Ecological dynamics

This ecological site is found in proximity to large rivers, which are slightly entrenched with moderate to high width to depth ratios, moderate to high sinuosity and have gravel or cobble channel material (Rosgen C4 type). The valleys in which they are found are considered Alluvial (type 8) and are mature, wide, gentle valley slopes with well-developed flood plain features adjacent to river terraces and/or glacial terraces-alluvial valley fills (Rosgen, 1994). Alluvial terraces and flood plains are the predominate landforms. The vegetation community is dominated in the overstory by Black cottonwood with some paper birch. There is little encroachment by conifer species and total cover is less than 5%. The understory has moderate to high shrub cover composed of redosier dogwood, common snowberry and Rocky mountain maple. A diverse assemblage of herbaceous forbs are present in low canopy cover at this ecological site.

This ecological site is found on the north and middle forks of the Flathead River, off Ole Creek and off creeks that feed into McDonald Creek. The Flathead River system exhibits disequilibrium sediment conditions and large amounts of woody debris (Malanson, 1991). There was severe flooding on all forks of this river in 1964, 1975 and 1989 that introduced massive amounts of new woody debris to the system, mobilized large amounts of sediment and created extensive new gravel bars (Malanson, 1991). These gravel bars would, with time without flooding, become vegetated through succession with herbaceous, willow and cottonwood seedlings. Other major floods on the Flathead River were in 1894, 1899, 1948, and 1954. Malanson (1991) conducted a study on correlations of species abundance with environmental variables on the North Fork of the Flathead River. Black cottonwood was found to occupy the most ecologically developed sites of gravel point bars. Factors that Black cottonwood presence were correlated with include soil organic matter and depth, woody debris, distance to head of bar, the size and location of each bar and its species richness. Salix species were correlated with less ecologically developed sites and correlated to elevation of stand, distance from the stand to the head of the bar, the amount of woody debris upstream, clast size, depth of fine sediment and species richness.

Walton (unpublished report, 2006) studied the floodplains of the Nyack floodplain on the Middle Fork of the Flathead River examining the relationships between riparian tree diversity, above ground biomass, stand age, and elevation. Their results indicate that the oldest stands on the Nyack floodplain are approximately 130 years old. The last major flood on the Nyack was in 1894, followed by several slightly less severe floods in 1916, 1964 and 1975. Cottonwoods had the most biomass in stands under approximately 110 years old, then Engelmann spruce dominates the biomass. In terms of individual stems, conifers which establish after 40 years, will dominate after approximately 55 years. The intermediate age stands of 40 years had the highest riparian tree diversity. Scouring frequency is strongly correlated to elevation above channel, with both biomass and stand age increasing linearly with elevation. Succession proceeds as follows: young stands (less than 35 years old) have higher total stem counts and are dominated by cottonwoods then willows. The taller cottonwood trees will shade out the shorter shrubby willow trees and eventually outcompete them. Alder, fir, larch and birch can briefly colonize the stands, but do not get large or live long due to competition. These stands have relatively low diversity. Cottonwoods are not shade tolerant and are great pioneer species. Older stands contain cottonwoods with Engelmann spruce, Grand fir and Douglas fir establishing and dominating after approximately 50 years. Engelmann spruce and fir are relatively shade tolerant conifers and can occupy the forest understory. The cottonwoods that do survive are large, greater than 50cm in DBH. As the stand matures, all size classes are represented and the youngest trees are dominated by Engelmann spruce. The oldest stands measured had only a few cottonwoods, while Engelmann spruce filled the largest DBH classes.

Due to the re-occurring potential of flooding for this ecological site, time for succession to conifers over a much longer time is limited. Therefore, this site is defined by the dominance of black cottonwood with little understory of conifers and is addressing a shorter timeframe of approximately 100 years. Potentially without flooding disturbance, this site could progress to one dominated by conifers, but we are defining this site as precluding that scenario and maintained by flooding disturbance.

The historic fire regime of black cottonwood within the larger vegetation community of Engelmann spruce-subalpine fir is 35 to >200 years (USFS, FEIS, Fire return interval table within black cottonwood species review webpage). This site is susceptible to fire during the late summer and fall. Black cottonwood and associated shrubs resprout after fire and are adapted to light and in some instances moderate intensity fires. Black cottonwood has moderate sprouting capability. Younger trees are more capable of sprouting than old, decadent ones. The higher the water table throughout the growing season, the greater the ability for sprouts to survive. So, timing of fire is important to resprouting capability. Community phase 1.2 is particularly damaged by fire because the cottonwood saplings do not have an extensive root system or thick bark. Numerous research projects throughout the West, have found cottonwood stands in decline. Braatne et. al (1996)attributed this decline to livestock grazing via consumption and trampling of seedlings, water diversion via drought stress, domestic settlement via clearing for development, exotic plants such as saltcedar and Russian-olive reducing recruitment, stream reservoirs, channelization which reduces meandering, agricultural clearing, gravel mining, direct harvesting and unnaturally high populations of beavers.

