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. 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 and transition model
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State 1 submodel, plant communities
State 1
Oak Savannah
This is the reference state for the Rhyolite Hills ecological site. It represents the historic range of variability in the plant communities with the periodic disturbance of fire and grazing. It is a mosaic of herbaceous plants, trees and woody understory vines and shrubs.
The dominant grasses are sideoats grama, dropseeds, silver bluestem and Scribner’s panicum with some areas of little bluestem. Other grasses include poverty oatgrass, perennial threeawn and sedges. Dominant forbs include sunflowers, western ragweed, heath aster. Legumes include 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. These woody species will increase in canopy cover in the absence of fire.
Woody canopy in the reference state is complex with some areas having dense tree canopy mixed with multiple prairie openings. For planning and management purposes, the average woody canopy in reference condition can be described as less than 40%.
The Rhyolite Hills site is estimated to produce between 500– 1,800 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%.
Midgrasses Trees Forbs = Shrubs
Minor components: Tallgrasses and cool season grass/grasslikes
Community 1.1
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
336 |
773 |
1211 |
| Tree |
112 |
258 |
404 |
| Forb |
56 |
129 |
202 |
| Shrub/Vine |
56 |
129 |
202 |
| Total |
560 |
1289 |
2019 |
State 2
Closed Canopy Woodland
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.
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.
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.
| Brush Management |
|
| Prescribed Burning |
|
| Firebreak |
|
| Prescribed Grazing |
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Additional community tables
