Ecological dynamics

The information in this ecological site description, including the state-and-transition model (STM), was developed based on historical data, current field data, professional experience, and a review of the scientific literature. As a result, all possible scenarios or plant species may not be included. Key indicator plant species, disturbances, and ecological processes are described to inform land management decisions.

The Riparian Woodland provisional ecological site in MLRA 52 Dry Grassland consists of four states: The Contemporary Reference State (1.0), the Herbaceous Understory State (2.0), the Invasive Tree State (3.0), and the Cropland State (4.0). Historically, plant communities associated with this ecological site evolved under the combined influences of climate, grazing, hydrology, and fire. Extreme climatic variability results in frequent droughts, which can have great influence on the relative contribution of species cover and production (Coupland, 1958, 1961; Biondini et al., 1998).

Hydrology, flooding in particular, is a crucial dynamic on this site. Annual flood events delivered water to the site, deposited sediment in some areas, and removed sediment in other areas. This natural erosion/deposition pattern facilitated a natural succession of plant communities. Pioneer species established on recent alluvial deposits, which gave way to more facultative and upland species as banks were built, soil developed, and water tables lowered. The process began anew when banks were eroded again and redeposited as the stream channel migrated back and forth across the flood plain. This natural cycle rejuvenated woodland stands, maintained high species diversity, and preserved the hydrologic function of the flood plain. Another phenomenon unique to this site are ice jams. In winter, the stream freezes over, then thaws during warmer weather, either during winter warming periods or spring thaw. The river ice breaks up into large pieces and floats downriver. When these ice flows are blocked by an obstruction, they accumulate and often are forced onto the floodplain. Ice jams can cause bank scouring and flooding, particularly on lower stream terraces. This creates bare sand and gravel bars which are colonized by pioneer species such as plains cottonwood.

Native grazers also shaped these plant communities. Bison (Bison bison) were the dominant historic grazer, but pronghorn (Antilocapra americana), elk (Cervus canadensis), and deer (Odocoileus spp.) were also common. Small mammals such as ground squirrels (Urocitellus spp.) also influenced this plant community (Salo et al., 2004). In addition, grasshoppers and periodic outbreaks of Rocky Mountain locusts (Melanoplus spretus; Lockwood, 2004) played an important role in the ecology of these communities.

The historic ecosystem also experienced relatively frequent lightning-caused fires, with estimated fire return intervals of 6 to 25 years (Bragg, 1995). Historically, Native Americans also set periodic fires. The majority of lightning-caused fires occurred in July and August, whereas Native Americans typically set fires during spring and fall to correspond with the movement of bison (Higgins, 1986). Generally, the riparian woodland ecosystem is resilient to fire and the historic fire return interval most likely had neutral or slightly positive effects on the plant community. However, long-term fire suppression in the 20th century removed periodic fire from the ecosystem altogether. Lack of periodic fires can result in an increase in litter accumulation, providing ideal conditions for seed germination and seedling establishment of invasive species. Heavy fuel loads associated with fire suppression also increase the risk of a severe, stand-replacing fire, which eliminates the woodland stand and exposes the site to bank destabilization and erosion.

The contemporary Riparian Woodland site is extremely altered, perhaps irreversibly, from its historic state. Most major rivers in MLRA 52 have been dammed for flood control, irrigation, or electric power generation, all of which significantly altered hydrology. Reduced flooding intensity and frequency have significantly reduced cottonwood regeneration. As a result, many stands are becoming decadent with very little seedling recruitment. Irrigation practices have removed water from streams and diverted it into canals. This has altered ground-water hydrology and stream recharge. On some rivers, unnaturally high flows caused by releasing irrigation water from storage reservoirs has triggered excessive downcutting and bank erosion. Non-native vegetation is also commonplace. Field investigations by Hanson et al. in 1995 and by NRCS in 2008 and 2015 were unable to identify a predominantly native herbaceous community. Due to the extreme alteration of this site, a contemporary reference state rather than a historic reference state is modeled for this provisional ecological site description. The implications of this alteration are not fully understood and require further investigation.

