Aspen Riparian 12-18 PZ
Scenario model
Current ecosystem state
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Management practices/drivers
Select a transition or restoration pathway
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Transition T1A
Invasion of the site by non-native plant species.
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Transition T2A
Time and lack of wildfire
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Transition T2B
Hydrologic alteration
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Restoration pathway R3A
Mechanical removal of conifers, prescribed fire, ungulate exclosure fencing
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Transition T3A
Hydrologic alteration
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Restoration pathway R4A
Restoration of hydrologic function
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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Description
This state represents the pristine historical reference conditions with no exotic species present. Healthy aspen stands are on this site type are heterogeneous with vigorous trees in all natural life stages from young saplings to mature. Stands will cycle through several development phases from shrub/grass communities through mature stands, with many phases often occurring on a single site. These dynamics are driven by an intact historical disturbance regime with periodic events acting to maintain a dynamic equilibrium with adjacent stream morphology and vegetative composition; helping to reduce conifer invasion; remove diseased and decadent mature trees and stimulate sucker reproduction. The resilience and resistance of the site is bolstered by positive feedbacks between aspen production and the formation of deep, mollic epipedons with high organic matter, nutrient content and water holding capacity; and negative feedbacks between stand maturity and disturbance frequency/magnitude.
Submodel
Description
This state is similar to the historical reference state yet with the introduction of non-native plant species and an increased presence of western juniper. Kentucky bluegrass is one of the most common and persistent invading herbaceous plants with others including cheatgrass (Bromus tectorum), Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare), houndstongue (Cynoglossum officinale), Whitetop and St. Johnswort (Hypericum perforatum) also common. Ecological process and function have not been altered fundamentally by this low level of invasion, yet resistance and resilience to disturbance are decreased. Vegetated communities include all historical functional and structural groups, yet composition and richness may be reduced. This state is common due to widespread invasion of Kentucky bluegrass and expansion of western juniper in the Western US.
Submodel
Description
In this state, conifer succession has advanced to overtop aspen and outcompete the species for light and soil moisture. On this site, western juniper is the most likely conifer invader, yet ponderosa pine (Pinus ponderosa), or Douglas fir (Pseudotsuga menziesii) may also encroach on favorable sites. Over mature, decadent aspen may still be present and aspen reproduction may occur in patches with adequate light and moisture. However, feedbacks have been altered and aspen will likely not successfully compete with conifers to regenerate successfully, recruit into the overstory, and regain forest structure without management interventions or altered disturbance regimes. Impacts and emerging feedbacks may include alterations to soil chemistry, changes in hydrologic cycling (including increased sublimation and translocation of snowfall by conifers), and persistent increases in understory shading favoring more shade tolerant conifer reproduction (Wall et al. 2001, LaMalfa and Ryle 2008). Heavy herbivory of sapling aspen, and/or low intensity fire that leaves more fire resistant conifers intact may further accelerate the loss of aspen to conifers. Several community phases may occur within this state with the common thread of conifer dominance.
Description
Streambanks have become unstable from loss of vegetation and the channel degrades becoming deeper and wider in the process. Subsurface flows are affected. The water table drops and storage of water for the late season flows is reduced. Plants well adapted to a drier climatic regime increase or invade and production drops. Channel widening and incision are common in this state as unstable banks and vegetation loss create a positive feedback loop that decreases resilience to runoff events. Abandoned floodplains transition into primary terraces and primary terraces transition toward secondary terraces which become dominated by drought adapted species that do not require a connection to the water table. Many community phases may exist within this state depending on the disturbance history and stand dynamics prior to incision.
Mechanism
Time and lack of wildfire allows juniper to overtop and outcompete aspen for light and moisture. Shade intolerant aspen die off and regeneration is dramatically decreased due to closed canopy conditions. On some sites ponderosa pine and/or Douglas fir may encroach and facilitate a similar dynamic in the absence of fire.
Mechanism
This transition may be the result of several disturbances that lower water tables beyond depths that support riparian woody vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness. These may include: alteration of streamflow by irrigation or impoundment leading to a lowering of the water table during times of year when riparian woody vegetation is dependent; removal of beaver; direct manipulation of channel morphology (namely straightening for agricultural or development purposes); removal of large woody debris or large woody debris sources, from channels or adjacent forests and significant alterations of upland watershed vegetation altering peak discharge or sediment loads.
Mechanism
Mechanical removal of conifers may release understory aspen from canopy shading. Prescribed fire may also be used to remove conifers and stimulate aspen regeneration. However, under circumstances where advanced decline has led to an absence of aspen regeneration, the use of prescribed fire may actually lead to further damage to the clone and may accelerate complete clone mortality. Depending on the degree of encroachment and aspen loss, these interventions may transition the site to any one of the current potential community phases. Under conditions of high native ungulate or livestock use of the stand, management actions such as fencing or jackstrawing may need to follow conifer removal to allow aspen regeneration to occur unimpeded.
Context dependence
Restoration options will be highly site specific and may not be possible in many circumstances.
Mechanism
This transition may be the result of several disturbances that lower water tables beyond depths that support riparian woody vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness. These may include: alteration of streamflow by irrigation or impoundment leading to a lowering of the water table during times of year when riparian woody vegetation is dependent; removal of beaver; direct manipulation of channel morphology (namely straightening for agricultural or development purposes); removal of large woody debris or large woody debris sources, from channels or adjacent forests and significant alterations of upland watershed vegetation altering peak discharge or sediment loads.
Mechanism
Restoration of hydrologic and biotic process and function through rehabilitation of channel and vegetation structure may be possible but will require considerable inputs, time and cost. This may require the placement of large woody debris, creation or removal of impoundments, alteration of water withdrawals, management changes to adjacent agricultural or grazing practices, or mechanical manipulation of stream channel courses among other intensive interventions.
Context dependence
Restoration options will be highly site specific and may not be possible in many circumstances.
Model keys
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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.