Sandy 25-32 PZ
Scenario model
Current ecosystem state
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Management practices/drivers
Select a transition or restoration pathway
- Transition T1A More details
- Transition T1B More details
- Transition T1C More details
- Restoration pathway R2A More details
- Transition T2A More details
- Transition T2B More details
- Restoration pathway R3A More details
- Restoration pathway R3B 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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Description
There are two communities in the Savannah State: the Midgrass Savannah Community (1.1) and the Woodland Savannah Community (1.2).
Prior to settlement, Sandy sites had a savannah appearance with open areas dominated by mid grasses (little bluestem and sideoats grama) interspersed with scattered mottes dominated by oaks. The Midgrass Savannah (1.1) may have up to 10 percent canopy cover while the Woodland Savannah (1.2) will have more than 10% woody canopy. Relatively frequent fires (7-12 year mean fire return interval) (Frost 1998) maintained the open areas by reducing cover shrub. 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 only 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 the canopy to reach the 20 percent level that indicates the shift to the Woodland Savannah Community (1.2). This transition is not dependent on the 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 of sufficient quantity to allow fire to reduce young brush and tree cover but not of sufficient quantity to create crown fires that would negatively affect single-stemmed mature trees. Therefore, the savannah would be relatively open for a short period following a fire, shrubs would begin to reestablish reducing the savannah appearance, fire would return in 10 years or less, this fire would reduce young shrubs and trees without harming mature trees returning the savannah appearance and shifting species composition back to dominance by little bluestem and other grasses.
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 negatively affect 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 Woodland Savannah Community (1.2) dominates the site with up to 30 percent woody cover. 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 Midgrass Savannah Community (1.1).
It was rare that a dense woodland would shift to a grassland or savannah. In order to do so, something would have to cause a widespread die-off of 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 (1.1) always has the 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.
Submodel
State 2
Shrubland State
Description
The Shrubland State (2.0) is characterized by trees, a significant shrub cover, and a shortgrass- understory. There are two communities in the Shrubland State (2.0): the Altered Savannah Community (2.1) and the Woodland Community (2.2). The two communities in the Shrubland State interact in much the same manner as the two communities in the Savannah State. The Altered Savannah Community (2.0) must be maintained with fire or brush control as it is under constant pressure by woody species to shift to the Woodland Community.
The hardwoods that made up a portion of the plant community in the Savannah State (1.0) may or may not be present in the Shrubland State (2.0). The transition to the Shrubland State will not cause a decrease in the number of hardwoods. However, the Shrubland State often occurs on lands that have been cleared of brush and trees at some point in the past. Trees were removed for lumber or firewood, in some cases to clear the land for pasture or farming. Rootplowing had the same effect as tillage, converting the site to a grassland immediately following plowing but leaving the site subject to rapid invasion by fast-growing shrub species. This transition may respond like agricultural conversion and may have been accompanied by shifts in soil chemistry and structure. Rootplowing is likely to shift the community to the Woodland Community (2.2). Once invasive woody species begin to establish, returning fully to the native community is difficult, but it is possible to return to a similar plant community.
The understory of the Shrubland State tends to be dominated by shortgrasses and lower-palatability forbs. The communities in the Shrubland State have a degraded herbaceous community when compared to the Savannah State. This is generally a result of long-term improper grazing management.
One factor that creates overgrazing on this site is the failure to reduce the stocking rate as woody cover increases. Increased woody cover results in fewer grazeable acres and less forage being available. Unless stocking rates are reduced, the stocking pressure on the remaining forage increases, which increases the likelihood of palatable plants being overgrazed, losing vigor, and being grazed out of the community. At the same time, less palatable plants gain a comparative advantage and increase their representation in species composition.
Submodel
State 3
Converted State
Description
The Converted State (3.0) includes cropland, tame pasture, hayland, rangeland, and go-back land. Agronomic practices are used with non-native forages in the Converted State (3.0) and to make changes between the communities in the Converted State (3.0). Cropland and tame pasture require repeated and continual inputs of fertilizer and weed control to maintain the Converted State.
Submodel
State 4
Eroded State
Description
This state is characterized by a single community, the Sparsely Vegetated Community (4.1).
The Sandy Ecological Site is one of the most, if not the most, erosive site in MLRA 82. Mismanagement (overgrazing, improper brush management, poorly managed farming) that leaves bare soil subject to raindrop impact, overland flow, or wind erosion will erode. Because sandy soils have weak chemical bonds to organic matter, the A horizon has higher erodibility. Once the A horizon has eroded, the other horizons are at increased risk of erosion.