In the early stages of succession, gravel and sand are deposited on alluvial bars by flooding or ice scouring. These bars are quickly colonized by pioneer species such as plains cottonwood (Populus deltoides) and sandbar willow (Salix exigua). In the absence of further disturbance, higher seral species will begin to establish and the plains cottonwood stand will begin to mature. Over time, banks will build up, the channel will migrate further from the site, and the water table will lower. A diverse understory with species such as redosier dogwood (Cornus sericea) will establish under a canopy of mature cottonwood trees. As the cottonwoods die out they are commonly replaced by box elder or an upland shrub/herbaceous community. Disturbance in the form of flooding, ice scouring, or stand-replacing fire can return the site to a pioneer community. Flooding disturbance has been greatly reduced due to dams and flood-control practices and is generally infrequent and limited to the lowest sandbars and terraces.

This site can be important for livestock grazing due to its high productivity and proximity to water. Improper grazing of this site can result in a reduction in the diversity of the understory and an increase in less palatable shrubs (Hansen et al., 1995). Improper grazing practices include any practices that do not allow sufficient opportunity for plants to physiologically recover from a grazing event or multiple grazing events within a given year and/or that do not provide adequate cover to prevent soil erosion over time. These practices may include, but are not limited to, overstocking, continuous grazing, and inadequate seasonal rotation moves over multiple years. The plant community will transition from a diverse shrub understory to one dominated by western snowberry. Prolonged severe grazing will eventually eliminate all shrubs and result in an herbaceous understory community. Once the stand has transitioned from a shrub-dominated understory to an herbaceous understory, returning it to its former state is very difficult (Hansen et al., 1995). Non-native and invasive species are common on this site. Introduced perennial grasses, such as Kentucky bluegrass (Poa pratensis) and smooth brome (Bromus inermis), are the most common grass species in all phases. Once established, they will displace native species and dominate the ecological functions of the site. Noxious weeds are a major concern on this site. Leafy spurge (Euphorbia esula), Canada thistle (Cirsium arvense), and hardheads (Acroptilon repens), better known as Russian knapweed, are common on this site and are capable of displacing native species. Russian olive (Elaeagnus angustifolia), an invasive tree species, is another common invader on this site. It will commonly establish in the understory, replace cottonwoods as they die out, become the dominant tree on the site, and prevent re-colonization by native tree species. Once established, Russian olive is very difficult to remove and may require extensive restoration practices, including tree removal, reestablishment of native tree/shrub species, and weed control.

The Riparian Woodland ecological site is often considered prime farmland. Many acres of this site have been cleared and converted to cropland, primarily irrigated hay. Common crop species include alfalfa, orchardgrass, and a grass/alfalfa mix. Annual crops, such as wheat, barley, and corn, are occasionally planted as part of a rotation or when renovating hay fields. Flood irrigation is common, and water is typically diverted from nearby streams and delivered to fields via canals. Irrigated cropland is extremely valuable in the region, and once the site is converted it is unlikely to be taken out of production.

The STM diagram suggests possible pathways that plant communities on this site may follow as a result of a given set of ecological processes and management. The site may also support states not displayed in the STM diagram. Landowners and land managers should seek guidance from local professionals before prescribing a particular management or treatment scenario. Plant community responses vary across this MLRA due to variability in weather, soils, and aspect. The contemporary reference state may not necessarily be the management goal. The lists of plant species and species composition values are provisional and are not intended to cover the full range of conditions, species, and responses for the site. Species composition by dry weight is provided when available and is considered provisional based on the sources identified in the narratives associated with each community phase.

State 1: Contemporary Reference State
The Contemporary Reference State contains five community phases. This state is not considered to be the historic natural state but instead is an evaluation of contemporary conditions given the extreme alterations to the abiotic and biotic components of the ecosystem. Flooding disturbance is a key dynamic on this site, but it has been severely altered by manmade structures. Frequent flooding created open alluvial bars where native cottonwood and willow species could colonize. Reduction of flooding has limited such disturbances to the lowest-elevation terraces. In general, this state was resilient to grazing and fire, although these factors could influence species composition in localized areas. Vegetation is characterized by cottonwood, redosier dogwood, and willow species. Plains cottonwood is the principle species, but narrowleaf cottonwood (Populus angustifolia) may also be present as elevations increase. Box elder may also occur in later stages of development. Following disturbance, this state will exhibit an increase in seedling cottonwoods and sandbar willow (Hansen et al., 1995). Herbaceous understory has been significantly altered and is predominantly non-native species such as smooth brome and Kentucky bluegrass. Noxious weeds are a common concern for all phases.

