Natural Resources
Conservation Service
Gravelly Sandy Loam 25-32 PZ
Scenario model
Current ecosystem state
Select a state
Management practices/drivers
Select a transition or restoration pathway
- Transition T1A More details
- Transition T1B More details
- Restoration pathway R2A More details
- Transition T2A More details
- Restoration pathway R3A More details
- Transition T3A More details
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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State 1
Savannah State
Savannah State
Description
There are two communities in the Savannah State: the Little Bluestem/Oak Savannah Community (1.1) and the Oak Woodland Community (1.2). The Little Bluestem/Oak Savannah Community occurred over the majority of this ecological site in a dynamically shifting mosaic over time with the Oak Woodland Community.
Prior to settlement, Gravelly Sandy Loam sites had a savannah appearance with open areas dominated by midgrasses (little bluestem and sideoats grama) interspersed with scattered mottes dominated by oaks. The Little Bluestem/Oak Savannah (1.1) may have up to 20 percent canopy cover while the Oak Woodland will have more than 20% woody canopy. Relatively frequent fires (7-12 year mean fire return interval) (Frost 1998) maintained the open areas by reducing canopy cover. Mature hardwoods found in the mottes were long-lived and resistant to ground fires.
Fires were natural or human-induced. When fires were frequent on the savannah, most fires burned the understory, leaving mottes of trees. Even with proper grazing and favorable climate conditions, lack of fire for 7-12 years will allow trees and shrubs to increase in canopy to reach the 20 percent level that indicates the shift to the Oak Woodland Community. This transition is not dependent on degradation of the herbaceous community, but on the lack of some form of brush control.
Shrub species would increase within the grassland portion of the savannah and within the understory of the mottes following fire. Fine fuels were continuous and sufficient in quantity to allow fire to reduce young brush and trees; quantity was not sufficient to create crown fires that would reduce the cover of single-stemmed mature trees. The savannah would be relatively open for a short period following a fire. However, shrubs would begin to reestablish reducing the savannah appearance.
Occasionally a site would not burn for a period long enough for trees to grow to a fire resistant stage within the grassland portion of the savannah. As these trees matured, the fine fuel understory would decrease, reducing the ability of fires to grow large enough (and hot enough) to reduce the canopy cover of mature trees. This long-term lack of fire (25 - 50 years) would allow large trees to fill in open areas shifting the site to a woodland appearance. Once the site had dense tree cover, the site would be resistant to fires and a very resilient woodland community would develop.
In the absence of fire, the Oak Woodland Community (1.2) dominated the site with a near closed canopy stand of hardwoods, including post oak and live oak. The two communities in the Savannah State shifted between one another depending on the frequency and intensity of fire, grazing, and drought. The primary influence on the understory is grazing management and the primary influence on the overstory is fire. This allows the understory and overstory to react independently, i.e., trees can increase to the point where they dominate a site even if the understory component remains vigorous and intact. Grazing management alone cannot maintain the site in the Little Bluestem/Oak Savannah Community (1.1).
It was rare that a dense woodland community would shift to a grassland or savannah community. In order to do so, widespread die-off would need to occur within the mature trees. This could occur due to disease or to a very hot fire that spread to the tree crowns, events that typically only occur every 300 to 1,000 years. Following a severe fire, the site would have a grassland appearance for a few years as shrubs and trees resprouted or grew from seed.
Shrubs and trees comprise a portion of both plant communities in the Savannah State (1.0), hence woody propagules are present. The Savannah State always has potential for shrub dominance without fire. Mann (2004) discussed the importance of human induced fire as an important factor in maintaining open grasslands before European settlement.
The relationship between the two communities in the Savannah State remains similar post-settlement. However, natural fires become less frequent and less widespread as human population density increases. “Cool”, slow-burning wildfires have become basically non-existent, because they are relatively easy to put out using modern firefighting equipment and techniques. Without fire, the reference savannah community becomes less resilient. Unless managers practice some method of brush control, shrub species will increase in the grassland portion of the savannah and in the understory of the oak mottes.
Brush control can play the role that natural fires played pre-settlement. However, it is difficult to manage in an ecological and economic matter on a small scale, as this site is rapidly repopulated by shrubs and trees without fire or brush management. Brush control may be prescribed fire, mechanical, chemical or biological control, or targeted grazing (generally by goats, although some instances exist in the Central Basin where exotic wildlife species or overpopulated white-tailed deer reduce woody cover). There are examples of this site being maintained as a savannah with introduced hay meadows and mottes of trees.
Dominant plant species
oak (Quercus), tree
little bluestem (Schizachyrium scoparium), grass
sideoats grama (Bouteloua curtipendula), grass
Submodel
State 2
Shrubland State
Shrubland State
Description
This state contains a single community, the Shrubland Community (2.1). The Shrubland State (2.0) is characterized by a shortgrass-dominated understory with scattered trees and shrubs. Bare ground, erosion, and water flow patterns will increase as the understory is degraded. Forage production will decline.
The hardwoods that made up a portion of the plant community in the Savannah State will likely be present in the Shrubland State unless they were removed by brush control or lumber harvest. The transition to the Shrubland State will probably not cause an increase in the number or biomass production of hardwoods. However, because the production of herbaceous species will decline, the relative species composition of shrubs and trees will increase. If the hardwoods were removed, they will be slow to reestablish.
Trees and shrubs remain a part of the community, but woody species composition may vary greatly. The woody component may be similar to the woody component in the Little Bluestem/Oak Savannah community (1.1). Mature oaks occur in mottes with scattered brush, though the woody component is more frequently made up of many species of widely scattered shrubs.
