Ecological dynamics
This willow riparian site complex is composed of several distinct plant community components which are adapted to various channel and bank configurations based on the depth and duration of seasonal surface and subsurface flows. The site occupies three riparian zones, the toe zone, bank zone and over bank zone. In the toe and bank zone, emergent sedge and spike rush dominated components are associated with the channel toe or shelf, alluvial bars and the lower bank (hydrology the bank full zone). Willow dominated plant community components in the overbank zone are associated with the upper bank, a variable low natural levee on top of the bank and a small portion of the adjacent active floodplain. It is a dynamic site with components responding to changes in channel configuration, sediment movement and flow regimes. The presence of beaver significantly contributes to the dynamic nature of the site promoting sediment deposition and water distribution.
The primary potential native plant community component located on the upper bank and over bank zone is dominated by willows. Periodic inundation and sediment deposition followed by flow subsidence and soil aeration provides ideal willow habitat. Soil aeration following inundation is a critical component for willow establishment and survival. The plant community is dominated by Booth’s willow. Yellow willow is secondary along with coyote willow and a variety other shrubs including redosier dogwood, rose and golden currant. Sedges made up a significant composition of the historic understory. The exact understory percentage is not presently known as reed canary grass and/or meadow foxtail readily invade. Vegetative composition of this component is approximately 70 percent shrubs, 25 percent grasses and 5 percent forbs. Approximate ground cover is 90 to 120 percent (basal and crown) with shrub canopy cover being 60-70 percent. Production averages 3000 lbs/acre in normal years. In the Cowardian system this primary plant community component is classified as a palustrine scrub/shrub wetland.
Two secondary potential plant community components on this site are located on alluvial gravel and silt point bars. Formed over time by annual inundation and sediment deposition they make up to 20 percent of the site complex. Silt portions of point bars are initially dominated by sedges and bulrush. Narrowleaf (coyote) willow, a rhizomatous species initially establishes on gravelly bars. These components are variable in production averaging 500 to over 800 lbs/acre in normal years. Bare ground on the bars is greater than 30 percent. These two plant community components are in a constant state of flux. In high seral state or condition they contribute significantly to stream aggradation, meander formation, channel stability and floodplain connectivity.
Located on narrow benches at the toe zone there is a third potential plant community component dominated by emergent vegetation. The narrow bench forms in areas where slow water velocities allow sediments to accumulates over gravel. Located below the average water elevation or baseflow the bench supports a potential plant community dominated by sedges, panicled bulrush and spike rush. Adjacent to areas of high bank stress, vegetation stabilization of sediment and gravel accumulation in this zone is critical to preventing bank failure and sloughing. Production averages 200 to over 500 lbs/acre on these very narrow areas. These areas make up less than 5 percent of the site complex. Water sedge, lakeshore sedge, awl fruit (sawbeak) sedge, lakeshore sedge, swordleaf rush, toad rush, panicled bulrush, common (creeping) spike rush, ovate spikerush an annual, American sloughgrass and water whorlgrass (brookgrass) are common grass plants in these components.
Although not part of the riparian site complex it needs to be emphasized that poorly drained wet meadow inclusions are common in this site complex. Even though willows tolerate periods of inundation they require aeration in the upper root zone and cannot survive on poorly drained wet meadow soils. On a larger scale, the site typically occurs in association with poorly drained wet meadow sedge sites, slightly higher tufted hairgrass meadow sites and terraced basin wildrye loamy bottom sites.
Range in Characteristics:
The extent and duration of surface flows and groundwater have a major effect on site composition and production. Just slight differences in stream and subsurface water elevations often one foot or less produce these effects. Willow densities are highest along perennial stream banks and natural bank levees where flows last well into the growing season and aeration occurs as spring flows subside. Willows decrease as primary channel depths increase and in upper drainages where perennial streams become seasonal and the depth to available groundwater increases. Understory community composition expresses these same changes where obligate sedge species are replaced consecutively by facultative wet, facultative and upland species as the site becomes drier. This is most apparent on dry swales where basin wildrye sites occur.
On wetland or hydric soils were water is at or near the surface throughout the majority of the growing season willows do not survive. Sedges, rushes and wetland grasses are well adapted to these wetland soils. Willows are particularly susceptible to poor drainage. In a high seral state the willow riparian site complex is intermixed on the active floodplain with the more extensive wet meadow and meadow sites. It occurs as discontinuous groupings on aerated banks of overflow channels, depression areas and abandon floodplain channel remnants.
Response to Disturbance - States:
When the condition of the site complex deteriorates as a result of improperly managed grazing willows rapidly decrease along with palatable grasses and sedges. Willows are severely impacted by heavy late summer-fall use when the protein content and palatability is greater than maturing grasses and grass-like plants. Both willow overstory and understory vegetation along with palatable channel toe and bar vegetation is reduced. Sedges and tufted hairgrass decrease. Reed canarygrass and meadow foxtail rapidly invade as long as the primary stream is connected to the floodplain and a high water table is present. Quackgrass and sod bluegrasses increase as the site becomes drier and the important channel stabilization function provided by willows and dense rooted sedges are lost.
