Ecological dynamics
Like many sites across the Great Plains, changes in disturbance initiated by permanent settlements has had a profound impact on the ecological dynamics of these sites. Historically, the site was influenced by periodic fires during all seasons of growth. These fires were often the result of dry lightning strikes and/or anthropogenic fires set by Native Americans. The fire frequency during the period prior to settlement is estimated between 2-5 years for the southern Great Plains. These frequent fires were often followed by grazing of migratory bison herds attracted to the new growth of grass. This led to a shifting mosaic between burned/grazed and unburned and ungrazed landscapes.
With the removal of fire and the introduction of conventional livestock fencing, the landscape began to change to more homogenous vegetation patterns and altered plant communities. Gently sloping sites like the Savannah site were often cleared and broken out for crop production also. This led to a highly fragmented, highly disturbed landscape we see today across much of the area.
Less fire tolerant woody species are more prevalent in many areas due to fire suppression and/or the use of strictly dormant season fires. Abusive grazing can shift the plant community to less palatable grass species and lead to an increase in opportunistic forbs. Heavy grazing can also reduce fine fuel load and limit the ability to conduct a prescribed fire.
State and Transitional Pathways:
The following diagram suggests some pathways that vegetation on the site might take in response to various treatments or natural stimuli over time. There may be other states that are not shown on this diagram. This information identifies the changes in plant communities that do occur due to management practices and natural factors. The plant communities described here are commonly observed on this ecological site. The local NRCS field office has information available to assist with planning and development of the plant community for specific purposes.
Changes in plant community makeup may be due to many factors. Change may occur slowly or in some cases, fairly rapidly. As vegetative changes occur, certain thresholds are crossed. A threshold means that once a certain point is reached during the transition of one community to another, a return to the previous state may not be possible without the input of some form of energy. This often means intervention with practices that are not part of natural processes. An example might be the application of herbicide to control some woody species to reduce their population and encourage more grass and forbs growth. Merely adjusting grazing practices would probably not accomplish any significant change in a plant community once certain thresholds are crossed. The amount of energy required to effect change in community would depend on the present vegetative state and the desired change.
State 1
Oak Savannah - Reference
This is the reference state for the Savannah ecological site. It represents the historic range of variability in the plant communities with the periodic disturbance of fire and grazing. It is dominated by a mosaic of oak savannah species and herbaceous plants.
Community 1.1
Tallgrass Savannah
The dominant grasses are big bluestem, Indiangrass, switchgrass and little bluestem. Other grasses include Texas cupgrass, sideoats grama, dropseeds, silver bluestem and Scribner’s panicum. Dominant forbs include sunflowers, western ragweed, heath aster, poppymallows and prairie petunia. Legumes include Illinois bundleflower, scurfpea, tick-clovers, trailing wildbeans, and native lespedezas. Woody species include blackjack oak, post oak, Shumard oak, chittamwood, redbud, poison ivy, greenbriar, coralberry, hackberry and American elm.
The Savannah site is estimated to produce between 2,500– 4,500 pounds of vegetative production per year in reference condition. Woody canopy in this community is complex with some areas having dense oak canopy mixed with multiple prairie openings. For planning and management purposes, the average woody canopy in reference condition can be described as 20% - 40%.
Tallgrasses Midgrasses Trees Forbs = Shrubs
Minor component : Cool season grass/grasslike
Table 7. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
1681 |
2354 |
3026 |
| Tree |
560 |
785 |
1009 |
| Forb |
280 |
392 |
504 |
| Shrub/Vine |
280 |
392 |
504 |
| Total |
2801 |
3923 |
5043 |
Community 1.2
Midgrass Savannah
Figure 10. Chigley soils. Murray County, OK
This community has shifted to predominately midgrass species such as sideoats grama, dropseeds, purpletop and Carolina joint-tail with some remnant tallgrasses such as little bluestem remaining. Woody species canopy cover may increase due to lack of fire. Cool season grasses and sedges increase as shade increases. Heavy grazing has reduced palatable tallgrasses and forbs and the community is at risk of transitioning to a closed canopy state. Woody canopy averages between 40% - 60%.
