This system is associated with perennial to intermittent or ephemeral streams throughout the northwestern Great Plains. In Montana, it occurs along smaller tributaries of the Yellowstone and Missouri rivers, as well as tributaries to the large floodplain rivers that feed them (e.g. the Milk, Marias, Musselshell, Powder, Clark’s Fork Yellowstone, Tongue, etc). In areas adjacent to the mountain ranges of central and southeastern Montana, and near the Rocky Mountain Front, it grades into Rocky Mountain Lower Montane-Foothill Riparian Woodland and Shrubland systems. This system is found on alluvial soils in highly variable landscape settings, from confined, deep cut ravines to wide, braided streambeds. Channel migration occurs in less-confined areas, but within a more narrow range than would occur in broad, alluvial floodplains. Typically, the rivers are wadeable by mid-summer.
The primary inputs of water to these systems include groundwater discharge, overland flow, and subsurface interflow from the adjacent upland. Flooding is the key ecosystem process, creating suitable sites for seed dispersal and seedling establishment, and controlling vegetation succession. Communities within this system range from riparian forests and shrublands to tallgrass wet meadows and gravel/sand flats. Dominant species are similar to those found in the Great Plains Floodplain System. In the western part of the system’s range in Montana, the dominant overstory species is black cottonwood (Populus balsamifera ssp. trichocarpa) with narrowleaf cottonwood (Populus angustifolia) and Plains 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 wetter systems, the understory is typically willow (Salix spp.) and redosier dogwood (Cornus stolonifera) with graminoids such as western wheatgrass (Pascopyrum smithii) and forbs like American licorice (Glycyrrhiza lepidota). In areas where the channel is incised, the understory may be dominated by big sagebrush (Artemisia tridentata) or silver sagebrush (Artemisia cana). Like floodplain systems, riparian systems are often subjected to overgrazing and/or agriculture and can be heavily degraded, with salt cedar (Tamarix ramosissima) and Russian olive (Eleagnus angustifolia) replacing native woody vegetation and regrowth. Groundwater depletion and lack of fire have resulted in additional species changes.
The primary inputs of water to these systems include groundwater discharge, overland flow, and subsurface interflow from the adjacent upland. Flooding is the key ecosystem process. It creates suitable sites for seed dispersal and seedling establishment, and controls vegetation succession. This system is associated with small rivers and perennial to intermittent or ephemeral streams that flow only during part of the year. In the Great Plains, the water source is primarily local precipitation and groundwater inflow (Decker, 2007); in systems receiving flow from central and southeastern mountain ranges, snowmelt and summer thunderstorms provide a significant portion of flows. The boundaries of these riparian areas extend beyond the limits of flooding into streamside vegetation (Gregory, 1991). They are important links between terrestrial and aquatic ecosystems, acting as ecotones between upland and wetland, and connecting ecological processes and plant communities.
Like the Western Great Plains Riparian system of Wyoming, Colorado and New Mexico, Montana’s Great Plains Riparian systems may include riparian forests or woodlands, as well as shrublands, tallgrass or mixedgrass wet meadows, herbaceous wetlands, and gravel/sand flats (Decker, 2007). Vegetation may be a mosaic of communities that are not always tree- or shrub-dominated. At lower elevations (e.g. along the Little Powder River and Mizpah Creek in southeastern Montana), forested communities may form galleries dominated by Plains cottonwood. Narrowleaf cottonwood occurs at intermediate elevations and black cottonwood tends to be prevalent at higher elevations (Hansen et al., 1996). Willows commonly associated with this system include sandbar willow (Salix exigua), yellow willow (Salix lutea), peachleaf willow (Salix amygdaloides), and diamondleaf willow (Salix planifolia). Other shrubs include redosier dogwood, western snowberry (Symphoricarpos occidentalis), chokecherry (Prunus virginiana), and woods rose (Rosa woodsii). In areas where the channel is incised, the understory may be dominated by big sagebrush or silver sagebrush. The herbaceous stratum is variable. Subirrigated areas may support tallgrass meadows dominated by big bluestem (Andropogon gerardii) or fresh water cordgrass (Spartina pectinata). Other graminoids include wooly sedge (Carex pellita), little bluestem (Schizachyrium scoparium), western wheatgrass, porcupine needlegrass (Hesperostipa spartea), northern dropseed (Sporobolus heterolepis), and panic grass (Panicum virgatum). American licorice and Canada goldenrod (Solidago canadensis) are common understory forbs within all cottonwood riparian systems. These sites are prone to invasion by exotic grasses and forbs, the most widely established being creeping bentgrass (Agrostis stolonifera), cheatgrass (Bromus tectorum), quackgrass (Agropyron repens), Canada thistle (Cirsium arvense), clovers (Melilotus species), leafy spurge (Euphorbia esula) and common dandelion (Taraxacum officinale).
