This system occurs along the Missouri and Yellowstone Rivers and their larger tributaries, including parts of the Little Missouri, Clark’s Fork Yellowstone, Powder, Tongue, Bighorn, Milk, and Musselshell rivers. These are the big perennial rivers of the region, with hydrologic dynamics largely driven by snowmelt and rainfall originating in their headwater watersheds, rather than local precipitation events. In the absence of disturbance, periodic flooding of fluvial and alluvial soils and channel migration will create depressions and backwaters that support a mosaic of wetland and riparian vegetation, whose composition and structure is sustained, altered and redistributed by hydrology. Dominant communities within this system range from floodplain forests to wet meadows to gravel/sand flats, linked by underlying soils and flooding regimes. In the western part of the system’s range in Montana, the overstory dominant species is black cottonwood (Populus balsamifera ssp. trichocarpa) with narrowleaf cottonwood (Populus angustifolia) and eastern cottonwood (Populus deltoides) occurring as co-dominants in the riparian/floodplain interface near the mountains. Further east, narrowleaf cottonwood and Plains cottonwood become dominant. In relatively undisturbed stands, willow (Salix species), redosier dogwood (Cornus sericea) and common chokecherry (Prunus virginiana) form a thick, multi-layered shrub understory, with a mixture of cool and warm season graminoid species below. Box elder (Acer negundo) and green ash (Fraxinus pennsylvanica) form a tree understory in mid-seral and late-seral stands.
In Montana, many occurrences are now degraded to the point where the cottonwood overstory is the only remaining natural component. The hydrology of these floodplain systems has been affected by dams, highways, railroads and agricultural ditches, and as a result, they have lost their characteristic wetland /riparian mosaic structure. This has resulted in a highly altered community consisting of relict cottonwood stands with little regeneration. The understory vegetation is dominated by non-native pasture grasses, legumes and other introduced forbs, or by the disclimax western snowberry (Symphoricarpos occidentalis) and rose (Rosa species) shrub community.
This system may occur as relatively broad and extensive forests, as seen along the lower stretches of the Missouri and Yellowstone Rivers, or more narrow bands, as seen along the Milk, Little Missouri, Tongue and Powder Rivers. The elevational range of this system is from 579-1,310 meters (1,900-4,300 feet) (Hansen et al., 1996). These are the big perennial rivers of the region, with hydrologic dynamics largely driven by snowmelt and rainfall originating in their headwater watersheds, rather than by local precipitation events. In an undisturbed state, periodic flooding of the fluvial and alluvial soils and channel migration will create alluvial bars, depressions and backwaters supporting zones or mosaics of wetland and riparian vegetation, whose composition and structure is sustained, altered and redistributed by hydrology. Soils can be Entisols, Inceptisols and Mollisols on older stabilized sites. Often there is up to 1 meter of soil overlying river gravels and cobble. Water tables can drop to within 1 meter of the soil surface in summer months.
In the western part of the system’s range in Montana, the overstory dominant species is black cottonwood, with narrowleaf cottonwood and Plains cottonwood occurring as co-dominants in the riparian/floodplain interface near the mountains. Further east, narrowleaf cottonwood and Plains cottonwood become dominant.
In less disturbed occurrences, willow species such as yellow willow (Salix lutea) planeleaf willow (Salix planifolia), and peachleaf willow (Salix amygdaloides) co-dominate the shrub layer with common chokecherry (Prunus virginiana), redosier dogwood (Cornus sericea), serviceberry (Amelanchier alnifolia) and currant (Ribes spps). Boxelder (Acer negundo), green ash (Fraxinus pennsylvanica), and American elm (Ulmus americana) are common in mid- to late-seral stands, and will generally succeed Plains cottonwood in the rivers of the Southeast. Sandbar willow (Salix exigua) and shrubby cinquefoil (Dasiphora fruticosa) frequently colonize recent alluvial bars in central and eastern Montana, while silverberry (Elaeagnus commutata), thinleaf alder (Alnus incana) and Drummond’s willow (Salix drummondiana) are colonizers in black cottonwood-dominated floodplains near the mountains and riparian/floodplain interface. Common graminoids associated with these floodplain systems include big bluestem (Andropogon gerardii), wooly sedge (Carex pellita), streamside wild rye (Elymus lanceolatus), old switch panicgrass (Panicum virgatum), western wheatgrass (Pascopyrum smithii), little bluestem (Schizachyrium scoparium), and sand dropseed (Sporobolus cryptandrus). Forbs include Drummond’s dryad (Dryas drummondii), yarrow (Achillea millefolium), starry solomon’s seal (Maianthemum stellatum) and aster (Symphyotrichum species). Because of the disturbance regimes typical in these systems, they are highly susceptible to invasion by exotic species. Russian olive (Elaeagnus angustifolia) and cheatgrass (Bromus tectorum) have become established in many stands, and leafy spurge ( Euphorbia esula) is a common invasive.
