Montana Field Guide

This group occurs along rivers and larger streams across the Great Plains region in Montana. It is most prevalent along the major prairie rivers in the eastern third of the state. Riparian forest and woodland communities in this groups are typically dominated or co-dominated by Eastern Cottonwood (Populus deltoides), Green Ash (Fraxinus pennsylvanica), Boxelder (Acer negundo), American Elm (Ulmus americana), and/or Peachleaf Willow (Salix amygdaloides). Communities dominated or co-dominated by Eastern Cottonwood are most common while those dominated or co-dominated by American Elm (Ulmus americana) are the least abundant. The tree canopy can be open to closed and is generally >25% and can range from about 30-50ft in height. The shrub and herbaceous strata can range from nearly absent to dense. Periodic flooding is important in establishing and maintaining stands of this group. Flooding regime (frequency, severity, duration) is also important in shaping the understory composition and structure. Scouring and deposition during flooding create areas of new bare mineral soil necessary for Eastern Cottonwood regeneration.

This Group is similar to the Great Plains Floodplain and the Great Plains Riparian Ecological Systems.

Riparian & Wetland Forest; Great Plains Region; Floodplains

Typical Dominants: Green Ash (Fraxinus pennsylvanica), American Elm (Ulmus americana), Eastern Cottonwood (Populus deltoides), Boxelder (Acer negundo), Peachleaf Willow (Salix amygdaloides)

In Montana, this group occurs east of the Continental Divide throughout the Great Plains Region along major River drainages and floodplains. It is most abundant in the eastern third of the state.

In Montana, G147 occurs in Level III Ecoregions 42 (Northwestern Glaciated Plains) and 43 (Northwestern Great Plains).

In Montana, G147 occurs within these Major Land Resource Areas: 52 - Brown Glaciated Plains, 53A - Northern Dark Brown Glaciated Plains, 53B - Central Dark Brown Glaciated Plains, 58A,B,C,D - Northern Rolling High Plains, and 60A,B - Pierre Shale Plains.

Small Patch and Large Patch

This group occurs along rivers and larger streams across the Great Plains region in Montana. Communities in this group 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 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. Periodic flooding is important in establishing and maintaining stands of this group. Flooding regime (frequency, severity, duration) is also important in shaping the understory composition and structure. Scouring and deposition during flooding create new areas of bare mineral soil necessary for Eastern Cottonwood regeneration. Soils in these communities 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.

Riparian forest and woodland communities in this group are typically dominated or co-dominated by Eastern Cottonwood (Populus deltoides), Green Ash (Fraxinus pennsylvanica), Boxelder (Acer negundo), American Elm (Ulmus americana), and/or Peachleaf Willow (Salix amygdaloides). Communities dominated or co-dominated by Eastern Cottonwood are most common while those dominated or co-dominated by American Elm (Ulmus americana) are the least abundant. The tree canopy can be open to closed and is generally >25% and can range from about 30-50ft in height. The shrub and herbaceous strata can range from nearly absent to dense. Common shrubs include Western Snowberry (Symphoricarpos occidentalis), Rose (Rosa spp), Red-osier Dogwood (Cornus sericea), Currants and Gooseberries (Ribes spp), Serviceberry (Amelanchier alnifolia) and Chokecherry (Prunus virginiana). Common graminoids associated with these floodplain systems include Big Bluestem (Andropogon gerardii), Woolly Sedge (Carex pellita), Thickspike Wheatgrass (Elymus lanceolatus), Switchgrass (Panicum virgatum), Western Wheatgrass (Pascopyrum smithii), and Little Bluestem (Schizachyrium scoparium).

Non-native grasses and forbs are common in many of these floodplain environments and may outcompete the native species leading to a conversion of the herbaceous undergrowth. Smooth Brome (Bromus inermis), Common Timothy (Phleum pratense), Redtop (Agrostis stolonifera) Kentucky Bluegrass (Poa pratensis), Quackgrass (Elymus repens), Sweetclover (Melilotus species), Clovers (Trifolium species), Canada Thistle (Cirsium arvense) Leafy Spurge (Euphorbia virgata) and Common Dandelion (Taraxacum officinale) are often present or abundant.

This group is represented by 2 Alliances and 6 associations in Montana within the National Vegetation Classification, which may encompass the range of communities found in the state. A Populus deltoides/Pascopyrum (Elymus) smithii Riparian Woodland Association is described for SD and WY and may occur in MT as well.

Periodic flooding is important in establishing and maintaining stands of this group. Flooding regime (frequency, severity, duration) is also important in shaping the understory composition and structure. Scouring and deposition during flooding create areas of new bare mineral soil necessary for Eastern Cottonwood regeneration. 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 (Symphoricarpos occidentalis) and Rose (Rosa spp) and a corresponding decrease in willow species, Red-osier Dogwood (Cornus sericea), Currant (Ribes spp), Serviceberry (Amelanchier alnifolia) and Chokecherry (Prunus virginiana).

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 Wyoming Big Sagebrush (Artemisia tridentata ssp. wyomingensis). In addition, exotic shrub species such as Salt-cedar (Tamarix spp) can invade disturbed floodplain systems. Russian Olive (Elaeagnus angustifolia) 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), Canada Thistle (Cirsium arvense) 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 species in these Great Plains floodplain habitats, creating a highly productive and important communities. They are specifically adapted to large flooding events that promote dispersion and colonization of newly formed alluvial bars, as such, maintaining the frequency and duration of the flows necessary for colonization to succeed is vital for their persistence on the landscape (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 (Tamarix spp) and Russian Olive that can drastically alter ecological processes. Salt-cedar is an early successional species with dispersal strategies and habitat requirements similar to native cottonwoods and willows (Lesica and Miles 2004). It can replace the native cottonwoods and willows 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 River (Citation?).

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. 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. 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.

Removing or limiting grazing will allow the understory vegetation to recover 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) as well as non-native forbs such as Sweetclover (Melilotus species), Clovers (Trifolium species), Canada Thistle (Cirsium arvense) and Common Dandelion (Taraxacum officinale) however, will persist on the site and may compete with existing populations of native graminoids and forbs.

J. Drake (2015)

S. Mincemoyer, L. Vance, C. McIntyre, T. Luna

12/6/2024