Fluvial processes play a key role in the dynamics of Great Plains streams. The nature of these processes is often indicated by channel morphology. Meandering channels generally have a shallow gradient, low flow variability, and sediment loads dominated by silt and finer particles, while braided channels are characterized by a steep gradient, high flow variability, and a sediment load dominated by sand and coarser particles (Friedman, 2002). Flooding is the key ecosystem process whereby establishment sites for riparian vegetation are created, seeds are dispersed and vegetative succession is controlled. However, since Euro-American settlement, natural fluvial processes have been disrupted in many of these systems by dams and diversions. Fire has been suppressed, agricultural activities have increased siltation rates and introduced both non-native species and chemical changes, and native grazers have been largely replaced by domestic cattle. Consequently, there has been a direct loss of woody plant diversity. Furthermore, both channel incision and channel widening have altered flooding regimes, leading to establishment of flood-intolerant species in many areas.
The quality and quantity of ground and surface water input into riparian areas is almost entirely determined by the condition of the surrounding landscape. Therefore, the integrity of riparian ecological systems is partly determined by processes operating in the surrounding landscape, especially in the local watershed. Different types of land use can alter surface runoff and recharge of local aquifers, and introduce excess nutrients, pollutants, or sediments. Great Plains riparian systems have been substantially impacted by the development of both groundwater and surface water for irrigation. Alteration of natural hydrological processes by dams, diversions, ditches, roads, etc., and abiotic resource consumption through groundwater pumping have considerably altered the presettlement condition of the Great Plains. Vegetation responds to hydrologic changes by shifting from wetland- and riparian-dependent species to more mesic and xeric species typical of adjacent uplands and/or encroaching into the stream channel. When periodic flooding is eliminated by water management, riparian areas may become dominated by late-seral communities due to the inability of pioneer species (e.g., cottonwood and willow) to regenerate (Decker, 2007). Pollution from agricultural runoff can introduce excess nutrients into riparian areas. Increased nutrients can alter species composition by allowing aggressive, invasive species to displace native species. Nutrient cycles may also be disrupted by water management that eliminates normal flooding cycles and prevents deposition of organic material from floodwaters (Decker, 2007). Riparian vegetation is also affected by climatic drought that reduces soil moisture in the unsaturated zone and decreases streamflows, which reduces recharge and lowers the alluvial water table (Friedman et al. 1997). The elimination of beavers from most of the plains watersheds probably decreased water storage and increased variability in plains streams (Friedman et al. 1997). The replacement of native grazers, especially bison, with fenced cattle has changed the regeneration patterns of cottonwood. In addition, salt cedar and Russian olive can drastically alter ecological processes in these plains riparian areas. 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.
Restoration strategies are dependent on the degree and type of disturbance event. Restoration efforts must first concentrate on restoring thestream's hydrology, so the floods can re-occur. In-stream habitat enhancement (e.g., additions of logs or boulders) should be employed after restoring natural processes or where short-term improvements in habitat are needed (e.g., for species in recovery).
Removingor eliminating grazing from this ecological system will allow the system to recover if hydric soils have not been lost due to extensive soil compaction, pugging, or down cutting of stream channels, and if there are existing populations of herbaceous native species (Carex, Juncus, and native grasses) that possess rhizomatous root systems capable of re-colonizing bare soils. The 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 (Trifolium species) 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 low enough that an adequate stand of native graminoids and forbs can become established on the site if reseeding efforts are used. In all cases, grazing should 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 riparian 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 seedbank.
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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