In Montana, many occurrences are now degraded to the point where the cottonwood overstory is the only remaining natural component. The hydrology of these floodplain systems has been affected by dams, highways, railroads and agricultural ditches. As a result, they have lost their characteristic wetland /riparian mosaic structure. This has resulted in a highly altered community consisting of relict cottonwood stands with little regeneration. In the system’s disturbed/altered hydrological state and/or under heavy grazing pressure, there will be an increase in shrub species such as western snowberry and rose and a corresponding decrease in willow species, redosier dogwood, currant, serviceberry and common chokecherry.
Successional processes create a community resembling adjacent upland communities; western snowberry and rose may persist, but will be joined by other native shrubs from adjacent upland communities, such as silver sagebrush (Artemisia cana)and big sagebrush (Artemisia tridentata). In addition, exotic shrub species such as salt cedar (Tamarix ramosissima) can invade disturbed floodplain systems. Russian olive has become a dominant overstory tree in many areas, shading out native species. In these disturbed floodplains, the understory vegetation is dominated by a mixture of pasture grasses such as smooth brome (Bromus inermis), common timothy (Phleum pratense), redtop (Agrostis stolonifera) and Kentucky bluegrass (Poa pratensis), as well as non-native forbs such as sweetclover (Melilotus species), clovers (Trifolium species), Canadian thistle (Cirsium canadensis) and common dandelion (Taraxacum officinale). Once exotic grasses become dominant, especially in the absence of episodic flooding, these systems cannot return to their original state without substantial management intervention.
Cottonwoods and willows are the dominant tree species in these Great Plains Floodplain Systems, creating a highly productive and important habitat type. Since they are specifically adapted to infrequent large flooding events that promote dispersion and colonization of newly formed alluvial bars, management efforts need to begin with identifying the frequency and duration of the flows necessary for colonization to succeed (Scott et al, 1994). Great Plains floodplain systems have been substantially impacted by the development of both groundwater and surface water for irrigation, isolating rivers from their adjacent floodplains. Unless water management can restore periodic flooding, floodplains and riparian areas may become dominated by late-seral communities, and nutrient cycles may be disrupted without floodwaters depositing organic material (Decker, 2007). In addition, management efforts need to target aggressive non-native shrubs like salt cedar and Russian olive that can drastically alter ecological processes. Tamarisk is an early successional species with dispersal strategies and habitat requirements similar to native cottonwood and willow (Lesica and Miles, 2004). It can replace the native cottonwood and willow where natural flow regimes have been altered. Early detection and rapid response are necessary to prevent its spread. Prescribed spring grazing by sheep has been effective in controlling leafy spurge along some rivers, particularly the upper Powder.
Restoration strategies are dependent on the degree and type of disturbance event. Restoration efforts must first concentrate on restoring hydrologic processes or simulating large flood events. Instream habitat enhancement (e.g., additions of logs or boulders) can be employed after restoring natural processes or where short-term improvements in habitat are needed (e.g., for species in recovery).
Removing or limiting grazing will allow the system to recover if hydric soils have not been lost due to soil compaction, pugging, or downcutting of stream channels, and if there are existing populations of herbaceous native species (Carex, Juncus, and native grasses) with rhizomatous root systems capable of re-colonizing bare soils. Presence of rhizomatous, highly adaptable exotic grasses such as Kentucky bluegrass (Poa pratensis), common timothy (Phleum pratense) and smooth brome (Bromus inermis) and pasture forbs such as clovers (Trifoliumspecies) and common dandelion (Taraxacum officinale) however, will persist on the site and may compete with existing populations of native graminiods and forbs. In these cases, land managers must decide if the exotic density is small enough that an adequate stand of native graminoids and forbs can become established on the site if reseeding efforts are used. With reseeding, grazing must be excluded for several years to allow adequate re-growth and recovery of existing shrubs and the herbaceous understory.
Because all major shrub species within this floodplain system are capable of re-sprouting and typically possess extensive, spreading root systems, modified land management practices in areas of low to moderate impact can minimize additional restoration needs. Vigor, health and degree of vegetative regeneration of existing trees and shrubs must be evaluated to determine if these components of the community are capable of recovery in an acceptable time frame. Intensive revegetation efforts should be limited to sites where a catastrophic wildfire or prolonged heavy grazing has destroyed existing trees, shrubs and the seed bank.
Decker, Karin. 2007. Western Great Plains riparian woodland and shrubland ecological system ecological integrity assessment. Ft. Collins, Colo: Colorado State University, Colorado Natural Heritage Program.
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