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 4 being a terminal state. Return to another state will be the result of site restoration efforts, not typical management changes.
Submodel
Mechanism
The driver for Transition T1A is lack of brush management coupled with overgrazing.
Overgrazing, lack of fire, and/or improper brush management will result in the site crossing a threshold to the Shrubland State (2.0) 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 look to 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.
Some of the increasing shrubs are introduced because their fruits and seeds are relished by wildlife that then spread them through droppings. If the management goal is to maintain the savannah state, managers need to take action when canopy cover is less than 10%.
Mechanism
This transition occurs when there is land clearing to remove the woody plant community. The transition to the Converted State (3.0) from the Savannah State (1.0) occurs when the grassland is cleared and planted to cropland or hayland. The Converted State includes cropland, hayland, tame pasture, and go-back land. The site is considered “go back land” during the period between cessation of active cropping, fertilization, and weed control and the return of native plants or escaped introduced plants.
Mechanism
This transition occurs when the Savannah site is subject to aggressive brush control, drought, and overgrazing. Broadcast brush control includes chaining, rootplowing, and chemical treatment. Seeding may or may not be done. 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 or active management of the herbaceous restoration process (range seeding).
An integrated approach of biological, mechanical and chemical brush control in combination with prescribed fire, proper grazing, and favorable growing conditions is the most economical means of creating and maintaining the desired plant community. A long-term prescribed fire program may sufficiently reduce brush density to a level below the threshold of the Savannah State (1.0). However, the fire program will have to be aggressive because many of the woody species on this site are resprouters following fire and fuel loading is marginal. Establishment of native grasses is difficult and dependent upon natural seeding from remnant patches and seed banks. If remnant populations of midgrasses and desirable forbs are not present at sufficient levels, range planting will be necessary to restore a desirable herbaceous plant community.
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. Re-growth of established woody plants will slow and it will become more difficult for new plants to establish. The extent to which the original Midgrass 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 |
Mechanism
The Shrubland State (2.0) is a very stable state, and transition to the Converted State (T2A) will require high energy input. The threshold for this transition occurs when the soil is plowed and the woody plant community is removed. The size and density of brush will require heavy equipment and energy-intensive practices (i.e. rootplowing, raking, roller-chopping, or heavy disking) to prepare a seedbed. The Converted State includes cropland, tame pasture, hayland, and “Go Back” land. The site is considered “Go Back land” during the period between cessation of active cropping, fertilization, and weed control and the return to the “native” states, even though the returning vegetation may be introduced plants.
Mechanism
The driver for this transition is non-selective brush control through chaining, rootplowing, or broadcast herbicides. This action removes the trees. Contributing drivers include heavy browsing by wildlife, sheep, and goats and overgrazing by cattle. The resprouting shrubs are generally not palatable forage. Severe soil degradation can result. A loss of vegetative cover can be 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. Mottes of trees may or may not survive this transition.
Mechanism
*Restoration to the Savannah State (1.0) is unlikely. Return to native communities in the Savannah State is more likely to be successful if soil chemistry and structure have not been heavily disturbed. Preservation of favorable soil microbes increases the likelihood of a return to reference-like conditions as does remnant seed sources. Converted sites may be returned to a community similar to the Savannah State through active restoration, including seedbed preparation and seeding of native grass and forb species.
Relevant conservation practices
| Practice | External resources |
|---|---|
|
Brush Management |
|
|
Prescribed Burning |
|
|
Prescribed Grazing |
|
|
Range Planting |
Mechanism
Restoration from the Converted State (3.0) can occur in the short term through active restoration or over the long-term due to the cessation of agronomic practices. Heavily disturbed soils are most likely to return to the Shrubland State (2.0).
Relevant conservation practices
| Practice | External resources |
|---|---|
|
Brush Management |
|
|
Prescribed Burning |
|
|
Prescribed Grazing |
|
|
Range Planting |
Mechanism
The driver for this transition is severe soil erosion and loss of soil properties. In some cases, this erosion can be extreme enough to result in the loss of the A (and even B) horizons. Mottes of trees may or may not survive this transition.
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The Ecosystem Dynamics Interpretive Tool is an information system framework developed by the USDA-ARS Jornada Experimental Range, USDA Natural Resources Conservation Service, and New Mexico State University.