Ecological dynamics
The foremost diagnostic feature of this site is the high clay content that begins at or near the surface and the shallow depth to bedrock (20 to 38 centimeters / 8 to 15 inches). The site does not receive additional moisture from higher adjacent areas and is subject to significant runoff. Relatively minor changes in local elevation can affect the plant community since the bedrock can fluctuate in depth. This site often occurs in complex with the Loamy Lowland and Loamy Upland sites.
This site developed with occasional fires being part of the ecological processes. It is presumed that the historic fires generally occurred every 3-4 years, were randomly distributed, and started by lightning at various times throughout the season when thunderstorms were likely to occur. It is also believed that pre-European inhabitants may have used fire as a management tool for attracting herds of large migratory herbivores (bison, elk, and/or deer.) The impact of fire over the past 100 years has been relatively insignificant due to the human control of wildfires and the lack of acceptance of prescribed fire as a management tool.
The degree of herbivory (feeding on herbaceous plants) has a significant impact on the dynamics of the site. Historically, periodic grazing by herds of large migratory herbivores was a primary influence. Secondary influences of herbivory by species such as grasshoppers and root feeding organisms impacted the vegetation historically and continue to this day. The management of herbivory by humans through grazing of domestic livestock and/or manipulation of wildlife populations has been a major influence on the ecological dynamics of the site. This management coupled with climate largely dictates the plant communities for the site.
The plant community for this site is dynamic due to the complex interaction of many ecological processes. The interpretive plant community for this site is the reference state. The reference state has been determined by the study of rangeland relic areas, areas protected from excessive disturbance and areas under long term rotational grazing strategies. Trends in plant community dynamics ranging from heavily grazed to lightly grazed areas, seasonal use pastures, and historical accounts have also been used.
The following is a diagram that illustrates the common plant communities that can occur on the site and the transition pathways among communities. The ecological processes will be discussed in more detail in the plant community descriptions following the diagram.
State 1
Native Tallgrass
This state comprises the communities within the range of natural variability under historic conditions and disturbance regimes. Patterns created by wildlife use and fire would have created a mosaic of communities across the landscape; however, warm-season tallgrasses are dominant, with a subdominant to minor contribution from native cool-season grasses, forbs, and shrubs.
Fire and bison herbivory were the dominant disturbance regimes that historically maintained the tallgrass dominance with a diverse forb component. Furthermore, bison grazing was closely linked to fire patterns as the animals preferred grazing burned areas offering lush regrowth devoid of decadence and of higher nutritive quality. Thus, historic plant communities were subjected to occasional burning and grazing, with substantial rest/recovery periods as the fuel load rebuilt to eventually start this process again. Fire return intervals of 3-4 years served to suppress woody species, particularly the various tree and shrub species prevalent in adjacent riparian corridors. The degree to which observed conditions represent this state largely depends on how closely the management has mimicked these past disturbance effects.
Big bluestem and little bluestem are used in this state and transition model as dominant; however many species of native grasses may be present on these sites. Green needlegrass (Nassella viridula), western wheatgrass (Pascopyrum smithii) and switchgrass (Panicum virgatum) may be common in some areas.
Community 1.1
Big bluestem – Little bluestem (Andropogon geradii – Schizachyrium scoparium)
This is the interpretive plant community and can be found on areas that are properly managed with prescribed grazing that allows for adequate recovery periods following each grazing event.
The plant community consists of 80-95% grasses and grass-likes, 5-10% forbs and 0-5% shrubs. Dominant grasses include Big bluestem, Little bluestem, Indiangrass, Switchgrass, and Tall dropseed. Other grasses and grass-likes are Canada wildrye, Kentucky bluegrass, Prairie junegrass, Sideoats grama, Scribner panicum, forbs and sedges.
This plant community is diverse, stable, and productive. Plant community dynamics, nutrient cycles, water cycles, and energy flow are functioning properly. Plant litter is properly distributed with negligible movement off-site and natural plant mortality is very low. This community is resistant to many disturbances except continuous, season-long heavy grazing, tillage, or non-use. Broadcast herbicide application will dramatically reduce forb diversity and abundance.
Community 1.2
Little bluestem - Switchgrass (Schizachyrium scoparium – Panicum virgatum)
Little bluestem has replaced Big bluestem as the dominant species. Other species, such as switchgrass, and western wheatgrass have also increased. While still within the range of natural variability, energy capture, nutrient cycling, and hydrology are not functioning at their full potential relative to the reference condition.
Pathway 1.1A
Community 1.1 to 1.2
Grazing management which does not provide adequate recovery periods will cause a shift from Big bluestem and Little bluestem towards less palatable species, particularly Switchgrass.
Pathway 1.2A
Community 1.2 to 1.1
Management that provides adequate recovery periods and does not annually prevent tallgrass seed set or otherwise impair vigor will facilitate a return to community phase 1.1.
