Ecological dynamics
Savannah sites developed under Northern Great Plains climatic conditions, light to severe grazing by bison and other large herbivores, sporadic natural or man-caused wildfire, and other biotic and abiotic factors that typically influence soil/site development. This continues to be a disturbance-driven site, by herbivory, fire, and variable climate. Changes occur in the plant communities due to short-term weather variations, impacts of native and/or exotic plant and animal species, and management actions. The landscape position and association with streams make this site somewhat less susceptible to fire, which allowed woody species to become more abundant than less sheltered sites in the MLRA.
One of the primary impacts to this site introduced by European-man is season-long continuous grazing by domestic livestock. This management practice causes the repeated removal of the growing point and excessive defoliation of the leaf area of individual tall warm season grasses. The resulting reduction of the plants ability to harvest sunlight depletes the root reserves, subsequently decreasing the root mass. This negatively impacts the plants' ability to compete for life sustaining nutrients, resulting in declining vigor, and eventual mortality. The space created in the vegetative community is then occupied by a species that evades the negative grazing impacts by a growing season adaptation (such as a cool season), a shorter structure or a reduced palatability mechanism.
The State and Transition Model (STM) is depicted below, and is made up of a Reference State, a Native/Invaded State, a Sod-busted State and an Invaded Woody State. Each state represents the crossing of a major ecological threshold due to alteration of the functional dynamic properties of the ecosystem. The main properties observed to determine this change are the soil and vegetative communities, and the hydrological cycle.
Each state may have one or more vegetative communities that fluctuate in species composition and abundance within the normal parameters of the state. Within each state, communities may degrade or recover in response to natural and man caused disturbances such as variation in the degree and timing of herbivory, presence or absence of fire, and climatic and local fluctuations in the precipitation regime. Periodic flooding and deposition events can cause a wide variability in plant communities and production on this site.
Interpretations are primarily based on the Reference State, and 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 have been interpreted from heavily grazed to lightly grazed areas, seasonal use pastures, and historical accounts. Plant communities, states, transitional pathways, and thresholds have been determined through similar studies and experience.
Growth of native cool season plants begins about April 1, and continues to about June 15. Native warm season plants begin growth about May 15, and continue to about August 15. Green up of cool season plants may occur in September and October if adequate moisture is available.
The following is a diagram that illustrates the common plant communities that can occur on the site and the transition pathways between communities.
State 1
Reference State
This state describes the range of vegetative community phases that occur on the Savannah site where the natural processes are mostly intact.
The Reference Community is a representation of the native plant community phase that occupies a site that has been minimally altered by management. The Degraded Savannah, the At-Risk, and the Excessive Litter Communities are the phases that result from management decisions that are unfavorable for a healthy Reference Community. The Ephemeral Forb Community is the result of a high intensity disturbance event.
High perennial grass cover and production allows for increased soil moisture retention, vegetative production, and overall soil quality.
Community 1.1
Tallgrass Savannah Community
Figure 9. Reference community for the Savannah ecosite
The Tallgrass Savannah Community is comprised of mid- and tall-grass native prairie species under a scattered canopy of deciduous trees, primarily oak, hickory, and elm. This community serves as a description of the native plant community that naturally occurs on the site when the natural disturbance regimes are intact, or closely mimicked by management practices. This phase is dynamic, with fluid relative abundance and spatial boundaries between the dominant structural vegetative groups. These fluctuations are primarily driven by different responses of the species to changes in precipitation timing and abundance, and fire and grazing events.
The potential vegetation consists of approximately 70 percent grasses and grass-like plants, 10 percent forbs, and 20 percent trees and shrubs. Big and little bluestem are the primary grass species in this community. Secondary species include switchgrass, indiangrass, and sideoats grama. The site has a moderately diverse forb population, including catclaw sensitive briar, sunflowers, and lespedezas.
This is a diverse, highly productive, resilient community, that is resistant to short term stresses such as drought and short periods of heavy stocking. The well-developed root systems support this resiliency when allowed adequate recovery periods between grazing events.
When exposed to long-term or frequent over-grazing events without adequate rest, this plant community will degrade.
The annual forage production of this community averages about 4,250 lbs per acre.
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Community 1.2
Degraded Savannah
Figure 11. Degraded Savannah community, eastern Kansas
Big bluestem, little bluestem, and sideoats grama lose productive capacity through loss of vigor and reproductive potential. Switchgrass, and Indiangrass are reduced to remnants.
As growing season defoliation continues, the more grazing evasive species such as hairy grama, Kentucky bluegrass, muhly, and tall dropseed increase. The woody component becomes more prominent, causing additional shade over the understory.
This community phase signals a significant loss of production. The change is due to continuous season-long grazing with inadequate recovery periods. The composition of the forb component favors less palatable species, and lespedeza and ragweed increase. Fewer high bio-mass producing deep rooted species result in a reduced fire-friendly fuel load.
While this plant community is less productive and less diverse than the representative plant community, it remains sustainable in regards to site/soil stability, watershed function, and biologic integrity.
