Ecological dynamics

The dominant visual aspect of this site is grasses and forbs with scattered clusters of shrubs. This site is commonly in complex with meadow and wet meadow range sites in the riparian zone. Composition by weight is approximately 75-85 percent grass and grass-likes, 3-8 percent forbs, and 10-20 percent shrubs.

During the last few thousand years, this riparian site has evolved in a semi-arid climate characterized by dry summers and cold, wet winters. The site has evolved on flood plains that have occasional to frequent and brief to very brief flooding. This flooding scours some areas and deposits sediment in others. This disturbance is necessary to provide exposed moist mineral soil with full sunlight to germinate willow seedlings. Deposition can bury parts of limbs and twigs which will also start new plants. Likewise, scouring can expose existing tree roots that will sprout. A water table at one foot or less in depth is necessary to provide season long moisture for the growth of the shrub (willow) species present.

Herbivory has historically occurred on this site at low levels of utilization. Herbivores include mule deer, Shiras moose, and Rocky Mountain elk.

Fire has played a role in maintaining the plant community by also providing bare mineral soil with full sunlight to germinate the willow species. Fire can cause sprouting of trees to provide regeneration. The fire frequency on the site is dependent on the frequency of fire on adjacent range sites and moisture in the fuels on the site. The normal fire frequency is 25-100 years.

The Historic Climax Plant Community (HCPC), the Reference State (State 1) moves through many phases depending on the natural and man-made forces that impact the community over time. State 1, described later, indicates some of these phases. The Reference Plant Community Phase is Phase A. This plant community is dominated by an overstory of the following willow species: Geyer, Booths, Wolf, and Drummond. The herbaceous layer is dominated by beaked sedge, water sedge, Baltic rush, bluejoint, and western polemonium. The plant species composition of Phase A is listed later under “Reference Plant Community Phase Plant Species Composition”.

The total annual production is 2800 pounds per acre (3111 kilograms per hectare) in a normal year. Production in a favorable year is 3600 pounds per acre (4000 kilograms per hectare). Production in an unfavorable year is 2000 pounds per acre (2222 kilograms per hectare). Structurally, grasses and grass-likes are very dominant followed by shrubs being more dominant than perennial forbs.


FUNCTION:

This site is suited for livestock grazing in late summer and fall. This site is usually heavily used by livestock, particularly in the summer. Prescribed grazing must be planned to avoid degradation of the site. Special management should be used to protect the adjacent water courses and the associated values. This is an important site for wildlife for food and cover. If the site is associated with perennial streams, a fisheries resource could also be present. Degradation of the site can occur with improper grazing management and down cutting of the adjacent stream course or upper watershed conditions that alter the flood frequency or duration. Hikers and fisherman often traverse the edges of this site. Picnickers and campers frequent the site in late summer and early fall.

Impacts on the Plant Community.

Influence of fire:

This site usually does not burn from wildfire. If a fire occurs, it usually does not adversely affect the plant community. Most plants including shrubs sprout back with sufficient moisture and/or during the next growing season.

Influence of improper grazing management:

Season-long grazing and/or excessive utilization can be very detrimental to this site. Fall use year after year, will result in excessive utilization on the willow regeneration. This may lead to a single age class stand of willows. Under this type of management, they will develop a “tunneled” appearance. Uncontrolled beaver populations may eliminate the overstory component. Due to improper grazing management grasses, forbs, and shrubs can all decline in the plant community. Shrubs usually increase initially, but with continued improper management, will decline.

Continued improper grazing management will result in a stand of forbs and exotic grasses such as Kentucky bluegrass, Reed canarygrass, and meadow foxtail, all with reduced vigor. The ability of the community to withstand seasonal flooding is reduced and down cutting of adjacent streams can result. This down cutting will lower the water table and thereby reduce the potential of the site. This site is particularly difficult to manage because animals seek out the site for shade and it is usually adjacent to water.

Proper grazing management that addresses frequency, duration, and intensity of grazing can maintain the integrity of the plant community and the water table on which it is dependent. Upstream watershed conditions must be maintained to have normal run-off events including moderate flooding.

Weather influences:

Because of the deep soils, the influence of the water table, seasonal flooding and run-on, the production of this site changes little during wet or dry precipitation years. The overall production can be adversely influenced with prolonged drought. Prolonged drought can also increase fire frequency. Overall plant composition is normally not affected when perennials have good vigor.

Below normal temperatures in the spring can have an adverse impact on total production regardless of the run-off and flooding. An early, hard freeze can occasionally kill some plants. An early frost can stop the growth of shrubs.

Influence of insects and disease:

Periodic disease and insect outbreaks can affect vegetation health. An outbreak of a particular insect is usually influenced by weather but no specific data for this site is available.


Influence of noxious and invasive species:

Annual and perennial invasive species can compete with desirable plants for moisture and nutrients. The result is reduced production and change in composition of the understory. There are several noxious or invasive plant species that are adapted to this site.

Influence of wildlife:

This site is important for many species of mammals and birds for food and life cycles. Total numbers are seldom high enough to adversely affect the plant community. The site is primarily used in the late spring, summer, and fall by big game. Many birds use the site for food, nesting, or brood rearing in the late spring, summer, and fall.

Watershed:

The largest threat to site degradation is a lowering of the water table and a change in flooding characteristics either from incised channels or upstream conditions. Off-site conditions can affect the gradient of adjacent stream channels that can affect the water table. If the perennial grass, sedge, and shrub cover is depleted, down cutting can be accelerated within the site. High run-off events from the adjacent uplands can severely damage or change the normal stream channel on the site. As the water table is lowered, productive potential is lost. Eventually the water table is below the root zone of the adapted perennial grasses, sedges, and shrubs. These plants are ultimately replaced by perennial forbs and shallow rooted grasses. Extreme down cutting and lowering of the water table can move the site across the threshold to a new, less productive site. Severe down-cutting can result in a plant community that resembles an upland site. Reducing the frequency of flooding can adversely affect the regeneration of willow, leading to an overstory of decadent older shrubs with no younger shrubs being established for stand replacement.


Plant Community and Sequence:

Transition pathways between common vegetation states and phases:
State 1.
Phase A to B. Develops with improper grazing management.
Phase B to A. Results from prescribed grazing.

State 1 to State 2. Develops through permanently lowering the late growing season water table to 20-40 inches. This can occur with continued improper grazing management. It may also occur with proper grazing on the site, but channel erosion may continue if poor off-site conditions cause frequent and/or severe flooding. The site has crossed the threshold. This state cannot be returned to State 1 without raising the water table. This might be done over time using “hard” structures or bio-engineered structures, but the plant community may take many years to approach the plant community in State 1.


State 2 to unknown site. Results from continued lowering of the water table to greater than 40 inches through down cutting of the stream channel. The site crosses the threshold and retrogresses to a new site with reduced potential due to significant loss of available soil moisture from the lowered water table. It occurs with continued improper grazing management or repeated significant run-off events. This state cannot be returned to State 1 without raising the water table. This might be done over time using “hard” structures or bio-engineered structures, but the plant community may take many years to approach the plant community in State 1.


Practice Limitations.

There are moderate to severe seeding limitations on this site due to difficulty in preparing an adequate seedbed. Elimination of existing vegetation prior to planting is difficult in wet seasons and high water table periods. Grade stabilization structures may be needed to prevent further down-cutting of the channel. Other options for rehabilitation may include application of fertilizer, prescribed grazing, and off-site livestock water development. Fencing of the site for better livestock control might also be a consideration.