Ecological dynamics
An ecological site is the product of all the environmental factors responsible for its development and it has a set of key characteristics that influence a site’s resilience to disturbance and resistance to invasives. Key characteristics include 1) climate (precipitation, temperature), 2) topography (aspect, slope, elevation, and landform), 3) hydrology (infiltration, runoff), 4) soils (depth, texture, structure, organic matter), 5) plant communities (functional groups, productivity), and 6) natural disturbance regime (fire, herbivory, etc.) (Caudle et al. 2013). Biotic factors that influence resilience include site productivity, species composition and structure, and population regulation and regeneration (Chambers et al 2013).
This ecological site is characteristics of an riparian area associated with the floodplain of a low gradient perennial stream. Riparian areas differ from adjacent uplands in vegetative composition and structure, geomorphology, hydrology, microclimate, and fuel characteristics. Composition is complex and is tied to 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 perennial stream channel. Moving perpendicular to the stream channel vegetation become more tolerant of dry soil conditions.
The Dry Meadow ecological site is characterized by a dominance of perennial bunchgrasses or grasslikes. Nevada bluegrass (Poa secunda) and alpine timothy (Phleum alpinum) are the most common, other species include sedges (Carex spp.), meadow barely (Hordeum brachyantherum), rush (Juncus spp.), mat muhly (Muhlenbergia richardsonis) and rhizomatous wheatgrasses (Elymus lanceolatus and Leymus triticoides).
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 patters are highly variable and unpredictable throughout this area. Dryland riparian areas are tolerant of prolonged drought conditions, however decline in overcall 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.
State 1
Reference State
This state is representative of the natural variability under pre-Euro settlement conditions. This site is highly productive and vegetation is tolerant of saturation in the soil profile.
Due to inherent soil fertility, high productivity, high percentage of rhizomatous plants and the relatively flat slopes, the state is fairly resistant following disturbance. Community phase changes are primarily driven by prolonged drought and periods of below normal temperatures. Fire has had little influence on the development of the site. This site normally burns in conjunction with an adjacent upland site. The estimate fire return intervals range from 20 to 40 years.
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.
Community 1.1
Reference community phase
This community phase is dominated by Nevada bluegrass and alpine timothy. Slender wheatgrass, streambank wheatgrass and carex species are also common. A variety of perennial forbs are present but do not account for a large part of the plant community by weight. Willows, woods rose, and silver sagebrush are present but are typically less than 5 percent by weight. Composition by weight is approximately 75 to 85 percent grass and grass-like, 10 to 20 percent forbs and 0 to 5 percent shrubs.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
785 |
1194 |
1849 |
| Forb |
146 |
224 |
336 |
| Shrub/Vine |
22 |
39 |
56 |
| Total |
953 |
1457 |
2241 |
Community 1.2
Community phase
This community phase is characterized by an increase in shrubs and other drought tolerant and/or disturbance tolerant plants. Species such as rocky mountain iris, Louisiana sagewort, basin wildrye, and juncus increase as Nevada bluegrass and alpine timothy decrease.
Resilience management. This community phase is at-risk of crossing an ecological threshold, if excessive utilization results in the complete removal of soil stabilizing herbaceous species the ability of the site to withstand seasonal flooding is reduced.
Pathway 1.1a
Community 1.1 to 1.2
Natural regeneration over time, absence of disturbance and prolonged drought favors shrubs and tap-rooted forbs over grasses.
Context dependence. Proper frequency, intensity and duration of herbivory may temporarily reduce herbaceous species, but if hydrology of the site is in-tack the community phase will recover without intervention.
Pathway 1.2a
Community 1.2 to 1.1
Release from drought and/or period wildfire reduce shrubs and long-lived forbs and favor dominance by deep-rooted perennial grasses.
State 2
Eroded State
This state is characterized by lowering of the water table and disconnection from the natural floodplain due to anthropogenic impacts such as road building, water diversion and up-stream channelization. Non native species are present and may be increasing or dominant. Seasonal water table is significantly lower than the reference state or absent. Season flooding frequency and duration is truncated spatially and temporally. Soil is active eroding.
Characteristics and indicators. If this site has experienced prolonged excessive utilization perennial grass will decline. Sedges, rushes and forbs increase due to lack of competition. Continued improper grazing management will result in dominance by Kentucky bluegrass, rushes, and Louisiana sagewort. Loss of deep-rooted and rhizomatous perennial grasses reduced the ability of the community to withstand seasonal flooding and results in down cutting of adjacent streams.
Resilience management. The drained state of the dry meadow is very stable. The ability to restore this state back to the reference state is dependent geomorphic characteristics and disturbance history. Resilience management includes maintaining cover of perennial native species and preventing dominance by non-native annuals.
Transition T2
State 1 to 2
Trigger: Significant anthropogenic disturbance (road building, water diversion and up-stream channelization) resulting in introduction of non-native species and loss of seasonal flooding.
Slow variables: Lowering of the water table and loss of seasonal flooding allows dryland species to establish and out compete native species that prefer saturation in the soil profile. This transition may also be coupled with introduction of non-natives, prolonged drought and/or inappropriate grazing management.
Threshold: Seasonal flooding is spatially and temporally truncated. Seasonal high water table is absent due to resulting from soil erosion and down cutting of the adjacent streams. Soil moisture is not sufficient to support vegetation characteristic of the reference state.
Constraints to recovery. Prolonged excessive utilization results in dominance by Kentucky bluegrass, sedges and rushes. Loss of deep-rooted perennial grasses reduced the ability of the community to withstand seasonal flooding, this results in active soil erosion and down cutting of adjacent streams. The drained state of the dry meadow is very stable.