In the case of drought, the return to more typical precipitation patterns will promote shift towards tallgrass species.
State 2
Native/Invaded Mix
This state can manifest in three ways: 1) the appearance of introduced cool-season grasses, 2) the expansion of shrubs and/or trees, or 3) some combination of these. Kentucky bluegrass and Smooth brome are the primary cool-season grass invaders in this region, commonly found in roadsides, disturbed areas, and pastures intentionally seeded for cool-season forage. Management practices and/or environmental conditions that are not favorable to native grass vigor may allow introduced grasses to invade the site thereby decreasing native diversity and abundance, particularly of forbs.
In the absence of the historic fire regime, woody species may also expand to become an influential component of the community. The invasive component tends to have very high resilience, is extremely difficult to eradicate, and what might be considered a new "contemporary" range of natural variability is seen as competition between the native grasses and introduced/woody species for space and resources.
Community 2.1
Subdominant Smooth brome – Kentucky bluegrass (Bromus inermis – Poa pratensis)
While native warm-season grasses still dominate the site, introduced cool-season species have established a foothold in the system and can be found interspersed throughout the stand. The stand may still have a native tallgrass appearance overall, but brome and/or bluegrass can be easily found.
Shrub/tree species may also have begun to expand into areas where they did not persist historically, but the overall appearance can vary depending on the propagation method of a particular species.
Community 2.2
Codominant Smooth brome – Kentucky bluegrass (Bromus inermis – Poa pratensis)
This community is comprised of a relatively even mix of native grasses and invasive species overall. This may manifest as a well-distributed interspersion of natives and invaders, as distinct patches wherein competitors dominate locally, or some combination. Forb diversity and abundance is further diminished.
Pathway 2.1A
Community 2.1 to 2.2
Management and/or environmental conditions have afforded a persisting competitive advantage to introduced cool-season grasses, and they begin to dominate the ecological dynamics of the site. The robust invasive component can quickly and effectively exploit opportunities to outcompete and displace natives. Repeated summer use of an area will place the bulk of stressor impacts on native plants, reducing native vigor and allowing invaders to thrive. Likewise, a climate pattern limiting natural moisture to the spring and fall months coincides with peak cool-season growth and may support a similar process.
Pathway 2.2A
Community 2.2 to 2.1
The native component remains in an abundance that can facilitate a return towards more historic conditions if management is modified to shift stressor impacts to the invasive species and promote warm-season grass vigor. Environmental conditions and/or disturbance regimes that strongly favor warm-season grasses can also trend the site towards the reference.
State 3
Invasive Dominant
Introduced cool-season invasion has progressed to the point that native species comprise a negligible portion of the community and the aggressively rhizomatous invasives preclude native germination and seedling survival. The native component may be completely absent, and the site resembles a seeded pasture. Alternatively, the dominant invasives may be woody species. Woody competitiveness for sunlight, water, space, and other resources continues to increase as desirable herbaceous species are shaded out, crowded out, or otherwise suppressed.
Community 3.1
Smooth brome – Kentucky bluegrass (Bromus inermis – Poa Pratensis)
This community is typically composed of Smooth brome with bluegrass interspersed among the brome tillers. Warm-season natives, if present, are sparse yet often conspicuous due to pronounced differences in growth habits and metabolic pathways.
Community structure and function have been dramatically simplified relative to the reference condition, and very few biotic functional groups are represented in amounts that would influence ecological function. The invasive grass root skein provides good site stability; however, replacement of the deeper roots and complex bunchgrass canopy with the shallower roots and erect tiller canopy of the invaders results in reduced interception and infiltration rates.
State 4
Annual/Pioneer
Nutrient cycling, hydrologic function, and/or soil stability have been severely altered, and possibly compromised. This is a highly variable state in which the specific plants observed will depend largely on the original community and the nature of the disturbance. This condition encompasses (but is not necessarily limited to) events such as severe fire impacts, heavy continuous grazing, heavy nutrient inputs, and abandoned cropland.
Community 4.1
Variable native and introduced
This community is heavily dominated by annual plants that thrive in disturbed areas and often includes annual ragweed, hoary verbena, or amaranths. It is also particularly vulnerable to noxious weed invasion with the most common species being Leafy spurge, Musk and Canada thistles.
State 5
Woody Dominant
Under historic disturbance regimes, frequent and uncontrolled fire and wildlife browsing served to keep woody species in check. However, in the absence of fire (either wild or prescribed), it's not uncommon for the woody trees and shrubs normally limited to riparian areas to expand into the floodplains, regardless of herbaceous community composition. Wildlife may introduce a seed source to areas not associated with a waterway, such as interdunal depressions.
Community 5.1
Native wooded overstory
Woody species have encroached and established, typically with species such as maples, cottonwood, boxelder, green ash, eastern red cedar and swamp oak.