Dominant plant species
oak (Quercus), tree
pricklypear (Opuntia), shrub
whitebrush (Aloysia gratissima), shrub
Texas persimmon (Diospyros texana), shrub
Submodel
Description
This state includes a single community, the Eroded Community (3.1), and it is characterized by significant soil loss.
Once the A Horizon is eroded (partially or completely), nutrient cycling is disrupted and difficult to restore. Loss of soil is accompanied by disruption (or loss) of soil mycorrhiza. Steeper portions of this site are subject to severe erosion when the plant cover is disturbed. Some examples of the site have eroded to granite bedrock.
Because of the availability of invasives with low palatability, this site rarely stays barren. There are examples that are degraded but not dominated by brush but these examples tend to be quickly reinvaded by annual forbs and grasses as well as low-palatability shrubs.
The Eroded State has the potential to be a terminal state. Due to the relatively high risk of severe soil erosion of the sandy loam soils, this site can erode to the point where there is a loss of soil functionality. When this level of erosion occurs, the site loses soil structure, soil fertility, organic matter, and/or soil microflora. There are examples of the loss of the A and B horizons and some with the soil eroded to bedrock. Once the site loses soil horizons or soil functions, it is very difficult or impossible to return the site to one of the other States, resulting in State 3 being an irreversible terminal state.
Dominant plant species
tridens (Tridens), grass
Submodel
Mechanism
The driver for Transition T1A is lack of brush management coupled with overgrazing. Overgrazing and/or improper brush management will result in the site crossing a threshold to the Shrubland State (2.0), which is characterized by shortgrasses, unpalatable grasses and forbs, annual grasses and forbs, and shrubby species. Bare ground, erosion, and water flow patterns will increase, and forage production will decline. Without regular fire, woody species will increase in size, density, and canopy cover, reducing production from herbaceous species. Woody species composition may vary greatly depending largely on management. Trees will be present if they were not historically removed. More frequently, the woody component is made up of many species of widely scattered shrubs. Overgrazing causes a loss of dominant midgrasses and forbs from the savannah. This transition is indicated by a decrease of little bluestem and sideoats grama to less than 10 percent of species composition of the herbaceous community. Once these species are lost from the community or present only in trace amounts (typically with low vigor), grazing management alone cannot create a shift back to the reference community. At this point, a threshold has been crossed indicating a change in state. Degradation of the herbaceous community combined with the aggressive nature of shrubs creates a loss in the savannah appearance of the site. The grassland portion is reduced and the trees exist in competition with aggressive shrubs. This competition limits the ability of trees to reproduce and increase. The aggressive nature of shrubs keeps the Savannah State (1.0) at high risk of transition to the Shrubland State (2.0). The possible exception would be the effective use of goats to target and suppress the shrubs. The trigger for this transition comes when shrubs reach reproductive capacity. Overgrazing, prolonged drought, no fire or brush management and a warming climate will provide a competitive advantage to shrubs.
Mechanism
This transition occurs when the Gravelly Sandy Loam site is subject to abusive brush control, drought, and overgrazing. The effects may be seen as a loss of vegetative cover, loss of soil, and destruction of soil structure or soil health. In some cases, this erosion can be extreme enough to result in the loss of the A (and even B) horizons.
Mechanism
The driver for Restoration Pathway R2A is fire and/or brush control combined with the restoration of the herbaceous community. Establishment of native grasses is difficult and dependent upon natural seeding from remnant patches and seed banks. This site is often too shallow for effective reseeding. A long-term prescribed fire program may sufficiently reduce brush density to a level below the threshold of the Savannah State (1.0). Proper grazing management and stocking rates maintain the herbaceous layer in this state. With proper grazing management, midgrasses can regain dominance on the site and undesirable trends in soil organic matter, fertility, temperature, and erosion can be arrested and reversed. The extent to which the original Little Bluestem/Oak Savannah Community (1.1) can be re-established will depend on the extent to which soil physical and chemical properties were altered during retrogression (Heitschmidt and Stuth 1991).
Relevant conservation practices
| Practice | External resources |
|---|---|
|
Brush Management |
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Prescribed Burning |
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Prescribed Grazing |
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Range Planting |
Mechanism
The driver for this transition is improper management. The effects may be seen in a loss of vegetative cover followed by a loss of soil. In some cases, this erosion can be extreme enough to result in the loss of the A (and even B) horizons. Severe soil degradation can be caused by abusive grazing or other soil-disturbing activities including rock harvest. This loss of soil results in or accompanies the loss of the vegetative cover associated with this site. Long-term drought may also trigger this transition from the Shrubland State (2.0) or exacerbate the effects of inappropriate grazing management on the Gravelly Sandy Loam site.
Mechanism
*The likelihood of returning to the Savannah State (1.0) is improbable and would require extensive and intensive restoration efforts. Range restoration techniques can be used to restore this site on high-value lands (such as mining reclamation). This will likely require replacement of topsoil and planting of native species, though the site is often too shallow for effective reseeding. A return to the reference condition should not be expected, and savannah conditions are only possible with continued inputs and management over a long period of time.
Relevant conservation practices
| Practice | External resources |
|---|---|
|
Brush Management |
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Prescribed Burning |
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Prescribed Grazing |
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Grazing Land Mechanical Treatment |
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Range Planting |
Mechanism
The Eroded State is frequently a terminal state unless range restoration processes are put in place. Restorations to the Shrubland State (2.0) are uncommon when there has been a pronounced degradation of soil chemistry and structure and a severe decline of native plant communities. Without fire or brush management, woody species may repopulate and eventually dominate the site.
Relevant conservation practices
| Practice | External resources |
|---|---|
|
Brush Management |
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Prescribed Burning |
|
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Prescribed Grazing |
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Grazing Land Mechanical Treatment |
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Range Planting |
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