Loamy Overflow
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition T1A More details
- Transition T1B More details
- Transition T2A More details
- Transition T2B More details
- Restoration pathway R3A More details
- Transition T3A More details
- Transition T4A More details
- Transition T4B More details
- Transition T5A 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
The Reference State represents the best estimate of the natural range of variability that dominated the dynamics in this ecological site prior to European settlement. The Reference State may exist, but is unlikely to function within the natural range of variability due to the spread of Kentucky bluegrass and smooth brome onto this site. In the Reference State, this site is dominated by warm- and cool-season grasses, and various shrub and tree species that are scattered across the site. Grazing or the lack of grazing, flooding or lack of flooding, fire, excessive haying, and invasion of non-native cool-season grasses are the major drivers of this State. Flooding and no fire on this site can cause a transition to the Woody Overstory State (4.0).
Submodel
Description
The Native/Invaded State is very similar to the Reference State (1.0). It is still dominated by native mid- and tall warm- and cool-season grasses, but invasive non-native cool-season grasses are now present in all community phases of this State. These non-native cool-season grasses can comprise up to 30 percent of total annual air-dry production. The primary disturbance mechanisms for this State include grazing by domestic livestock, infrequent fires, and flooding. Timing and intensity of grazing events coupled with weather dictate the dynamics that occur within this State. The cool-season native grass can decline and an increase in introduced sodgrasses will occur. Many times, the Native/Invaded State appears as a mosaic of community phases caused primarily by continuous season-long grazing.
This State represents the more common range of variability that exists with higher levels of grazing management but in the absence of periodic fire followed by short-term intensive grazing. The Native/Invaded State is dominated by cool- and warm-season grasses. It can be found on areas that are properly managed with grazing and prescribed burning, and sometimes on areas that receive occasional short periods of rest. Warm-season species can decline and a corresponding increase in cool-season grasses will occur.
Submodel
Description
This State is the result of invasion and dominance of introduced species. The Invaded State is characterized by the dominance of Kentucky bluegrass and smooth brome, and an increasing thatch layer that effectively blocks introduction of other plants into the system. Plant litter accumulation tends to favor the more shade-tolerant, introduced grass species. The nutrient cycle is also impaired, and the result is typically a higher level of nitrogen, which also favors the introduced species. Increasing plant litter decreases the amount of sunlight reaching plant crowns, thereby shifting competitive advantage to shade-tolerant introduced grass species. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the introduced grass species. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in the invasive grass dominance. Preliminary studies would tend to indicate this threshold may exist when Kentucky bluegrass exceeds 30 percent and native grasses represent less than 40 percent of the plant community composition. Plant communities dominated by Kentucky bluegrass have significantly less cover and diversity of native grasses and forb species (Toledo, D., et al., 2014).
Submodel
Description
The Wooded Overstory State is the result of the establishment of a tree overstory and shrub mid-story canopy. This State is more common on the lower reaches of the site. The dynamics of the Wooded Overstory State are largely due to flooding and successional changes, starting with cottonwood and shrub establishment, and eventually the development of a green ash and boxelder plant community. The successional process can restart following another flooding event. Water control structures which limit flooding, livestock grazing, heavy wildlife browse, fire, the introduction of non-native, cool-season grasses, and encroachment of juniper can alter the dynamics of this site, resulting in old remnant stands of trees with little, if any regeneration. Downcutting of the drainageway or stream channel will eventually leave the overflow site on a higher and drier landscape position. In this case the site will be described as the Loamy Terrace Ecological Site (R064XY028NE).
Submodel
Description
The Woody Invaded State developed as a result of continuous season-long grazing, or continuous seasonal grazing and no fire combined with the invasion and establishment of Russian olive, saltcedar, or juniper trees on this Loamy Overflow ecological site. With time, the cottonwood, boxelder, and ash trees that survive become mature, with little or no regeneration. This is due mainly to grazing of seedlings and saplings. Wildlife browse can also contribute to the loss of native tree and shrub regeneration. Grazing that limits regeneration also results in a reduction of the desirable native herbaceous species, often resulting in a dominance of species such as bluegrass and smooth brome, and forbs such as western ragweed, Canada thistle, burdock, and hound’s tongue.
Submodel
Mechanism
Continuous season-long grazing and/or excessive haying, or non-use and no fire, and the invasion of non-native cool-season grasses will transition the Reference Plant Community (1.1) to the Native/Invaded State (2.0).
Mechanism
Flooding, followed by long-term prescribed grazing, and no fire are necessary to shift this plant community to the Cottonwood/Prunus-Snowberry/Forbs/Grasses Plant Community (4.1). Flooding reduces herbaceous competition through scouring of the soil surface and provides a site for regeneration to occur. Once a flooding event occurs during the proper time, a long-term period of prescribed grazing is necessary to establish and maintain a woody plant community. Grazing during the mid-summer growing season typically has an adverse effect on woody regeneration. This State is more likely to occur and persist on the mid- to lower reaches of a drainage.
Mechanism
Heavy, continuous grazing, excessive haying, or long-term light grazing will cause a transition the Invaded State (3.0). Extended periods of non-use and no fire will also result in the expansion of non-native cool-season grasses on this site. The ecological threshold can be identified by the percentage of non-native cool-season species in the plant community. Preliminary studies would tend to indicate this threshold may exist when Kentucky bluegrass exceeds 30 percent of the plant community composition and native grasses represent less than 40 percent (Toledo, D., et al., 2014). Smooth brome is assumed to follow a similar ecological threshold but is not documented scientifically.
Mechanism
Flooding and no fire, followed by prescribed grazing and including proper stocking, change in season of use, and adequate time for recovery will likely transition this site to the Woody Overstory State (4.0). Timed grazing is very important and must be followed for many years for saplings to attain a height at which livestock will not damage or kill the trees. Wildlife browse can also be a concern if the management objective is to improve the overstory canopy. The Wooded Overstory State (4.0) is more likely to occur on the mid- to lower reaches of a drainageway.
Mechanism
Long-term prescribed grazing (moderate stocking levels coupled with adequate recovery periods, or other grazing systems such as high density, low-frequency intended to treat specific species dominance, or periodic light to moderate stocking levels, possibly including periodic rest) may lead this plant community phase over a threshold to the Native/Invaded State (2.0). Prescribed burning may also be needed to suppress cool-season invasive grasses. This will likely take a long period of time, possibly up to ten years or more, and recovery may not be attainable. Success depends upon whether native reproductive propagules remain intact on the site.
Mechanism
Encroachment of juniper trees from upland sites, or invasion of non-native trees, no fire for extended periods of time, continuous season-long grazing, or continuous seasonal grazing and time will cause a transition to the Wooded Invaded State (5.0). Canopy cover increases as trees increase in size, which alters microclimate and reduces fine fuel amounts, resulting in reduced fire intensity and frequency.
Mechanism
Fire, invasion of non-native, cool-season grasses, excessive haying, continuous season-long grazing, or continuous seasonal grazing resulting in little woody regeneration, and time will transition this State to the Native/Invaded State (2.0).
Mechanism
Encroachment of juniper from upland sites, and/or invasion of non-native trees, coupled with no fire and continuous season-long grazing or continuous seasonal grazing will transition this State (4.0) to the Wooded Invaded State (5.0).
Model keys
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