Community Phase 1.1: Seedling Cottonwood – Sandbar Willow Phase
The Seedling Cottonwood – Sandbar Willow Phase is dominated by seedling cottonwoods and sandbar willow. The understory is very dense. Sandbar willow is by far the most abundant species. Other shrub species that may be present at low cover are yellow willow (Salix lutea) and redosier dogwood. Common forbs include Canada goldenrod (Solidago canadensis) and American licorice (Glycyrrhiza lepidota). Graminoids are commonly dominated by non-native species, such as smooth brome, Kentucky bluegrass, and quackgrass (Elymus repens). Other graminoid species that may be present are woolly sedge (Carex pellita), Canada wildrye (Elymus canadensis), and rush (Juncus spp.) (Hansen et al., 1995).

Community Phase Pathway 1.1a
Lack of flooding disturbance, natural plant growth, bank building, or a combination of these factors shift the Seedling Cottonwood – Sandbar Willow Phase (1.1) to the Pole Cottonwood – Yellow Willow Phase (1.2). Time periods with little or no disturbances permit natural growth of the overstory. The stream channel typically is migrating away from the site at this time. Distance to the water table increases, and some shade-tolerant shrubs start to inhabit the understory.

Community Phase 1.2: Pole Cottonwood – Yellow Willow Phase
The Pole Cottonwood – Yellow Willow Phase consists of mostly pole-stage cottonwoods and some sapling cottonwoods. In this phase, the stream channel has moved farther away and the ground-water table has lowered. The tree canopy is typically 60 to 80 percent, with tree diameters of 3 to 11 inches diameter at breast height (DBH). Higher seral species such as yellow willow dominate the understory, and redosier dogwood is becoming more common. Other shrub species that may be present are sandbar willow, Woods’ rose (Rosa woodsii), and silver buffaloberry (Shepherdia argentea). The herbaceous understory is typically dominated by non-native species such as smooth brome and Kentucky bluegrass. Native species such as Canada wildrye may also be present. Canada goldenrod, American licorice, and fleabane (Erigeron spp.) are common forbs.
 
Community Phase Pathway 1.2a
Flooding, bank scouring by ice jams, or a combination of these factors shift the Pole Cottonwood – Yellow Willow Phase (1.2) to the Seedling Cottonwood – Sandbar Willow Phase (1.1). Flood control has reduced the occurrence of this transition, and it is typically confined to lower terraces near the channel.

Community Phase Pathway 1.2b
Decades of low disturbance, bank building, and lowering of the water table transition the Pole Cottonwood – Yellow Willow Phase (1.2) to the Mature Cottonwood – Dogwood Phase (1.3). The cottonwood overstory matures and begins to self-thin. The canopy starts to open, promoting understory growth.

Community Phase 1.3: Mature Cottonwood – Dogwood Phase
In the Mature Cottonwood – Dogwood Phase, the overstory consists of mature, old-growth cottonwoods. The tree canopy varies from 40 to 90 percent, with tree diameters of 12 inches or more DBH. The understory is characterized by a dense and diverse shrub layer. Common shrubs are redosier dogwood, chokecherry (Prunus virginiana), Woods’ rose, yellow willow, currant (Ribes spp.), and western snowberry. As time progresses, the cottonwoods begin to die off and the stand thins. Other native trees such as box elder begin to inhabit the understory and will eventually replace the cottonwood stand (Hansen et al., 1995). The herbaceous layer remains dominated by non-native species. The most common are smooth brome and Kentucky bluegrass, but quackgrass and annual bromes may also occur.

The approximate canopy cover by species for the Mature Cottonwood – Dogwood Phase is as follows:

Percent canopy cover by species*
Cottonwood spp. 65%
Redosier Dogwood 55%
Common Chokecherry 20%
Western Snowberry 1%
Woods’ Rose 10%
Other Native Shrubs 30%
Graminoids 20%
Perennial Forbs 40%

* Estimated from Hansen et al. (1995)

Community Phase Pathway 1.3a
Extremely long periods of low disturbance, bank building, and lowering of the water table transition the Mature Cottonwood – Dogwood Phase (1.3) to the Box Elder/Shrub Phase. The exact length of time required for this transition is unknown, but is estimated to be 80 to 100 years. The majority of the cottonwood stand has died out and has been replaced by box elder and facultative shrubs. At this point, the site is transitioning to a more upland site rather than a riparian site.

Community Phase Pathway 1.3b
Channel migration associated with major flooding or a stand-replacing fire transitions the Mature Cottonwood – Dogwood Phase (1.3) to the Seedling Cottonwood – Sandbar Willow Phase (1.1). A major flood event that causes the river channel to migrate or a stand-replacing fire that exposes the bank to scouring and collapse will effectively revert the site back to an alluvial bar and begin the process of succession over again.

