Riverine Riparian

Description

This site is made up from the consolidated and unconsolidated depositional sediments found immediatly adjacent to perennial streams like the Rio Puerco. Plant community structure and function are determined largely by the hydrology of the stream, the frequency of flooding and ponding and the occasional complete altering of the stream channel position and function by flood events as the stream channel constantly seeks equilibrium with its flow regime and constraining landscape features. Plant communities in this system fluctuate widely across the site in responce to routine disturbence but reestablish quickly in predictable patterns in relation to available water and depositional features.

Potential vegetation on this site is dominated by tall and mid cool-season perennial grasses and cottonwood species, which are adapted to occasional periodic flooding and overflow and a fluctuating water table. Other significant vegetation includes a variety of riparian shrubs and forbs.

The overstory is dominated by cottonwood and other native shrubs. The understory is comprised of a sparse mix of native grasses, shrubs and forbs. Occasional flooding is important for establishment and maintenance of cottonwood and willows along the streambanks. Fire is not common on this site due to the moist nature of the soils and vegetation, but may occur in rare events.

Cottonwood species of various age classes are the primary tree species, while basin big sagebrush and a variety of riparian shrubs comprise the main shrub species. A variety of forbs also occurs in this state and plant diversity is high.

Willow has flexible branches that are able to bend with the water force during flow events without too much damage to the plants (Karrenberg et al. 2002; McBride & Strahan 1984; Anderson 2006). Also, Branches can re-sprout if buried by sediment and they may also regenerate vegetatively from broken stems and roots (Anderson 2006). Willows and mesic forbs often are found on the upper edge of the flood plain away from the active channel because it is an intermediate location with access to the water table below high flow. Willow seeds are non-dormant and quickly loose viability. Seedlings generally establish close enough to a water supply and far enough from the channel to be protected from scouring during floods (Anderson 2006). Initially willows grow faster than cottonwood, but given time cottonwood overtops the willows. Seedlings of willow and cottonwood require the same germination conditions, bare, moist soil so they often germinate together. Yearly variations in flow and large floods that scour vegetation and deposit sediment on floodplains are microsites for willow and cottonwood seeds. The adaptations of willows and cottonwoods are: they produce a large number of seeds, the seedlings have a high growth rate, stem fragments can regenerate, and willow root systems are extensive and allow the plant to anchor and bind the soil (Karrenberg et al. 2002).

Submodel

Description

This community is dominated by a canopy of cottonwood and some willows, with an understory of young cottonwood, willow, and other shrubs. The herbaceous understory consists of a mix of the native herbaceous species with non-native species present. The herbaceous understory consists of a mix of the native herbaceous species with non-native grasses, including ripgut brome (Bromus diandrus), cheatgrass (Bromus tectorum) and Kentucky bluegrass (Poa pratensis). Overstory canopy cover would fluctuate with flood events under natural conditions.

Submodel

Description

Tamarisk (saltcedar) and/or Russian olive dominate or co-dominate the overstory with native trees. Native shrubs and grasses can occur in the understory, but non-native annuals are present.

Russian Olive and Tamarisk can readily establish and replace the willows and cottonwood. Tamarisk is more tolerant of drought and salinity than native species (Horton et al. 2001). The timing of seed dispersal is also different for tamarisk than native shrubs. Tamarisk produces seed from April to October (Horton et al. 2001) and with high summer flows could be at a seed dispersal advantage over cottonwoods and willows, which produce seed from February to April (Braatne et al. 1996). Tamarisk seedlings can establish midsummer on fresh sediment deposits from runoff during summer rain storms, months after cottonwood and willows dispersed seed (Stromberg et al. 2007). Tamarisk invasion can create a feedback loop that is difficult to reverse. Once tamarisk invades and begins to replace willows and cottonwoods, bare soil begins to decrease and shade increases, further decreasing the chance that cottonwoods and willows will regenerate in the site.

The introduction of non-native trees and shrubs could have also aided in channel stabilization. A series of years with lower flow could have also allowed for stabilization, establishing riparian veg (native and non-native). Coyote willow, and tamarisk are known for their bank stabilization capacity.

Submodel