Rocky Mountain Foothill Streamside Woodland

This ecological system is found throughout the Rocky Mountain and Colorado Plateau regions within a broad elevational range from approximately 900 to 2800 m. This system often occurs as a mosaic of multiple communities that are tree-dominated with a diverse shrub component. It is dependent on a natural hydrologic regime, especially annual to episodic flooding. Occurrences are found within the flood zone of rivers, on islands, sand or cobble bars, and immediate streambanks. It can form large, wide occurrences on mid-channel islands in larger rivers or narrow bands on small, rocky canyon tributaries and well-drained benches. It is also typically found in backwater channels and other perennially wet but less scoured sites, such as floodplains swales and irrigation ditches. In some locations, occurrences extend into moderately high intermountain basins where the adjacent vegetation is sage steppe. Dominant trees may include Acer negundo, Populus angustifolia, Populus deltoides, Populus fremontii, Pseudotsuga menziesii, Picea pungens, Salix amygdaloides, or Juniperus scopulorum. Dominant shrubs include Acer glabrum, Alnus incana, Betula occidentalis, Cornus sericea, Crataegus rivularis, Forestiera pubescens, Prunus virginiana, Rhus trilobata, Salix monticola, Salix drummondiana, Salix exigua, Salix irrorata, Salix lucida, Shepherdia argentea, or Symphoricarpos spp. Exotic trees of Elaeagnus angustifolia and Tamarix spp. are common in some stands. Generally, the upland vegetation surrounding this riparian system is different and ranges from grasslands to forests. In the Wyoming Basins, the high-elevation Populus angustifolia-dominated rivers are included here, including along the North Platte, Sweetwater, and Laramie rivers. In these situations, Populus angustifolia is extending down into the sage steppe zone of the basins.

Dominant trees may include Acer negundo, Populus angustifolia, Populus deltoides, Populus fremontii, Pseudotsuga menziesii, Picea pungens, Salix amygdaloides, or Juniperus scopulorum. Dominant shrubs include Acer glabrum, Alnus incana, Betula occidentalis, Cornus sericea, Crataegus rivularis, Forestiera pubescens, Prunus virginiana, Rhus trilobata, Salix monticola, Salix drummondiana, Salix exigua, Salix irrorata, Salix lucida, Shepherdia argentea, or Symphoricarpos spp. Exotic trees of Elaeagnus angustifolia and Tamarix spp. are common in some stands. Generally, the upland vegetation surrounding this riparian system is different and ranges from grasslands to forests.

This system is dependent on a natural hydrologic regime, especially annual to episodic flooding. It is found within the flood zone of rivers, on islands, sand or cobble bars, and immediate streambanks. It can form large, wide occurrences on mid-channel islands in larger rivers or narrow bands on small, rocky canyon tributaries and well-drained benches. It is also typically found in backwater channels and other perennially wet but less scoured sites, such as floodplains swales and irrigation ditches. It may also occur in upland areas of mesic swales and hillslopes below seeps and springs. The climate of this system is continental with typically cold winters and hot summers. Surface water is generally high for variable periods. Soils are typically alluvial deposits of sand, clays, silts and cobbles that are highly stratified with depth due to flood scour and deposition. Highly stratified profiles consist of alternating layers of clay loam and organic material with coarser sand or thin layers of sandy loam over very coarse alluvium. Soils are fine-textured with organic material over coarser alluvium. Some soils are more developed due to a slightly more stable environment and greater input of organic matter.

This ecological system contains early-, mid- and late-seral riparian plant associations. It also contains non-obligate riparian species. Cottonwood communities are early-, mid- or late-seral, depending on the age class of the trees and the associated species of the occurrence (Kittel et al. 1999b). Cottonwoods, however, do not reach a climax stage as defined by Daubenmire (1952). Mature cottonwood occurrences do not regenerate in place, but regenerate by "moving" up and down a river reach and regeneration is often associated with flooding events. Over time a healthy riparian area supports all stages of cottonwood communities (Kittel et al. 1999b).

