Warm Desert Mountain Streamside Woodland

This ecological system occurs in foothill and mountain canyons and valleys of the warm desert regions of the southwestern U.S. and adjacent Mexico, and consists of mid-to low-elevation (1100-1800 m) riparian corridors along perennial and seasonally intermittent streams. Rivers include upper portions of the Gila, Santa Cruz, Salt, San Pedro, and tributaries of the lower Colorado River (below the Grand Canyon), the lower Rio Grande and Pecos (up to its confluence with Rio Hondo) that occur in the desert portions of their range. The vegetation is a mix of riparian woodlands and shrublands. Dominant trees include Acer negundo, Populus deltoides ssp. wislizeni, Populus fremontii, Platanus wrightii, Juglans major, Fraxinus velutina, and Sapindus saponaria. Occasionally Populus angustifolia may come in from higher elevations. Shrub dominants include Salix exigua, Shepherdia argentea, Prunus spp., Alnus oblongifolia, and Baccharis salicifolia. Vegetation is dependent upon annual or periodic flooding and associated sediment scour and/or annual rise in the water table for growth and reproduction. In Texas, woody species that may be dominant include Celtis laevigata var. reticulata, Fraxinus velutina, Juglans major, Juglans microcarpa, Populus deltoides ssp. wislizeni, Populus fremontii, Salix gooddingii, Sapindus saponaria var. drummondii, and Ungnadia speciosa. Shrubs commonly encountered include Acacia constricta, Acacia greggii, Baccharis salicifolia, Brickellia californica, Cephalanthus occidentalis, Fallugia paradoxa, Mimosa aculeaticarpa var. biuncifera, Prosopis glandulosa, Rhus microphylla, and Salix gooddingii. Some sites with sparse woody overstory may be dominated by grasses such as Aristida spp., Bothriochloa laguroides ssp. torreyana, Bouteloua curtipendula, Bouteloua gracilis, Distichlis spicata, Muhlenbergia porteri, Muhlenbergia rigens, Pleuraphis mutica, and Sporobolus airoides.

The vegetation is a mix of riparian woodlands and shrublands. Dominant trees include Acer negundo, Populus deltoides ssp. wislizeni, Populus fremontii, Platanus wrightii, Juglans major, Fraxinus velutina, and Sapindus saponaria. Occasionally Populus angustifolia may come in from higher elevations. Shrub dominants include Salix exigua, Shepherdia argentea, Prunus spp., Alnus oblongifolia, and Baccharis salicifolia. Vegetation is dependent upon annual or periodic flooding and associated sediment scour and/or annual rise in the water table for growth and reproduction. In Texas, woody species that may be dominant include Celtis laevigata var. reticulata, Fraxinus velutina, Juglans major, Juglans microcarpa, Populus deltoides ssp. wislizeni, Populus fremontii, Salix gooddingii, Sapindus saponaria var. drummondii, and Ungnadia speciosa. Shrubs commonly encountered include Acacia constricta, Acacia greggii, Baccharis salicifolia, Brickellia californica, Cephalanthus occidentalis, Fallugia paradoxa, Mimosa aculeaticarpa var. biuncifera, Prosopis glandulosa, Rhus microphylla, and Salix gooddingii. Some sites with sparse woody overstory may be dominated by grasses such as Aristida spp., Bothriochloa laguroides ssp. torreyana, Bouteloua curtipendula, Bouteloua gracilis, Distichlis spicata, Muhlenbergia porteri, Muhlenbergia rigens, Pleuraphis mutica, and Sporobolus airoides.

This ecological system occurs in foothill and mountain canyons and valleys of the warm desert regions of the southwestern U.S. and adjacent Mexico, and consists of mid- to low-elevation (1100-1800 m) riparian corridors and their associated perennial and seasonally intermittent streams. Some occurrences originate as, or receive flow from, headwater streams supported by surface runoff and shallow groundwater seepage; others originate at montane springs.

The hydrologic regime is naturally highly variable temporally and spatially among the streams of this ecosystem. Where present, bedrock formations that force alluvial and basin-fill groundwater to the surface and spring discharges from bedrock aquifers provide flows unaffected by rainfall and snowmelt. Otherwise, stream and river flows are subject to wide fluctuations in where they occur, at what magnitudes, and when and how often as a result of the wide variation in where and when precipitation takes place (cool versus warm season), what form the precipitation takes (rain versus snow), and where and when snowmelt takes place (e.g., Abell et al. 2000, Izbicki and Michel 2004, Levick et al. 2008, Miller et al. 2010a). Intense runoff associated with intense rainfall events are highly erosive, resulting in rapid reconfiguration of aquatic and riparian macrohabitats particularly along reaches with sand and gravel substrates. As a result of this intense regime of fluvial disturbance, occurrences of this ecosystem contain early-, mid- and late-seral riparian plant associations.

Conversion of this type has commonly come from bridge crossings and road installation, agricultural conversion, and drowning by reservoir creation. Dewatering of streams through groundwater pumping and upstream diversions. Conversion to non-native-dominated types such as tamarisk and Russian olive. Common stressors and threats include concentrated grazing, cutting of woody vegetation, development, river channelization, diversion of flows, wildfire suppression, exotic species, unregulated recreation (both motorized and nonmotorized), road building, mining, pollution, channel dredging, bank armoring, and construction of dams. 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. Road crossings and dams can constrict flows and cause increased bank erosion. Reductions in flows can reduce the production of gravel and sand bars and thereby limit cottonwood and willow regeneration.

Forecasts for 2060 show monthly maximum temperature to increase at least two standard deviations above the 20th-century baseline values (1900-1979). Increases in July maximum temperature range from 2.5-8.6°F. This result is likely to counter any increase in precipitation via higher evapotranspiration and lower soil moisture levels. The increases in monthly minimum temperature (i.e., night-time temperature) are severe. For every month, 85-99% of the Mojave Basin and Range ecoregion (MBR) is projected to exceed one standard deviation beyond the 20th century baseline. For midcentury summers - July thru October - models predict 80-95% of the region will experience monthly minimum temperatures two standard deviations beyond baseline values; with extremes reaching a 9.6°F increase. This may be related to cloud cover associated with increased precipitation forecasts; in other words, increased night-time cloud cover will reduce radiative cooling at night. Overall, there is no clear spatial pattern to the area that is not expected to experience these changes, although portions of the southern MBR more frequently experience values closer to the range of historic climatic variability (Comer et al. 2013b).

Potential climate change effects could include the following (edited excerpt from Comer et al. 2013b): "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). They include: higher evapotranspiration rates leading to an earlier, more rapid seasonal drying-down of riparian occurrences; earlier snowmelt and a smaller snowpack in watersheds which moves and reduces the height of the peak flood and earlier in the spring; 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.

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."

This system occurs in southern Arizona, New Mexico, and adjacent Mexico, as well as in the desert mountain ranges of southeastern California, at low elevations. It also occurs in southern Nevada and western Texas.