The montane deciduous alluvium floodplain ecological site is important to wildlife for cover, shade and food for a variety of species. Large mammals use this site depending on the season. The black cottonwood canopy provides nesting sites for bald eagles and ospreys and great blue herons and later Canada geese. Woodpeckers, great horned owls, wood ducks and raccoons nest in trunk cavities. Black cottonwood and red-osier dogwood are eaten by moose and used by beaver for food and building materials (Hansen, 1995). Waterfowl, small birds and mammals use the understory species for food and cover. This site is very important in providing thermal cover, debris recruitment, and streambank stability. Redosier dogwood and other shrub species are excellent in controlling erosion along streams (Hansen, 1995).

Black cottonwood is a fast growing native deciduous tree that lives 100-200 years (USFS FEIS). It has shallow spreading roots with well-developed laterals and are typically ectomycorrhizal. Establishment is by wind or water dispersed seed and establishment is episodic often creating multi-aged stands. Seeds are copious and produced yearly though need both moisture and mineral soil for establishment which occurs in 5-10 year intervals. Seedlings are able to withstand inundation and sediment deposition, which removes competitors. In fact, it requires water with dissolved oxygen content and is very shade intolerant. Ice jams during spring thaw and break up can scour away seedlings and fire will kill them as well as saplings. Black cottonwood trees are frequently damaged by fire even of low severity. Severe fire kills or top kills older trees which may be more resistant to lower severity fires by their thick bark. It sprouts post fire and is considered a fire “endurer”. Fire increases light to the forest floor and exposes mineral soil which cottonwoods need for seedling establishment, therefore it can also be a fire “invader”. Scott et. al (1996) summarized geomorphic processes that influence the type of cottonwood stand to be established. The montane deciduous alluvium floodplain site falls in the meandering fluvial process with frequent moderate flows on point bars that produce moderate numbers of even-aged stands, arranged in narrow, arcuate bands. These have a strong left-bank, right-bank asymmetry in ages based on meander pattern. Often there is directional flood training of stems.

Black cottonwood is categorized as a facultative wetland (FACW) species in the wetland indicator status system (species that usually occur in wetlands (67-99% estimated probability), but is occasionally found in non-wetlands), with medium energy value (retains usable energy value moderately well during fall and winter) and protein value (retains digestible protein value moderately well during fall and winter). It rates poor for cattle and horse forage (not relished and normally consumed to only a small degree or not at all) and fair for sheep (moderately relished and consumed to a moderate degree). For wildlife such as elk, mule deer, whitetail deer and antelope it is rated poor food value (available but the plant is consumed to only a small degree or not at all). It is rated good food value for upland game birds, small non-game birds and small mammals, but fair for waterfowl. It is good whitetail deer cover and small non-game birds and small mammals (readily utilized for cover when available), but fair for elk and mule deer and upland game bird and waterfowl (moderately utilized for cover when available).

Redosier dogwood is a FACW species, with medium energy value and Low protein value (poor retention of digestible protein value during fall and winter). It is fair for cattle and sheep forage palatability, but poor for horses. It is rate good food value for mule and whitetail deer (readily to moderately available in the plant’s range and consumed to a high degree). It is fair for antelope and poor for elk (readily to moderately available in the plant’s range but consumed only to a moderate degree). It is fair food value for upland game birds, waterfowl, small non-game birds and small mammals. It is fair cover value for elk, mule deer, whitetail deer, upland game bird, waterfowl, small non-game bird and small mammals.

Common snowberry is a FACU (species that usually occur in non-wetlands, but occasionally found in wetlands), has medium energy value and medium protein value. It is fair forage palatability for cattle and sheep, but poor for horses. It is considered fair in food value for elk, mule deer, whitetail deer and antelope. It is fair food value for upland game birds, waterfowl, small non-game birds and small mammals. It is fair cover value for mule and whitetail deer, but poor for elk (rarely or never utilized for cover when available). It is good cover for upland game birds, waterfowl, small non-game birds and small mammals.

Rocky Mountain maple is a FAC species (species equally likely to occur in wetlands or non-wetlands), with medium energy value and low protein value. It is fair for sheep forage palatability and poor for cattle and horses. It is rated good food value for mule deer, fair to elk and whitetail deer and poor for antelope. It is fair food value for upland game birds, small non-game birds and small mammals, but poor for waterfowl. It is fair cover value for elk, mule, whitetail deer, upland game birds, waterfowl, small non-game birds and small mammals.

Western meadowrue is a FACU species with low energy and low protein value. It is fair forage palatability for sheep, but poor for cattle and horses. It is considered fair food value for elk, mule and whitetail deer and poor for antelope.

