Ecological dynamics
This ecological site occurs on the floodplain of a low gradient stream. Riparian areas differ from adjacent uplands in vegetative composition and structure, geomorphology, hydrology, microclimate, and fuel characteristics. Composition is complex and is tied closely to hydrologic processes, geomorphology, and use history. This site is highly productive and the vegetation is tolerant of saturation in the soil profile. Vegetation serves as an indicator of soil moisture gradient. Species classified as obligate or facultative wet occur closest to the stream channel. Moving perpendicular to the stream channel vegetation become more tolerant of dry soil conditions.
This site is characterized by a dominance of perennial bunchgrasses or grasslikes. Nevada bluegrass (Poa secunda) is dominant. Sedges and rushes are common. The amount of Nevada bluegrass in relation to sedge depends on the extent and duration of lateral subsurface water flows. Sedges increase on areas of meadows receiving subsurface flows late into the growing season. Conversely, Nevada bluegrass increases on areas of meadows with limited drainage areas.
This ecological site occupies a small percentage of the landscape, but provides critical habitat for a variety of wildlife. Ecological diversity of this system is maintained by natural disturbance regimes, including flooding and drought periods. Fire regimes in riparian areas are related to the fire regime of the adjacent upland community, although its suggested that fire frequency and severity was generally lower and than adjacent communities (Dwire and Kauffman 2003).
The presence of invasive species has the ability to significantly alter disturbance regimes from their natural range of variation. Precipitation patterns are highly variable and unpredictable throughout this area. Dryland riparian areas are tolerant of prolonged drought conditions, however decline in overall vegetative cover and production is expected during drought periods. Inappropriate management and upstream disturbance can lead to bank destabilization (Fleischner 1994) and soil erosion. Where stream channels become deeper and wider, causing water to flow faster (DJ Krueper, 1996), the water table is lowered, flooding is reduced, and a more drought tolerant plant community can invade. With time, the site can transition to sod forming bluegrasses such as Kentucky bluegrass.
State 1
Reference State
This represents the historical reference state in pristine conditions with no exotic species present. Variability in depth to water table and seasonal fluctuations support native facultative wetland 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.
Characteristics and indicators. Depth to water table and species composition with any complex of meadow sites can be highly variable. Conditions may vary within the site at a given location due to a minor changes in soils, flooding frequency and duration, seasonal water table fluctuations, and competition between plants that are mostly rhizomatous.
Dominant plant species
-
bluegrass (Poa), grass
-
sedge (Carex), grass
-
rush (Juncus), grass
Community 1.1
Reference Plant Community
The reference plant community is dominated by Nevada bluegrass. Sedges, rushes, and a variety of other grasses are common. Proportion of the individual species shifts with drought and flooding cycles and proximity to a stream channel. An assortment of perennial forbs are present but do not account for a large part of the plant community by weight. Silver sagebrush and rose are present but are typically less than 5 percent by weight.
Dominant plant species
-
bluegrass (Poa), grass
-
sedge (Carex), grass
-
rush (Juncus), grass
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
1877 |
2085 |
2292 |
| Forb |
84 |
95 |
101 |
| Shrub/Vine |
56 |
62 |
73 |
| Total |
2017 |
2242 |
2466 |
State 2
Current Potential State
This state is similar to the reference state yet includes a component of non-native species such as Kentucky bluegrass (Poa pratensis), common timothy (Phleum pretense), and meadow foxtail (Alopecurus pratensis). Ecological process and function have not been altered fundamentally by this low level of invasion, yet resistance and resilience are decreased. Erosion processes are still within a historical range of variation, yet with continued vegetation loss the site risks a transition to an alternative state. Variability in depth to water table and seasonal fluctuations support native vegetation. 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 non-native meadow grasses in the Western US. Prolonged improperly managed grazing will promote the spread of Kentucky bluegrass and increase cover of shrubs (State 3). Further improperly managed grazing will increase bareground, increase erosion and risk a transition to a drained state (State 4).
State 3
Invaded State
Soil compaction, trampling and sustained overutilization has altered vegetated composition and increased bare ground. Relative to the current potential state, composition of wetland facultative species has been reduced and silver sage has increased. Much of the Nevada bluegrass, sedge, and rush cover has been replaced by Kentucky bluegrass. The state may also be invaded by exotic annual grasses and forbs and exotic tap rooted perennials. Soil erosion and vegetation pedestalling is often present. Banks are moderately stable, hydrology may be altered with somewhat lowered water tables.
State 4
Incised State
Sustained disturbance may lead to unstable stream banks, entrenched channels and headcuts. Active floodplains and primary terraces will become disconnected from the channel and evolve into high terraces with significantly lowered water tables. This will often lead to the replacement of facultative wetland communities with mountain big sagebrush communities. Plant community composition within this state will vary and may depend on adjacent vegetation types, water table levels, past disturbance history, drought and current management.
Transition T1.1
State 1 to 2
Invasion of non-native meadow grasses
Transition T2.1
State 2 to 3
Sustained improperly managed grazing during times of year when soils are most susceptible to compaction, and when graminoids are most prone to damage by trampling and over utilization.
Transition T2.2
State 2 to 4
This transition may be the result of several disturbances that lower water tables beyond depths that support facultative wetland vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness.
Transition T3.1
State 3 to 4
This transition may be the result of several disturbances that lower water tables beyond depths that support facultative wetland vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness.
