Gravelly Hills
Scenario model
Current ecosystem state
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Management practices/drivers
Select a transition or restoration pathway
- Transition T1A More details
- Restoration pathway R2A More details
- Transition T2A More details
- Restoration pathway R3A 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 plant communities and ecological dynamics of the Gravelly Hills ecological site. This state includes the biotic communities that become established on the ecological site under the natural disturbance regime prior to pre-European settlement. The main pathways on this site are fire and climate (drought/wet cycles). Drought is frequent on this site. The reference plant community of the Gravelly Hills site is grass-dominated and supports a mixture of warm- and cool-season grasses, widely spaced shrubs and trees, and a minor component of forbs. Sideoats grama and black grama are the dominant grass species. Common shrubs include Bigelow sagebrush, broom snakeweed, longleaf ephedra, feather dalea, and yucca. Sparse stands of pinyon-juniper (P-J) may be widely scattered across the site with 3-5% canopy cover of pinyon and juniper (P-J). The reference state is self-sustaining and resistant to change due to high resistance to natural disturbances and high resilience following natural disturbances. When natural disturbances occur, the rate of recovery is variable due to disturbance intensity. Once invasive plants establish, return to the reference state may not be possible.
Submodel
Description
This state (Encroached State) is characterized by a noticeable increase in P-J and decreased grass cover and production compared to the Reference State. Grass cover consists mainly of patchy blue grama, ring muhly, galleta, threeawn, and dropseed. Other species that may increase include pingue and broom snakeweed. Grass cover is patchy, large bare areas may be present under P-J canopies. Evidence of erosion such as small rills and pedestalled plants may be present. 10 to 15% tree species canopy cover is usually characterized of this state.
Submodel
Description
This state is characterized by reduced cover and production of grasses and accelerated erosion. P-J and shrubs dominate. About 25% tree canopy cover is thought to be the threshold for the transition from the P-J Encroached State to the Eroded State. Arnold (1964) analysis of possible explanations of these threshold include allelopathy, shade, precipitation interception by the canopy, and litter cover forming a physical barrier to germinating plants. Other explanations offered to explain the reduced under-canopy vegetation include root competition for soil moisture, and possible chemical properties of one-seed juniper litter (Arnold, 1964). Broom snakeweed and pingue are often found at increased densities. This state is characterized by soil loss from large, interconnected bare areas and hydrologic events carrying sediment off-site. Evidence of erosion such as rills, gullies, and pedestalled plants is common. The Eroded State is characterized by competition for nutrients, water, and light in the interstitial spaces; multiple age classes of P-J; and sparse understory. Fire return interval in Eroded State may exceed 100 years.
Submodel
Mechanism
This transition is from the native shrub and perennial grass state (reference state), to a state that is dominated by pinyon and juniper (Encroached State). The resulting decreased competition by perennial grasses and forbs facilitates the encroachment of pinyon and juniper. Events include time without disturbance, drought, insect herbivory, continuous season long grazing of perennial grasses, and tree invasion. As tree canopy density increase, perennial grass and forb cover is reduced and composition has changed, bare ground will increase in size and frequency, accelerating erosion, increasing run-off and further affecting the watershed functionality. This transition also favors the establishment of invasive annual species such as cheatgrass. In wet years, high grass cover may suppress P-J seedlings by competing directly for available soil moisture; lower grass cover may facilitate P-J establishment.
Mechanism
Restoration Pathway from Encroached State to Reference State (R2A). Removal of P-J will be necessary to reduce competition for resources. PJ removal could be from fire; treatments; drought; and/or insect/pathogen outbreaks. Prescribed grazing will help ensure adequate rest following PJ control and will assist in the establishment and maintenance of grass cover, seeding may be necessary depend on herbaceous cover present on the specific site. This pathway requires lots of energy input into the system.
Mechanism
When this transition to state 3 (Eroded State) occurs the site has lost much of its expected resistance and resilience. At this point natural and/or management actions have decreased the understory to a point where erosion increases. Lack of from fire, insects, and drought could cause the tree canopy to close, effectively reducing the herbaceous understory and facilitating the transition. Improper grazing and or increase surface disturbance combined with periods of drought can facilitate this transition since soil stability is lost and susceptibility to soil loss increases.
Mechanism
Restoration Pathway from Eroded State to Reference State (R3A). Removal of P-J will be necessary to reduce competition for resources. Erosion control structures in conjunction with seeding will be necessary to reestablish hydrology and grass dominance. Prescribed grazing will help ensure adequate rest following seeding and P-J removal and will assist in the establishment and maintenance of grass cover. The degree to which this site is capable of recovery is dependent on the extent of soil degradation.
Model keys
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