Transition T1A
State 1 to 2
In the presence of introduced cool-season grasses, environmental conditions and/or management that reduces native vigor and stand resilience, and frees up resources (space, sunlight, nutrients, water) will allow for colonization of Kentucky bluegrass and Smooth brome. Likewise, similar processes may also allow for woody species to expand.
Transition T1B
State 1 to 4
There are many possible triggers for this transition that may occur as acute events (e.g. plowing) or cumulative impacts of chronic events (e.g. long-term undermanaged grazing.) The absence of deep-rooted perennial cover exposes the site to topsoil loss, open nutrient cycle, and free space which collectively allow for opportunistic annual species to dominate.
Transition T1C
State 1 to 5
All herbaceous communities are vulnerable to woody encroachment in the absence of fire and/or browsing and hoof action impacts. This is particularly prominent in areas adjacent to riparian corridors which supply a constant seed source. As tree establishment progresses, the conditions grow increasingly favorable for woody germination and growth.
Restoration pathway R2A
State 2 to 1
Eradication of introduced cool-season grasses from this site will require long-term, targeted management efforts to create an adverse environment during the spring and late fall when bluegrass and brome are most actively growing, with favorable conditions during the summer to promote native warm-season species. Targeted practices such as prescribed burning, flash grazing, and herbicide are often employed at strategic times of the year to set back undesirable species. The combination of practices should strive to mimic the historic disturbance regimes to which the desirable native species are best adapted.
Transition T2A
State 2 to 3
If the conditions which initiated and fomented the colonization and expansion of cool- season invasion are not removed or mitigated, stand composition will continue to shift in this direction and begin to resemble a monoculture of brome and/or bluegrass. Due to the dense rhizomatous root mat of brome and bluegrass, native species suffer decreasing opportunities to contribute propagules, and individual plants lost are not replaced by desirable natives.
Transition T2B
State 2 to 4
There are many possible triggers for this transition that may occur as acute events (e.g. plowing) or cumulative impacts of chronic events (e.g. long-term undermanaged grazing.) The absence of deep-rooted perennial cover exposes the site to topsoil loss, open nutrient cycle, and free space which collectively allow for opportunistic annual species to dominate.
Transition T2C
State 2 to 5
All herbaceous communities are vulnerable to woody encroachment in the absence of fire and/or browsing impacts. This is particularly prominent in areas adjacent to riparian corridors which supply a constant seed source. As tree establishment progresses, the conditions grow increasingly favorable for woody germination and growth.
Transition T3C
State 3 to 2
Aggressive intervening actions will be required to simultaneously recolonize native grasses and suppress vigor in undesirable species. Restoration follows the same principles as the R2A pathway but may also require native range seeding if the latent seedbank is inadequate.
Transition T3B
State 3 to 4
Nutrient cycling, hydrologic function, and/or soil stability have been severely altered, and possibly compromised. This is a highly variable state in which the specific plants observed will depend largely on the original community and the nature of the disturbance.
Transition T3A
State 3 to 5
All herbaceous communities are vulnerable to woody encroachment in the absence of fire and/or browsing impacts. This is particularly prominent in areas adjacent to riparian corridors which supply a constant seed source. As tree establishment progresses, the conditions grow increasingly favorable for woody germination and growth.
Restoration pathway R4A
State 4 to 1
Restoration strategies will depend on the nature of the disturbance and the viability of the seedbank. On pastures, changes to gazing management and favorable moisture conditions may produce a perennial community. However, in abandoned cropland range seeding will likely be necessary to recolonize desirable perennial species.
Transition T4B
State 4 to 2
As the site matures from the annual / pioneer stage, it will transition over to a mix of native and invaded species depending on the seed sources available. In many cases this will be a mix of Smooth brome and Kentucky bluegrass for these sites.
Transition T4C
State 4 to 3
The annual / pioneer stage can transition to an invasive dominant stage if the seed source available after the disturbance is predominantly non-native species.
Restoration pathway R5A
State 5 to 1
The combination of tree size, reduced herbaceous understory, and more mesic conditions makes it increasingly difficult for natural disturbances to restore/maintain the historic tallgrass community, and mature woodlands can no longer be restored with fire. Intensive brush management will be required to mechanically remove the established overstory. Woody control and maintenance will be an ongoing process and may also require chemical methods if sprouting species are present.
Transition T5B
State 5 to 2
Any type of natural act or management practices that kill off the woody species can transition the woody dominant site back to a native / invaded mix as the remaining herbaceous plants and seed source thrive due to the introduction of more sunlight and less woody competition.
Transition T5C
State 5 to 3
Any type of natural act or management practices that kill off the woody species can transition the woody dominant site back to invasive dominant regime as the remaining herbaceous plants and seed source thrive due to the introduction of more sunlight and less woody competition.
