This ecological system is found throughout the Rocky Mountain and Colorado Plateau regions. In Montana, it ranges from approximately 945 to 2,042 meters (3,100 to 6,700 feet), characteristically occurring 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 on 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 boxelder maple (Acer negundo), narrowleaf cottonwood (Populus angustifolia), Plains cottonwood (Populus deltoides), Douglas-fir (Pseudotsuga menziesii), peachleaf willow (Salix amygdaloides), or Rocky Mountain juniper (Juniperus scopulorum). Dominant shrubs include Rocky Mountain maple (Acer glabrum), thinleaf alder (Alnus incana), river birch (Betula occidentalis), redoiser dogwood (Cornus sericea), hawthorn (Crataegus spp.), chokecherry (Prunus virginiana), skunkbush sumac (Rhus trilobata), Drummond’s willow (Salix drummondiana), sandbar willow (Salix exigua), Pacific willow (Salix lucida), rose (Rosa species), silver buffaloberry (Shepherdia argentea), or snowberry (Symphoricarpos species). Exotic trees of Russian olive (Elaeagnus angustifolia) and saltcedar (Tamarix species) may invade some stands in southeastern and south-central Montana.
Because of the frequent disturbance regime, this system usually occurs as a mosaic of shrub and tree dominant communities. Dominant trees may include boxelder maple, narrowleaf cottonwood, Plains cottonwood, Douglas-fir, peachleaf willow, or Rocky Mountain juniper. In central and eastern Montana, narrowleaf cottonwood frequently dominates the overstory. Dominant shrubs include Rocky Mountain maple, thinleaf alder, river birch, redoiser dogwood, hawthorn, chokecherry, skunkbush, Drummond’s willow, sandbar willow, Pacific willow, silver buffaloberry, rose or snowberry. Russian olive and saltcedar may invade some stands in southeastern and south-central Montana.
The herbaceous understory usually includes colonizing native forbs such as yarrow (Achillea millefolium), Canada goldenrod (Solidago canadensis), American licorice (Glycyrrhiza lepidota), Canada horseweed (Conyza canadensis) and exotics such as Canada thistle (Cirsium arvense) and common dandelion (Taraxacum officinale). Exotic grasses such as redtop (Agrostis stolonifera), Canada bluegrass (Poa compressa), Kentucky bluegrass (Poa pratensis), common timothy (Phleum pratense) and reed canarygrass (Phalaris arundinacea) can dominate the graminoid layer if this system adjoins cultivated areas or disturbed upland communities. Generally, some stands may have a small component of native graminoid species like slimstem reedgrass (Calamagrostis stricta) or wheatgrasses (Elymus species) (Hansen et al., 1995). Wet meadow pataches adjoining or asscociated with this system often contain woolly sedge (Carex pellita), clustered field sedge (Carex praegracilis), Baltic rush (Juncus balticus), and bluejoint reedgrass (Calamagrostis canadensis).
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