This ecological system is found throughout the Rocky Mountain and Colorado Plateau regions. In Montana, sites occur at elevations of 609-1,219 meters (2,000-4,000 feet) west of the Continental Divide. East of the Continental Divide, this system ranges up to 1,676 meters (5,500 feet). It generally comprises a mosaic of multiple communities that are tree-dominated with a diverse shrub component. It is dependent on a natural hydrologic regime with annual to episodic flooding, so it is usually found within the flood zone of rivers, on islands, sand or cobble bars, and along 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. Black cottonwood (Populus balsamifera ssp. trichocarpa) is the key indicator species. Other dominant trees may include boxelder maple (Acer negundo), narrowleaf cottonwood (Populus angustifolia), eastern 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), hawthorne (Crataegus species), chokecherry (Prunus virginiana), skunkbush sumac (Rhus trilobata), willows (Salix species), rose (Rosa species), silver buffaloberry (Shepherdia argentea), or snowberry (Symphoricarpos species).
Black cottonwood (Populus balsamifera ssp. trichocarpa) is the key indicator species. Several other tree species can be mixed in the canopy, including boxelder maple (Acer negundo), narrowleaf cottonwood (Populus angustifolia), eastern cottonwood (Populus deltoides), Douglas-fir (Pseudotsuga menziesii), peachleaf willow (Salix amygdaloides), or Mountain juniper (Juniperus scopulorum). Quaking aspen (Populus tremuloides), paper birch (Betula papyrifera), water birch (Betula occidentalis) and white spruce (Picea glauca) also occur. Grand fir (Abies grandis), western red cedar (Thuja plicata), and western hemlock (Tsuga heterophylla) are commonly co-dominant canopy species in western Montana occurrences, particularly in lower montane riparian zones. Shrub understory components include red-oiser dogwood (Cornus sericea), Rocky Mountain maple (Acer glabrum), thinleaf alder (Alnus incana), devil’s club (Oplopanax horridus), and common snowberry (Symphoricarpos albus). Other shrubs may include currant (Ribes species), Douglas hawthorn (Crataegus douglasii), plane leaf willow (Salix planifolia) yellow willow (Salix lutea), Woods’ rose (Rosa woodsii), alder buckthorn (Rhamnus alnifolia), and common chokecherry (Prunus virginiana). Shrubby cinquefoil (Dasiphora fruticosa), Drummond’s willow (Salix drummondii), and sandbar willow (Salix exigua) are often present on recent alluvial bars.
Dominant graminoid vegetation in the herbaceous stratum includes bluejoint reedgrass (Calamagrostis canadensis), and to a much lesser extent, blue wildrye (Elymus glaucus) and Bebb’s sedge (Carex bebbii). Common forbs include yarrow (Achillea millefolium), fireweed (Chamerion angustifolium), swamp willow herb (Epilobium palustre), common cowparsnip (Heracleum maximum), aster (Symphyotrichum species), western meadow rue (Thalictrum occidentale), Canada goldenrod (Solidago canadensis), starry solomon’s seal (Maianthemum stellatum), clasping-leaf twisted stalk (Streptopus amplexicaulus) and western sagewort (Artemisia ludoviciana). Fern and fern ally cover is often high and includes species such as American ladyfern (Athyrium filix-femina), oak fern (Gymnocarpium dryopteris), and horsetails (Equisetum species).
Flooding in these systems influences community composition by transporting sediments and creating establishment sites for colonization. Plants have acquired adaptive traits to survive in these high-energy flood-disturbance settings. Many plants have flexible, resilient stems and specialized cells to hold oxygen so that they can survive large flood events; some have reproductive adaptations like water-dispersed seeds and are able to sprout quickly from damaged stumps.
Grazing, timber harvest, recreation and residential development can all alter structure, composition, and function of this system. Poor grazing practices can result in increased erosion and channel downcutting, limiting the overbank flows that drive succession. Where grazing is excessive, shrub cover will decrease, resulting in a more open canopy. Continued heavy grazing can completely eliminate cottonwood regeneration, and herbaceous vegetation will eventually transition to a system dominated by grasses such as redtop (Agrostis stolonifera), fowl bluegrass (Poa palustris), Canada bluegrass (Poa compressa), Kentucky bluegrass (Poa pratensis) and other exotic perennial forbs. Any activity that alters the hydrology of these systems (e.g., improperly sized culverts, land clearing and compaction, water diversion and withdrawal, and rip-rap installation) can eventually lead to a loss of characteristic disturbance-prone vegetation communities.
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