Community Phase Pathway 1.3c
Improper grazing management transitions the Mature Cottonwood – Dogwood Phase (1.3) to the Mature Cottonwood – Snowberry Phase (1.5). Improper grazing management will reduce the diversity of the shrub understory, reducing canopy cover of redosier dogwood and chokecherry. The understory will become dominated by western snowberry and Wood’s rose (Hansen et al., 1995).

Community Phase 1.4: Box Elder/Shrub Phase
The Box Elder/Shrub Phase occurs after extremely long periods without disturbance by flooding. The channel has migrated away from the site, and the ground-water table is no longer a significant contributor to site moisture. Cottonwoods are rare or absent, and the site is on the verge of transitioning to a drier ecological site. Common shrubs may include chokecherry, snowberry, or buffaloberry. Herbaceous species are dominated by non-native species but begin to shift to more drought-tolerant species, such as Kentucky bluegrass and field brome (Bromus arvensis). Unless the channel begins to migrate back toward the site, this phase will eventually transition to a drier site, such as Overflow or Swale.

Community Phase Pathway 1.4a
Channel migration in combination with major flooding or a stand-replacing fire transitions the Box Elder/Shrub Phase to the Seedling Cottonwood – Sandbar Willow Phase (1.1). A major flood event that causes the river channel to migrate back to the site in combination with bank slumping or a stand-replacing fire that exposes the bank to scouring and collapse will effectively revert the site back to an alluvial bar and begin the process of succession over again.

Community Phase 1.5: Mature Cottonwood – Snowberry Phase
The Mature Cottonwood – Snowberry Phase occurs due to improper grazing management of the Mature Cottonwood – Dogwood Phase. The diversity of the shrub understory is significantly reduced, and desirable species such as redosier dogwood have been replaced by less desirable species such as western snowberry and Woods’ rose. The overstory consists of mature, old-growth cottonwoods. The tree canopy varies from 40 to 90 percent, with tree diameters of 12 inches or more DBH. As time progresses, the cottonwoods begin to die off and are replaced by other trees, such as box elder. The herbaceous layer is dominated by non-native species, particularly smooth brome.

Community Phase Pathway 1.5a
Proper grazing management will return the Mature Cottonwood – Snowberry Phase (1.5) to the Mature Cottonwood – Dogwood Phase (1.3).


Community Phase Pathway 1.5b
Channel migration associated with major flooding or a stand-replacing fire transitions the Mature Cottonwood – Snowberry Phase (1.5) to the Seedling Cottonwood – Sandbar Willow Phase (1.1). A major flood event that causes the river channel to migrate or a stand-replacing fire that exposes the bank to scouring and collapse will effectively revert the site back to an alluvial bar and begin the process of succession over again.

Transition T1A
Prolonged improper grazing management in combination with the absence of flooding disturbance transitions the Contemporary Reference State (1) to the Herbaceous Understory State (2). Prolonged improper grazing will eventually completely remove the shrub understory layer, leaving an understory of herbaceous vegetation. Long periods of low disturbance have built up banks and lowered the water table.

Transition T1B
Clearcutting, tillage or application of herbicide, and seeding of cultivated crops (such as wheat, barley or introduced hay) transitions the Contemporary Reference State (1) to the Cropland State (4).

Transition T1C
Establishment of invasive tree species, particularly Russian olive, transitions the Contemporary Reference State (1) to the Invasive Tree State (3).

State 2: Herbaceous Understory State
The Herbaceous Understory State (2) occurs when the shrub understory has been removed due to long-term improper grazing practices. The overstory typically consists of a mature, open cottonwood stand but may include other trees in some cases. The tree canopy varies from 40 to 90 percent, with tree diameters of 12 inches or more DBH. The understory consists primarily of non-native grasses, particularly smooth brome. Noxious weeds such as leafy spurge and Canada thistle may be prominent in some cases. Once the site has converted from a shrub-dominated understory to one dominated by introduced herbaceous species, returning it to its former state is very difficult. A drastic change in management is needed, and it most likely will be labor intensive and costly (Hansen et al., 1995).

Community Phase 2.1: Native Tree/Smooth Brome Phase
The Native Tree/Smooth Brome Phase consists of a mature, open tree overstory with a non-native grass understory. The overstory is typically plains cottonwood but may also include box elder, peachleaf willow (Salix amygdaloides), or other native tree species. Narrowleaf cottonwood commonly replaces plains cottonwood as elevation increases. The understory is dominated by the introduced grass smooth brome but may also include other species, such as Kentucky bluegrass and quackgrass.