State 1.0: This represents the historic state in which there are no weedy species present at the site. The community is the same composition as currently but without any weedy species.
Community Phase 1.1: This represents mature black cottonwood in the overstory with a significant presence of shorter statured paper birch present. The understory is multistoried with medium statured shrubs common snowberry and redosier dogwood and the low statured shrub thimbleberry and a diverse herbaceous layer with western sweet cicely and western meadowrue. Cowardin Classification: Paustrine: Class=forested wetland; subclass=broad-leaved deciduous; Water regime= temporarily flooded to intermittently flooded.


State 2.0: This represents the current reference state in which the same dominant species exist as in State 1.0 but there is a low presence of weedy species.
Community Phase 2.1: This represents the reference state in which tall, mature black cottonwood dominate the overstory with a minor amount of paper birch. The understory has a mixture of shrubs including: redosier dogwood (Cornus sericea ssp. sericea), common snowberry (Symphoricarpos albus), and Rocky mountain maple (Acer glabrum). There is a mixture of herbaceous forbs. There can be small gap dynamics in which one or two trees die, opening the canopy and allowing sunlight into the forest floor. Cowardin Classification: Paustrine: Class=forested wetland; subclass=broad-leaved deciduous; Water regime= temporarily flooded to intermittently flooded.

Community Phase 2.2: This represents the community post disturbance from flooding or fire. It represents an early seral stage. If the flooding event or fire is substantial, than the site will not have trees. The mineral soil will allow for re-establishment by herbaceous forbs, grasses and resprouting shrubs. Tree species, especially black cottonwood, will become established on the mineral soil. Over time, the trees will over-top the shrubs and herbaceous species, and dominate. This phase is very susceptible to flooding and may be eliminated after just a year or two of growth. Ice can also damage this phase by mechanical damage (bending, shearing or pulling out of young stems or tilting or pushing over of mature trees) or flooding. Seedlings and sapling black cottonwoods are particularly susceptible to fire since their bark is thin and the root system shallow. Young black 0cottonwoods are very susceptible to defoliation by livestock, wildlife and beaver use, and can be eliminated. Cowardin: System=palustrine; Class=forested wetland; Subclass=broad-leaved deciduous; Water Regime (nontidal) = seasonally flooded.

Community Phase 2.3: This represents the community in competitive exclusion phase in which the tree seedlings have grown to sapling and pole sized trees and are competing with each other for resources. The canopy cover is very high, shading out the forest floor and limiting the understory to shade-tolerant species. Cowardin Classification: Paustrine: Class=forested wetland; subclass=broad-leaved deciduous; Water regime= temporarily flooded to intermittently flooded.

State 3.0 Native with introduced annuals state:
3.1: This represents the community after heavy grazing resulting in a decrease of shrubs species except western snowberry and rose species and an introduction of weedy species such as timothy and Kentucky bluegrass. This represents a moderately disturbed secondary successional stage. Cowardin Classification: Paustrine: Class=forested wetland; subclass=broad-leaved deciduous; Water regime= temporarily flooded to intermittently flooded.

3.2: This represents the community after severe grazing in which all shrub species have been eliminated and the cover of weedy species has increased dramatically. Secondary successional state. This depauperate state has only an overstory of black cottonwood and an understory of wood’s rose, common timothy and Kentucky bluegrass. Cowardin Classification: Paustrine: Class=forested wetland; subclass=broad-leaved deciduous; Water regime= temporarily flooded to intermittently flooded.

Community Phase Pathway 2.1.A: This pathway represents flooding disturbance intense enough to kill trees and set the site back to mineral soil and colonizing herbaceous species, resprouting shrubs and tree seedlings.


Community Phase Pathway 2.2A: This pathway represents time without disturbance.


Community Phase Pathway 2.3A: This pathway represents time without disturbance.


Community Phase Pathway 2.3B: This pathway represents flooding disturbance intense enough to kill trees and set the site back to mineral soil and colonizing herbaceous species, resrouting shrubs and tree seedlings.

Transition 1: This represents the pathway from the historic state 1.0 without any weedy species present in the vegetation community to State 2.0 in which there are very low covers of weedy species present in the vegetation community.

Transition 2: This represents the pathway from State 2.0 the current reference state in which the cover of weedy species is very low to State 3.0 in which there has been a dramatic increase of weedy species through heavy continuous livestock grazing.

Community Phase Pathway 3.1.A: This pathway represents the community after heavy grazing with a concomitant loss of shrub species except western snowberry and rose species.

Community Phase Pathway 3.2A: This pathway represents the community after cessation heavy grazing or establishment of prescribed grazing methods in which there is an increase of shrub species such as serviceberry, redosier dogwood and western snowberry and a decrease in shrub species such as western snowberry and rose species.