Community 1.3
At-Risk Native Community
Figure 12. At Risk Savannah community, eastern Kansas
In this plant community, the more palatable tall warm-season grasses have been reduced to remnant populations by continued defoliation during their critical growth periods. Grazing-evasive warm-season and cool-season grasses increase significantly. Tall dropseed, warm season shortgrasses, and Kentucky bluegrass become dominant. Shade tolerant annuals increase. Sedges, rosette panicum, and buckbrush populations expand, and the palatable forb component is only a trace.
Soil health is affected by reduced efficiency in the nutrient, mineral, and hydrologic cycles as a result of decreases in plant litter and rooting depths. This may result in formation of a compacted layer in the pockets of deeper soil, and total annual vegetative production declines significantly. Without a management change, this community is at-risk to degrade to the Native/Invaded State.
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Community 1.4
Excessive Litter Community
The Excessive Litter Community Phase describes the response of the community to the removal of the natural disturbances of herbivory and fire. As the undisturbed duff layer deepens, infiltration of the precipitation is interrupted and evaporation increases significantly, simulating drought-like conditions.
Community 1.5
Ephemeral Forb Community
This community describes the flush of forbs that occurs in response to a major disturbance, or combination of disturbances. Growing season wildfire followed by hail, extreme prolonged drought, or extreme defoliation by herbivores are all examples of these disturbances. The native warm-season grasses re-establish dominance with-in a few years of the event.
Pathway CP 1.1-1.2
Community 1.1 to 1.2
Tallgrass Savannah Community
A shift from the Reference community to the Degraded Savannah community occurs with continuous season long grazing and inadequate recovery periods during the growing season.
Pathway CP 1.1-1.4
Community 1.1 to 1.4
Prolonged interruption of the natural disturbances of herbivory and fire will result in conversion from this community to the Excessive Litter Community.
Pathway CP 1.1-1.5
Community 1.1 to 1.5
A high-impact disturbance event or combination of events causing excessive defoliation of the vegetation, i.e. a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event or long-term drought, etc.
Pathway CP 1.2-1.1
Community 1.2 to 1.1
Tallgrass Savannah Community
A shift from the Degraded Savannah community toward the Reference community can be achieved through prescribed grazing. Applying grazing pressure during the growth period of the undesirable cool season grasses, and allowing rest during the warm season growing season favors our desired species. This grazing regime will enable the more deeply rooted tall warm season grasses to out compete the shallow rooted grazing evasive warm season and the cool season grasses.
Appropriately timed prescribed fire will accelerate this process.
| Prescribed Burning |
|
| Access Control |
|
| Prescribed Grazing |
|
Pathway CP 1.2-1.3
Community 1.2 to 1.3
Maintaining continuous season long grazing or haying with inadequate recovery periods during the growing season further degrades the site to the At-Risk Community.
Pathway CP 1.2-1.4
Community 1.2 to 1.4
Prolonged interruption of the natural disturbances of herbivory and fire will result in conversion from this community to the Excessive Litter Community.
Pathway CP 1.2-1.5
Community 1.2 to 1.5
A high-impact disturbance event, or combination of events causing excessive defoliation of the vegetation, i.e. a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event, or long-term drought, etc.
Pathway CP 1.3-1.2
Community 1.3 to 1.2
Reversing the downward trend to the previous community can be achieved with prescribed grazing early and late in the growing season to reduce undesirable cool season grasses. Targeting the peak growth period of cool season grasses with high intensity grazing events followed by rest will allow the tall native warm season grasses to rejuvenate. Appropriately timed prescribed fire will accelerate this process.
| Access Control |
|
| Prescribed Grazing |
|
Pathway CP 1.3-1.4
Community 1.3 to 1.4
Prolonged interruption of the natural disturbances of herbivory and fire will result in conversion from this community to the Excessive Litter Community.
Pathway CP 1.3-1.5
Community 1.3 to 1.5
A high-impact disturbance event, or combination of events causing excessive defoliation of the vegetation, i.e. a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event, or long-term drought, etc.
Pathway CP 1.4-1.1
Community 1.4 to 1.1
Re-introduction of the natural processes of herbivory and fire will allow the vegetation to return to the previous community.
| Prescribed Burning |
|
| Prescribed Grazing |
|
Pathway CP 1.4-1.2
Community 1.4 to 1.2
Re-introduction of the natural processes of herbivory and fire will allow the vegetation to return to the previous community.
Pathway CP 1.4-1.3
Community 1.4 to 1.3
Re-introduction of the natural processes of herbivory and fire will allow the vegetation to return to the previous community.
Pathway CP 1.4-1.5
Community 1.4 to 1.5
A high-impact disturbance event, or combination of events causing excessive defoliation of the vegetation, i.e. a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event, or long-term drought, etc.
Pathway CP 1.5-1.1
Community 1.5 to 1.1
Restoration occurs naturally once the disturbance event has subsided. Allowing growing season rest will accelerate the recovery.
Pathway CP 1.5-1.2
Community 1.5 to 1.2
Restoration occurs naturally once the disturbance event has subsided. Allowing growing season rest will accelerate the recovery.