With loss of willows and lack of bank and toe cover, channels rapidly degrade during runoff events. Floodplain connectivity is lost, flows become concentrated, velocities increase and erosion accelerates. The primary channel becomes deeper and wider in essence becoming a transportation reach. As the water table drops subsurface flows and storage of water for late season flows are reduced, the site becomes drier and production decreases. The initial rapid down cutting is followed by continued degradation as the incised channel widens. With widening of the incised channel an entrenched narrow flood plain slowly develops. Sod bluegrasses occupy the narrow moist incised floodplain. Basin big sagebrush, rabbitbrush, annuals and noxious deep rooted forbs and juniper when a seed source is present invade and occupy the higher isolated bank.
On lower elevation bottomlands channel straightening, deepening and drainage practices are often implemented to use the excellent floodplain soils for intense agriculture activities, transportation corridors and urban development. With a narrowed steeper reach, stabilization practices, revegetation, and channel alterations may be needed to support the progression of natural processes and promote the development of an entrenched stable narrow floodplain. The goal is to reduce the erosive force of high flood flows and contain flows within the channel. Upstream water storage and withdrawals for irrigation on the floodplain and adjoining terraces may also be modified to support rehabilitation.
In upland areas where intensive agriculture practices are not feasible or desired, the natural restoration of floodplain functions and production may be an alternative. If all structural and functional plant species are still on site, proper grazing management may help support the development of a well vegetated stable entrenched floodplain followed by slow aggregation and channel narrowing. Willows re-establish on the incised floodplain and with the maintenance of adequate fall vegetative cover on channel bars and toes sediment is retained during spring run-off. Banks are protected. In time and with adequate upstream sediment delivery the initial wide floodplain is reconnected.
The state and transition model below represents an approximation of ecological states resulting from the disturbance dynamics described above. Further work is needed to better understand the transitions and thresholds that result in these alternative states.
State 1
Reference State
This represents the reference state in pristine conditions. Erosion and deposition processes are within a historical range of variation, variability in depth to water table and seasonal fluctuations support native vegetation and vegetated communities include all historical functional and structural groups. The historical disturbance regime is intact and driven primarily by climate which influences drought and flood cycles. The resilience and resistance of the site is bolstered by negative feedbacks between vegetation establishment and hydrologic processes that maintains a dynamic equilibrium with geomorphological processes.
Community 1.1
Reference Plant Community
The reference native willow riparian site complex is composed of several distinct plant community components which are adapted to various channel and bank configurations based on the depth and duration of seasonal surface and subsurface flows. The site occupies three riparian zones, the toe zone, bank zone and over bank zone. In the toe and bank zone, emergent sedge and spike rush dominated components are associated with the channel toe or shelf, alluvial bars and the lower bank (hydrology the bank full zone). Willow dominated plant community components in the overbank zone are associated with the upper bank, a variable low natural levee on top of the bank and a small portion of the adjacent active floodplain. It is a dynamic site with components responding to changes in channel configuration, sediment movement and flow regimes. The presence of beaver significantly contributes to the dynamic nature of the site promoting sediment deposition and water distribution.
The primary potential native plant community component located on the upper bank and over bank zone is dominated by willows. Periodic inundation and sediment deposition followed by flow subsidence and soil aeration provides ideal willow habitat. Soil aeration following inundation is a critical component for willow establishment and survival. The plant community is dominated by Booth’s willow. Yellow willow is secondary along with coyote willow and a variety other shrubs including redosier dogwood, rose and golden currant. Sedges made up a significant composition of the historic understory. The exact understory percentage is not presently known as reed canary grass and/or meadow foxtail readily invade. Vegetative composition of this component is approximately 70 percent shrubs, 25 percent grasses and 5 percent forbs. Approximate ground cover is 90 to 120 percent (basal and crown) with shrub canopy cover being 60 to 70 percent. Production averages 3000 lbs/acre in normal years. In the Cowardian system this primary plant community component is classified as a palustrine scrub/shrub wetland.
Two secondary potential plant community components on this site are located on alluvial gravel and silt point bars. Formed over time by annual inundation and sediment deposition they make up to 20 percent of the site complex. Silt portions of point bars are initially dominated by sedges and bulrush. Narrowleaf (coyote) willow, a rhizomatous species initially establishes on gravelly bars. These components are variable in production averaging 500 to over 800 lbs/acre in normal years. Bare ground on the bars is greater than 30 percent. These two plant community components are in a constant state of flux. In high seral state or condition they contribute significantly to stream aggradation, meander formation, channel stability and floodplain connectivity.
Located on narrow benches at the toe zone there is a third potential plant community component dominated by emergent vegetation. The narrow bench forms in areas where slow water velocities allow sediments to accumulates over gravel. Located below the average water elevation or baseflow the bench supports a potential plant community dominated by sedges, panicled bulrush and spike rush. Adjacent to areas of high bank stress, vegetation stabilization of sediment and gravel accumulation in this zone is critical to preventing bank failure and sloughing. Production averages 200 to over 500 lbs/acre on these very narrow areas. These areas make up less than 5 percent of the site complex. Water sedge, lakeshore sedge, awl fruit (sawbeak) sedge, lakeshore sedge, swordleaf rush, toad rush, panicled bulrush, common (creeping) spike rush, ovate spikerush an annual, American sloughgrass and water whorlgrass (brookgrass) are common grass plants in these components.
