Great Basin Foothill Streamside Woodland

This system occurs in mountain ranges of the Great Basin and along the eastern slope of the Sierra Nevada within a broad elevation range from about 1220 m (4000 feet) to over 2135 m (7000 feet). This system often occurs as a mosaic of multiple communities that are tree-dominated with a diverse shrub component. The variety of plant associations connected to this system reflects elevation, stream gradient, floodplain width, and flooding events. Dominant trees may include Abies lowiana, Alnus incana, Betula occidentalis, Populus angustifolia, Populus balsamifera ssp. trichocarpa, Populus fremontii, Salix laevigata, Salix gooddingii, and Pseudotsuga menziesii. Dominant shrubs include Artemisia cana, Cornus sericea, Salix exigua, Salix lasiolepis, Salix lemmonii, or Salix lutea. Herbaceous layers are often dominated by species of Carex and Juncus, and perennial grasses and mesic forbs such Deschampsia cespitosa, Elymus trachycaulus, Glyceria striata, Iris missouriensis, Maianthemum stellatum, or Thalictrum fendleri. Introduced forage species such as Agrostis stolonifera, Poa pratensis, Phleum pratense, and the weedy annual Bromus tectorum are often present in disturbed stands. These are disturbance-driven systems that require flooding, scour and deposition for germination and maintenance. Livestock grazing is a major influence in altering structure, composition, and function of the system.

Dominant trees may include Abies lowiana (= Abies concolor var. lowiana), Alnus incana, Betula occidentalis, Populus angustifolia, Populus balsamifera ssp. trichocarpa, Populus fremontii, Salix laevigata, Salix gooddingii, and Pseudotsuga menziesii. Dominant shrubs include Artemisia cana, Cornus sericea, Salix exigua, Salix lasiolepis, Salix lemmonii, or Salix lutea. Herbaceous layers are often dominated by species of Carex and Juncus, and perennial grasses and mesic forbs such Deschampsia cespitosa, Elymus trachycaulus, Glyceria striata, Iris missouriensis, Maianthemum stellatum, or Thalictrum fendleri. Introduced forage species such as Agrostis stolonifera, Poa pratensis, Phleum pratense, and the weedy annual Bromus tectorum are often present in disturbed stands.

This system is found in low-elevation canyons and draws, on floodplains, steep-sided canyons, or narrow V-shaped valleys with rocky substrates. This includes both perennial and intermittent streams. Sites are typically subject to temporary flooding during spring or late winter runoff. Overbank flooding and some gravel areas are required for regeneration of these riparian forests and woodlands, especially for cottonwoods.

The hydrologic regime is naturally highly variable temporally and spatially among the streams and rivers of this system. Where present, spring discharges from bedrock aquifers provide flows unaffected by rainfall and snowmelt. Otherwise, stream and river flows - where they occur, at what magnitudes, and when and how often - are subject to wide fluctuations as a result of the wide variation in where and when precipitation takes place, what form the precipitation takes (rain versus snow), and where and when snowmelt takes place (e.g., Abell et al. 2000, Levick et al. 2008, Miller et al. 2010a). Intense runoff associated with intense rainfall events is highly erosive, resulting in rapid reconfiguration of aquatic and riparian macrohabitats particularly along reaches with sand and gravel substrates. Fire disturbances occur in riparian zones, but are generally less severe and less often than in neighboring uplands (Reeves et al. 2005).

Conversion of this type has commonly comes from agricultural development, road development, changes in hydrology either by flooding reaches under reservoirs or complete draining of reaches by 100% upstream diversion by dams and other flood-control activities. Riparian areas and their aquatic communities are directly affected by concentrated grazing, cutting of woody vegetation for timber and firewood, residential development, river channelization, regulation or diversion of flows, wildfire suppression, trapping (principally beaver), exotic species (both terrestrial and aquatic plants and animals), unregulated recreation (both motorized and nonmotorized), road building, mining, pollution, farming, channel dredging, bank armoring, and construction of dams and levees. These same communities are indirectly affected by human activities across their surrounding watersheds that alter watershed runoff and groundwater recharge and discharge via altered ground cover and water diversions and withdrawals, or cause pollution, including from atmospheric deposition.

Invasive plant species may be one of the greatest agents of change in occurrences of this system. Invasive plant species such as salt-cedar and Russian-olive have invaded nearly all of the riparian systems to varying degrees and can convert many miles of riparian zone into undesirable monotypes.

By 2060, models forecast substantial increases in maximum temperatures for all months of the year, with the greatest increases concentrated during the summer. July and August monthly maximum temperatures are projected to increase by 5.5° and 6.5°F, respectively, more than two standard deviations above the average values from the 80-year baseline (1900-1979), where as November and December minimum temperatures only increase by one standard deviation beyond the baseline values (Comer et al. 2013a). Potential climate change effects could include the following (edited excerpt from Comer et al. 2013a): "The forecasted changes in temperature and precipitation patterns would be expected to result in several effects on riparian resources in the ecoregion, as discussed by Melack et al. (1997), Field et al. (1999), Mote (2006), Christensen and Lettenmaier (2007), Chambers and Pellant (2008), Brown and Mote (2009), Covich (2009), Das et al. (2009), Dettinger et al. (2009), McCabe and Wolock (2009), Cayan et al. (2010), Miller et al. (2010a), USBOR (2011). These include: higher evapotranspiration rates leading to an earlier, more rapid seasonal drying-down of riparian occurrences; increased water stress in basin-floor phreatophyte communities; shrinkage of areas of perennial flow/open water, coupled with higher water temperatures at locations/times when water temperatures are not controlled by groundwater discharges or snowmelt; persistence of these hydrologic conditions later into the fall or early winter; reduced groundwater recharge in the mountains and reduced recharge to basin-fill deposits along the mountain-front/basin-fill interface; and more erosive mid/late-summer runoff events in those areas experiencing increased July/August precipitation, potentially with associated channel down-cutting and expanded deposition of the eroded sediment in lower-elevation gravel fans. Warmer winters will likely decrease mortality among insect and fungal pests, leading to an increase in morbidity and mortality among overstory trees such as cottonwood and willows, which are prone to disease and pest damage already. As smaller water sources dry and become unusable, wildlife, domestic livestock, and humans will increase use of larger or more stable water sources.

Based on the ways in which these hydrologic factors affect ecological dynamics in riparian resources, persistence of these hydro-meteorological impacts over multiple decades could result in several long-term impacts at both high and low elevations, as discussed by many of the authors cited above, and also by Harper and Peckarsky (2006), Hultine et al. (2007), Martin (2007), Chambers and Wisdom (2009), Jackson et al. (2009), and Seavy et al. (2009). These include: loss of riparian vegetation at lower elevations where the frequency and spatial extent of seasonal flows determines the spatial limits of this vegetation; loss of basin-floor phreatophyte (deep-rooted plants that obtain water from groundwater sources) communities as a result of lower near-surface ground elevations; declines in the spatial extent and biodiversity of perennial streams and open waters as a result of shrinkage and warmer temperatures; reduced discharge to springs and seeps as a result of reduced aquifer recharge; a continuation of normal "warm-season" aquatic ecological dynamics later into the fall as a result of seasonally normal (baseline) overnight near-freezing temperatures becoming less common in many areas until later in the fall; and a possible de-coupling of the places and timing of emergence of insects, the plants on which they depend, and the animals that feed on the insects, as individual species respond to different cues from air and water temperatures, water availability, and flow conditions."

Occurs in mountain ranges of the Great Basin and along the eastern slope of the Sierra Nevada within a broad elevation range from about 1220 m (4000 feet) to over 2135 m (7000 feet).