Pathway 1.1A
Community 1.1 to 1.2
Abusive grazing practices can lead to a reduction in palatable tallgrasses and forbs. Coupled with the absence of fire or brush management, the site may shift to the Midgrass Savannah community.
Pathway 1.2A
Community 1.2 to 1.1
With adequate rest from grazing and implementation of prescribed fire and/or selective brush management, the site may shift back to the Tallgrass Savannah community.
State 2
Closed Canopy Woodland
Figure 11. Naru soils. Murray County, OK
This state is often the result of fire suppression for multiple years. Non fire-tolerant woody species such as elms, hackberry and juniper have increased and created a shaded environment with a heavy accumulation of leaf litter. Ecosystem processes are significantly altered and the herbaceous community is dominated by shade tolerant understory species. Greenbriar, grape and other shrubs and vines may create a dense understory layer.
State 3
Converted Land Use
Figure 12. Chigley soils in fescue. Johnston County, OK
This state represents a change in land use from rangeland to pastureland. The soil structure and biology has been altered and the site is dominated by introduced species. Management of introduced forages requires more inputs than native grasses. Careful consideration should be taken prior to planting to ensure the result meets the desired use. Ratings for forage yields can be found under the non-irrigated crop yield section in web soil survey. As with any fertility management program, current soil tests should be taken before planting and subsequent fertilization of introduced pastures.
The most common forage species on these sites include Bermudagrass and Old World Bluestems(eg. KR Bluestem).
Without brush management, woody species such as mesquites, junipers, elms, or honey locust may invade these sites.
There may be opportunities to plant native grass species on these sites to restore the reference plant communities. The success of this type of restoration is highly variable and depends on the remaining soil resources and past management. This type of endeavor often requires site specific planning and evaluation. However, the species described in the reference state are a good resource for initial planning of any restoration project.
State 4
Abandoned/Invaded
Figure 13. Chigley soils. Johnston County, OK
This state describes the invaded, woody dominated plant community of the Savannah site. The ecological processes are dominated by woody species including mesquite, honey locust, elm, hackberry and juniper species. Some herbaceous plants persist under the woody canopy or in interspaces. Usually, shade tolerant species like Texas wintergrass and sedges are prominent herbaceous components in this community.
Transition T1A
State 1 to 2
In the absence of fire or other forms of brush management, woody species will continually increase on these sites. As woody plants begin to dominate ecological processes, the site will transition to the Closed Canopy Woodland state.
Transition T1B
State 1 to 3
Many of these sites were cultivated for crop production during the settlement of the area. Many of them have been planted to monocultures of introduced forage grasses in recent years. When these sites are cultivated, the soil properties are significantly altered from the reference state. Soil structure, hydrology and biology have been impacted and the site is transitioned to the Converted Land Use state.
Restoration pathway R2A
State 2 to 1
At this point it will take significant inputs to remove woody species and restore the grass dominated pasture. However, it may be achieved through prescribed fire or brush management and a prescribed grazing plan which allows ample rest for the re-establishment of grasses. Effectiveness of prescribed fire depends upon fine fuel load and continuity and burning conditions. Selective thinning may be required in order to maintain the reference savannah dynamics without detrimental impacts to the hardwood community.
Transition T3A
State 3 to 4
These sites are prone to invasion by juniper, mesquite and honey locust. Without brush management, brush invasion may increase to the point where the site transitions to the Abandoned/Invaded state. At this point it will take significant inputs to remove woody species and restore the herbaceous plant community.
Transition T4A
State 4 to 3
At this point it will take significant inputs to remove woody species and restore the grass dominated pasture. However, it may be achieved through brush management and a prescribed grazing plan which allows ample rest for the re-establishment of grasses.