Although not part of the riparian site complex it needs to be emphasized that poorly drained wet meadow inclusions are common in this site complex. Even though willows tolerate periods of inundation they require aeration in the upper root zone and cannot survive on poorly drained wet meadow soils. On a larger scale, the site typically occurs in association with poorly drained wet meadow sedge sites, slightly higher tufted hairgrass meadow sites and terraced basin wildrye loamy bottom sites.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Shrub/Vine |
1400 |
2100 |
2800 |
| Grass/Grasslike |
500 |
750 |
1000 |
| Forb |
100 |
150 |
200 |
| Total |
2000 |
3000 |
4000 |
State 2
Current Potential State
This state is similar to the reference state yet includes a component of invasive species such as reed canary grass and meadow foxtail. Ecological process and function have not been altered fundamentally by this low level of invasion, yet resistance and resilience is decreased. Erosion and deposition processes are still within a historical range of variation, yet are at risk of transitioning to a less stable state, variability in depth to water table and seasonal fluctuations support native vegetation and vegetated communities include all historical functional and structural groups, yet composition and richness may be reduced. The historical disturbance regime is intact and driven primarily by climate which influences drought and flood cycles. The resilience and resistance of the site is bolstered by negative feedbacks between vegetation establishment and hydrologic processes that maintains a dynamic equilibrium with geomorphological processes. This state is common due to widespread invasion of reed canary grass in the Western US.
State 3
Moderately stable channel, water table present
Relative to the Current Potential State, much of the willow cover and original understory has been lost, the primary channel is becoming unstable, and the floodplain is becoming disconnected yet the water table is still present. Chanel types are E, sinuosity is greater than 1.5, gradient is less than 0.2/100 ft and width to depth ratio is less than 12. With further deterioration, erosion and bank instability will increase and risk a transition to State 4. Decreased bank stability make the site vulnerable to channel widening and incision during large runoff events and transitioning to State 4. Reed canarygrass may provide bank stability of overhanging banks, yet it declines where connectivity to the water table is not available. This state also includes stable analogue channels that have reformed following channel incision, widening and the creation new floodplains.
State 4
Unstable Banks, Disconnected Floodplain
This state is characterized by straightened reaches with higher gradients, reduced sinuosity, unstable banks and disconnected floodplains. Chanel types are C, sinuosity is less than 1.5, gradient is greater than 0.2/100 ft and width to depth ratio is greater than 12. Reed canarygrass and meadow foxtail have become strongly dominant where willows and other woody riparian vegetation have been reduced. This leads to bank erosion and reduced stream shading (less than one percent in mid-summer). 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. Point bars may still support narrowleaf willow and are characterized by gravels, as sand and silt has been lost. Abandoned floodplains transition into terraces and are dominated by drought adapted species that do not require a connection to the water table to persist. Plant communities within this state will vary depending on water table levels, past disturbance history, fire, drought and current management. Currently, insufficient data exists to model the dynamics between these communities but possible phases include:
Quackgrass-Kentucky bluegrass (higher water table or irrigated);
Basin big sagebrush/Kentucky bluegrass-Annuals (primary channel deeply incised, loss of floodplain connectivity and overland flows);
Basin big sagebrush(western juniper)/Annuals (water table lowered, primary channel deeply incised, floodplain disconnected);
Annuals-Deep rooted biennial and perennial forbs (water table lowered, primary channel deeply incised, floodplain disconnected);
Transition T1A
State 1 to 2
Invasion of reed canarygrass and meadow foxtail into the site.
Transition T2A
State 2 to 3
Improperly managed grazing during times of year when willow is most vulnerable to decline or most susceptible to overuse. Removal of willow by mechanical means.
Transition T2B
State 2 to 4
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.
Restoration pathway R3A
State 3 to 2
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. Restoration options will be highly site specific and may not be possible in many circumstances.
Transition T3A
State 3 to 4
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, destabilize banks, 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; sustained improperly managed grazing for many seasons; and significant alterations of upland watershed vegetation altering peak discharge or sediment loads. This state will be more vulnerable to these changes compared to state 2 given less stable banks and lower cover of riparian woody vegetation.
Restoration pathway R4A
State 4 to 2
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. Restoration options will be highly site specific and may not be possible in many circumstances.
Restoration pathway R4B
State 4 to 3
Given time, if natural channel evolution processes are allowed to take place, and sediment loads are adequate, the stream will form an entrenched floodplain at a lower depth then the original. The original floodplain will become a terrace, disconnected from the water table and supporting drought adapted plant species. The resulting riparian area will be more confined and of significantly less extent than originally and the capacity of the basin to store water will be reduced considerably.