Community Phase 2.2: Native Tree/Noxious Weeds Phase
The Native Tree/Noxious Weed Phase consists of a mature, open tree overstory with a noxious weed understory. This community develops when removal of the understory by improper grazing occurs in proximity to a noxious weed seed source. The overstory is typically plains cottonwood but may also include box elder, peachleaf willow, or other native tree species. Narrowleaf cottonwood commonly replaces plains cottonwood as elevation increases. The understory is dominated by noxious weeds, particularly leafy spurge, Canada thistle, and Russian knapweed.

Transition T2A
Establishment of invasive tree species, particularly Russian olive, transitions the Herbaceous Understory State (2) to the Invasive Tree State (3).

Transition T2B
Clear cutting, tillage or application of herbicide, and seeding of cultivated crops (such as wheat, barley, or introduced hay) transitions the Herbaceous Understory State (2) to the Cropland State (4).

Restoration Pathway R2A
Proper grazing management, tree/shrub planting, and intensive weed management can transition the Herbaceous Understory State (2) back to the Contemporary Reference State (1). A change in management alone may not be sufficient. Replanting of desirable species and intensive weed management practices are generally needed in conjunction with proper grazing management. These restoration methods are labor intensive and costly and may not be a practical in all situations.

State 3: Invasive Tree State
The Invasive Tree State occurs when invasive tree species, particularly Russian olive, establish and dominate the site. Russian olive is a highly competitive tree native to southern Europe and western Asia. It is widely established and commonly forms dense thickets at the exclusion of native species (Hansen et al., 1995). In more open stands, Russian olive may occur in conjunction with widely scattered cottonwood stands and native shrubs, such as snowberry and Woods’ rose. As the cottonwood stand dies out, Russian olive will replace it at the exclusion of the typical native tree species.

Community Phase 3.1: Russian Olive/Shrub Phase
The Russian Olive/Shrub Phase occurs in open stands on older alluvial bars and terraces. The overstory is dominated by Russian olive; however, an open, decadent stand of cottonwoods may be present in some cases. The understory is sparse, and diversity is low. Primary understory species are snowberry and Woods’ rose. Herbaceous species are dominated by non-native grasses, particularly smooth brome.

Community Phase 3.1: Russian Olive/Noxious Weed Phase
The Russian Olive/Noxious Weed Phase occurs on flood plains and terraces, particularly where soils are slightly to moderately saline. Under these conditions, Russian olive typically forms a dense thicket at the exclusion of native tree species. The understory is extremely suppressed and is predominately noxious weeds. Canada thistle and leafy spurge are common understory species.


Transition T3A
Removal of invasive tree species transitions the Invasive Tree State (3) to the Herbaceous Understory State (2). Typically, tree/shrub planting is required to reestablish native woody species. When clearing the Russian Olive/Noxious Weed Phase (4.2), intensive weed management is critical. Removal of the overstory will release understory growth and cause noxious weed populations to increase exponentially. This transition is very costly and labor intensive and may not be practical in all situations.

Transition T3B
Clear cutting, tillage or application of herbicide, and seeding of cultivated crops (such as wheat, barley, or introduced hay) transitions the Invasive Tree State (3) to the Cropland State (4).

State 4: Cropland State
The Cropland State (4) occurs when land is put into cultivation. Deep, fertile soils and favorable moisture conditions make the Riparian Woodland ecological site prime farmland. Additionally, its proximity to perennial streams makes it ideal for irrigation. Because of this, many acres of the Riparian Woodland ecological site have been cleared and converted to farmland. It is commonly planted to non-native perennial species and irrigated for production of hay. Common species include alfalfa, orchardgrass, and grass/alfalfa mixes. Annual crops, such as wheat and barley, are commonly planted in rotation with perennial species at 5- to 15-year intervals. Silage corn is grown is some cases, but this crop is of limited extent. Flood irrigation is most common but center pivot sprinklers are also used. Several major storage reservoirs and large networks of irrigation canals are present in much of the Milk and Missouri River valleys. Cropping and irrigation projects have vastly altered vegetation and hydrology on much of the Riparian Woodland ecological site. Once the site is converted to production agriculture, land values increase significantly and it is unlikely that the site will be converted back to natural vegetation.