Conversion of this type has commonly come from conversion to agricultural development, road development, changes in hydrology either by flooding reaches under reservoirs or complete draining of reaches by 100% upstream diversion by dams, ditches and other flood-control or translocations activities. Conversion may also come from downcutting of the stream channel where it is susceptible (e.g., wide alluvial bottoms), resulting in desertification or "uplandification" of the former floodplain (J. Tuhy pers. comm. 2013). Historic and contemporary land-use practices have impacted hydrologic, geomorphic, and biotic structure and function of riparian areas in the Rocky Mountains. Human land uses both within the riparian area as well as in adjacent and upland areas have fragmented many riparian reaches which has reduced connectivity between riparian reaches along the same stream as well as a disconnect between riparian areas and their once adjacent upland areas. Adjacent and upstream land uses can contribute excess nutrients and pollutants into riparian areas. Reservoirs, water diversions, ditches, roads, and human land uses in the contributing watershed can have a substantial impact on the hydrology regime. Altered hydrologic regime: reduced frequency and magnitude of flood events, and different timing of flood events, due to upstream dams/diversions, warmer/drier conditions in watersheds (J. Tuhy pers. comm. 2013).

Management effects on woody riparian vegetation can have direct impact, e.g., removal of vegetation by dam construction, roads, logging, or they can be indirect, e.g., removing beavers from a watershed, removing large woody debris, or construction of a weir dam for fish habitat. Continuous heavy livestock grazing can be major influence in altering structure, composition, and function of the community (Elmore and Kauffman 1994, Patten 1998, Flenniken et al. 2001). In general, continuous heavy livestock or native ungulate use leads to less woody cover and an increase in sod-forming grasses particularly on fine-textured soils. Undesirable forb species, such as Urtica and Equisetum, increase with livestock use. Non-native plants or animals, which can have wide-ranging impacts, also tend to increase with these stressors (Patten 1998). Downcutting is a common side effect of heavy grazing or poor road placement and once a stream starts downcutting the larger floodplain is abandoned and the likelihood of riparian vegetation regeneration is greatly reduced, thus reducing and degrading the size of the riparian area (R. Rondeau pers. comm. 2013).

Invasion and dominance of non-native woody plants (e.g., tamarisk, Russian olive); narrowing and armoring of the channel, and artificial accumulations of sediment adjacent to the channel (levees), caused by the woody invasives, which can lead to reduction in suitable sites for cottonwood regeneration: fewer colonizable sand/mud bars, or available sand/mud bars colonized by woody invasives instead of natives (J. Tuhy pers. comm. 2013).

Average temperature has already increased roughly 1.5°F compared to the 1960-1979 baseline period in the southwestern US, including the southern Rocky Mountains (Karl et al. 2009). Predictions are for 3.5-5.5°F increase in temperatures by mid-century (Karl et al. 2009). Predictions suggest an increase in probability of droughts, and that droughts will be exacerbated by warmer temperatures. Increased temperatures will drive declines in spring snowpack and Colorado River flow (Karl et al. 2009). For the higher elevations, in areas where it snows, a warmer climate means major changes in the timing of runoff: streamflow increases in winter and early spring, and then decreases in late spring, summer, and fall. This shift in streamflow timing has already been observed over the past 50 years (Peterson et al. 2008), with the peak of spring runoff shifting from a few days earlier in some places to as much as 25 to 30 days earlier in others (Stewart et al. 2004). This trend is projected to continue, with runoff shifting 20 to 40 days earlier within this century. Reductions in summer water availability are expected to see reductions of about 10% in colder regions such as the Rocky Mountains (Karl et al. 2009). Moreover, increased flood risk in the southern Rocky Mountains is likely to result from a combination of decreased snow cover on the lower slopes of high mountains, and an increased fraction of winter precipitation falling as rain and therefore running off more rapidly (Knowles et al. 2006). The increase in rain on snow events will also result in rapid runoff and flooding (Bales et al. 2006).

Potential climate change effects could include: a shift away from cottonwood-dominated reaches due to shift in timing of high flows and seed distribution of Populus spp. (Merritt and Wohl 2002); and lower streamflows in late summer and early fall leading to earlier senescence of vegetation, which may shift species composition to more drought-tolerant and heat-tolerant species such as tamarix; lower groundwater tables due to less recharge and lower streamflows, which may result in loss of deep-rooted riparian species (Comer et al. 2013a). Warmer summer temperatures may increase the number and strength of thunder storms which may in turn result in increased flash flooding, increasing sediment runoff and scour of stream channels, which may increase stream geomorphology heterogeneity and consequently riparian habitat diversity (Parsons et al. 2005, Stromberg et al. 2010a). The 2000-2012 below-average precipitation pattern in much of the region killed many mature cottonwood trees and with an increase in intensity and frequency of droughts, more die-back is expected (R. Rondeau pers. comm. 2013).

This system is found throughout the lower montane Rocky Mountain and Colorado Plateau regions within a broad elevation range from approximately 900 to 2800 m. It is also found in the island mountain ranges of central and eastern Montana.