Pathway CP 1.5-1.3
Community 1.5 to 1.3
Restoration occurs naturally once the disturbance event has subsided. Allowing growing season rest will accelerate the recovery.
State 2
Native/Invaded State
This state has been degraded from the Reference State and much of the native warm-season grass community has been replaced by less desirable plants. The loss of tall and mid- warm-season grasses has negatively impacted energy flow and nutrient cycling. The woody component has expanded significantly, and shading due to the overstory has increased. Water infiltration is reduced due to the shallow root system and rapid runoff characteristics of the grazing-evasive plant communities. Alteration of the soil, and the hydrological processes make return to reference state unlikely.
The Invasive Woodies and the Eastern Redcedar are the communities in this state.
Community 2.1
Invasive Woodies
This plant community represents a shift from the Reference State across a plant community threshold. With continued grazing pressure annual bromes, hairy grama, Kentucky bluegrass, sedges, and dropseed will become the dominant grass and grasslike species, with only trace remnants of the more palatable warm-season grasses such as little bluestem and sideoats grama. Continuous and heavy grazing pressure will maintain this plant community in a sod-bound condition. Forb richness and diversity has decreased. Without focused management, the woody component will expand significantly. The increased light interception from the overstory creates a more shade tolerant environment. Kentucky bluegrass, sedges, broomsedge bluestem and pupletop become the dominant herbaceous species.
With the decline and loss of deeper penetrating root systems, a compacted layer may form in the soil profile below the more shallow replacement root systems.
Community 2.2
Eastern Redcedar Monoculture
If eastern redcedar is present on the site, it will often completely dominate and replace the existing woody community, creating a monoculture that eliminates most of the understory as well as the overstory.
Pathway CP 2.1-2.2
Community 2.1 to 2.2
Interruption of the natural fire regime can allow eastern redcedar to dominate to the point of becoming a monoculture.
Pathway CP 2.2-2.1
Community 2.2 to 2.1
Implementation of an aggressive prescribed burning program will eliminate the encroaching eastern redcedar. Mechanical removal will also aid in reversing the invasion.
| Brush Management |
|
| Prescribed Burning |
|
State 3
Sod-busted State
This threshold is crossed as a result of mechanical disturbance to facilitate production agriculture. If farming operations are suspended, the site can be abandoned, which will result in the Naturally Reclaimed Community, or be re-seeded to a desired perennial forage mixture, which is described as the Re-seeded Community.
Permanent alterations of the soil community and the hydrological cycle make restoration to the original native Reference Community extremely difficult, if not impossible. Formation of a compacted plow pan in the soil profile is likely.
Community 3.1
Re-Seeded Grass
This plant community does not contain native remnants, and varies considerably depending on the seed mixture, the degree of soil erosion, the age of the stand, nitrogen fertilizer use, and past grazing management. Prescribed grazing with adequate recovery periods will be needed to maintain productivity and desirable species.
Native range and seeded grasslands are ecologically different, and should be managed separately. Factors such as functional group, species, stand density, and improved varieties all impact the production level and palatability of the seedings. Species diversity is often limited, and when grazed in conjunction with native rangelands, uneven forage utilization may occur.
Total annual production during an average year varies significantly depending on precipitation, management and grass species seeded.
Community 3.2
Natural Reclamation
This plant community consists of annual and perennial weeds and less desirable grasses.
These sites have been farmed and abandoned without being reseeded. Soil organic matter/carbon reserves are reduced, soil structure is changed, and a plow-pan or compacted layer can be formed which decreases water infiltration. Residual synthetic chemicals may remain from farming operations. In early successional stages, this community is not stable. Erosion is a concern.
Total annual production during an average year varies significantly depending on the succession stage of the plant community and any management applied to the system.
Transition T1-2
State 1 to 2
Heavy grazing or haying without adequate recovery periods will cause this state to lose a significant proportion of tall and mid- warm-season grass species and cross a threshold to the Native/Invaded State. Water infiltration and other hydrologic functions will be reduced due to the root matting presence of sod-forming grasses. With the decline and loss of deeper penetrating root systems, soil structure and biological integrity are catastrophically degraded to the point that recovery is unlikely. Once this occurs, it is highly unlikely that grazing management alone will return the community to the Reference State. Additionally, increasing woody encroachment also results from overgrazing and lack of fire. The expansion of the overstory blocks more of the sunlight, creating a more competitive environment for a shade tolerant understory. Mechanical removal with follow-up stump treatment of the root-sprouting species and implementation of a prescribed burning regime will alleviate the invasion of woodies.
Transition T1-3
State 1 to 3
The Reference State is significantly altered by mechanical tillage to allow the site to be placed into production agriculture. The disruption to the plant community, the soil and the hydrology of the system make restoration to a true reference state unlikely.
Transition T
State 2 to 3
The state is significantly altered by mechanical tillage to allow the site to be placed into production agriculture. The disruption to the plant community, the soil and the hydrology of the system make restoration to a true reference state unlikely.
