturbance regime levels and frequencies, or periodic light to moderate grazing will convert this plant community to the 1.1 Reference Plant Community Phase.
State 2
Native Invaded State
This state represents the range of variability that exists with reduced vigor and production of the dominant species as a result of grazing-induced disturbance and the introduction of non-native species. This state is dominated by cool-season grasses. It can be found on areas that are impacted by extended periods of heavy, continuous grazing. Grazing tolerant species become dominant, and non-native species are present.
Community 2.1
Foxtail Barley-Inland Saltgrass
This plant community developed with heavy, continuous grazing where adequate recovery periods between grazing events were not allowed. Patches of inland saltgrass sod are typical and foxtail barley and fowl bluegrass are well distributed throughout the community. Nuttall’s alkaligrass and western wheatgrass have been greatly reduced in production and vigor, but may persist in remnant amounts. This plant community is resistant to change due to the grazing tolerance of inland saltgrass and increased surface salts. A significant amount of production and diversity has been lost when compared to the 1.1 Reference Plant Community Phase. Loss of key cool-season grasses and increased bare ground have negatively impacted energy flow and nutrient cycling. Water infiltration is reduced significantly due to the massive shallow root system “root pan,” characteristic of inland saltgrass and increased bare ground.
It will take a long time to bring this plant community back to the Reference State (State 1) with management alone. Renovation (mechanical and chemical inputs) is typically not effective due to high salt content of the soil and saltgrass persistence.
Community 2.2
Curly Dock-Foxtail Barley
This plant community can be reached with heavy, continuous grazing coupled with compaction due to grazing when the soil is saturated. This plant community can also result from long-term ponding and occasional subsequent drying, as when this site is developed for a water source. The dominant vegetation includes pioneer annual grasses, forbs, invaders, and early successional biennial and perennial species. Grasses may include foxtail barley, which may become dominant, along with fowl bluegrass, Nuttall’s alkaligrass, and western wheatgrass.
The dominant forbs include curly dock, curlycup gumweed (Grindelia squarrosa), kochia (Bassia scoparia), cocklebur, and other early successional salt tolerant species. The community is susceptible to non-native species due to severe soil disturbances and the relatively high percent of bare ground. This plant community is resistant to change as long as soil disturbance or severe vegetation defoliation persist, thus holding back secondary plant succession. Secondary succession is highly variable, depending upon availability and diversity of a viable seed bank of higher successional species within the existing plant community and neighboring plant communities.
Pathway 2.1A
Community 2.1 to 2.2
Heavy, continuous grazing which includes herbivory at moderate to heavy levels at the same time of year each year without adequate recovery periods, or during periods of below normal precipitation shifts this community to the 2.2 Curly Dock-Foxtail Barley Plant Community Phase.
Pathway 2.2A
Community 2.2 to 2.1
Prescribed grazing (alternating season of use and providing adequate recovery periods) or periodic light to moderate grazing will convert this plant community to the 2.1 Foxtail Barley-Inland Saltgrass Plant Community Phase.
Transition T1A & T1B
State 1 to 2
Heavy, continuous grazing (stocking levels well above carrying capacity for extended portions of the growing season, and often at the same time of year each year, typically beginning early in the season) and invasion of non-native plant species will convert the 1.1 Common Spikerush-Pale Dock Plant Communtity Subphase within the Reference State (State 1) to the 2.2 Curly Dock-Foxtail Barley Plant Community Phase within the Native/Invaded State (State 2).
Heavy, continuous grazing (stocking levels well above carrying capacity for extended portions of the growing season, and often at the same time of year each year, typically beginning early in the season) and invasion of non-native plant species will convert the 1.2 Western Wheatgrass-Inland Saltgrass Plant Community Phase within the Reference State (State 1) to the 2.1 Foxtail Barley-Inland Saltgrass Plant Community Phase within the Native/Invaded State (State 2).
Restoration pathway T2A
State 2 to 1
Long-term prescribed grazing (moderate stocking levels coupled with adequate recovery periods, or other grazing systems such as high-density, low-frequency intended to treat specific species dominance, or periodic light to moderate stocking levels possibly including periodic rest) may lead the Native/Invaded State (State 2) over a threshold to the Reference State (State 1).
