Ecological dynamics

Reference State
Community Phase 1.1: willow shrub carr/ rich & productive understory
Without disturbance, the Reference State would have been dominated by woody plants. The proportion of woody plants in relation to herbaceous understory species would have been influenced by the time and type of natural of disturbance that most recently took place. Possible natural disturbances would have included beaver and/or moose consumption of willow, deciduous wood pathogens (i.e. insects) reducing particular species, wildfires, and extreme run-off causing flooding or diversion of existing drainages. Such disturbances would have temporarily decreased the woody component and allowed an increase in herbs. All of these influences tended to have very long return intervals. Without such disturbance, woody plants would have increased at the expense of the understory because of the overtopping shade they create.

T1a: Transition from State 1 to State 2 (Reference State to Willow Shrub Carr/ Diminished Understory State)
The simultaneous introduction of exotic species, both plants and animals, and possible extinctions of native flora and fauna, along with climate change, has caused State 1 to transition to State 2. Reversal of such historic changes (i.e. a return pathway) back to State 1 is not practical.

Willow Shrub Carr/ Diminished Understory State
State 2 is very similar to State 1 in form and function, with the exception of the presence of non-native plants and animals, possible extinctions of native species, and a different climate. State 2 is a description of the ecological site shortly following Euro-American settlement. This state can be regarded as the current potential. State 2 can fluctuate between two willow-dominated phases: one that is relatively undisturbed with a rich and productive understory (2.2), and another where the understory is reduced due to the heavy livestock grazing (2.1). Phase 2.1 is a willow (Salix spp.) stand with a reduced understory due to the heavy livestock grazing. This Phase was also produced by horses and mules belonging to early European settlers and travelers (2.2a). These areas usually have surface water available, an attractant to most animals in the vicinity, which tend to pass through these stands at least once daily to feed, get water, and find shade. Heavy use of this part of the landscape was common. Because of the high resilience of these sub-irrigated habitats, partial recovery of the understory was attained in many instances and accelerated soil erosion arrested. Sustainable use was approached in such instances (2.1a).

Community Phase 2.1: increased willows & unpalatable forbs/ diminished understory
Periods of heavy livestock and/or trailstock grazing of the herbaceous understory, along with near-extirpation of beaver and moose, causes this plant community to experience an increase in the proportion of woody plants at the expense of the herbaceous understory. Sedges, unpalatable forbs, and woody species are increased.

Community Pathway 2.1a:
A reduction in livestock numbers and limiting seasons of use allows the understory component to rejuvenate.

Community Phase 2.2: willow shrub carr/ rich & productive understory
This plant community has regained the understory components following a period of reduction in livestock use.

Community Pathway 2.2a:
Heavy continuous season long grazing by livestock reduces the palatable understory species. This occurred in the past when these sites were common places for travelers to feed and water their animals.

T2a: Transition from State 2 to State 3 (Willow Shrub Carr/ Diminished Understory State to Xerified Shrubland State)
Excessive season-long livestock (or trailstock use in the past) involves high intensities of forage utilization, trampling, and bedding. Salting was common on such locations. When ground cover is reduced, accelerated soil erosion is possible. These impacts, along with logging in the watersheds above, results in accelerated channel down-cutting, more extreme flooding, and changes in drainage patterns. The overall result is a xerification of the site and lignification of its vegetation (an increase in woody vegetation). The approach to this transition is indicated by changes in species composition – primarily an increase in woody vegetation. The trigger causing this transition is stream down-cutting due to extreme hydrologic events.

Xerified Shrubland State
Where control of grazing intensity isn’t been achieved earlier and excessive use by livestock prevails, the vegetation takes on more of the character of that of drier sites at low elevations. As the water table is lowered, the stature of the willows and other riparian shrubs declines, allowing upland species such as rubber rabbitbrush (Ericameria nauseosus), Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) to fill in these sites (3.2). Wildfire followed by continued heavy livestock grazing (3.2a) will temporarily remove the shrub and palatable herbaceous component, leaving annuals and biennials such as lesser burdock (Arctium minus), rough cocklebur (Xanthium strumarium), horehound (Marrubium vulgare), houndstongue (Cynoglossum officinale), stickseed (Hackelia spp.), Canada thistle (Cirsium arvense), and Scotch cottonthistle (Onopordum acanthium) to flourish (3.1). The recovery of moose and beaver puts more pressure on the remaining willows. However, if enough willow and other deciduous shrubs survive previous herbivory to allow beaver dam building, and thus re-ponding of these sites (R3a), it may be possible for the original mesic species to re-establish and for the site to return to State 2. A return to heavy livestock grazing and logging will negatively impact the resiliency of this State.

Community Phase 3.1: unpalatable annuals & biennials dominance
This plant community is dominated by assorted unpalatable annuals and biennials that gained dominance following wildfire and heavy continuous season long grazing. Some of the species may include burdock, cocklebur, horehound, houndstongue, stickseed, Canada thistle, and Scotch cottonthistle.

Community Pathway 3.1a:
Previous channelization and consequent lowering of the water table will over time lead to xerification of this site. This occurs because channelization moves water through the site rather than allowing it to infiltrate the soil and be retained for season-long plant growth.

Community Phase 3.2: Wyoming big sagebrush/ rubber rabbitbrush
This plant community is dominated by Wyoming big sagebrush and rubber rabbitbrush due to a lowering of the water table and subsequent xerification of the site.

Community Pathway 3.2a:
In the event of wildfire followed by heavy grazing pressure by livestock the site will convert to one dominated by assorted unpalatable annual and biennial forbs.

T3a: Transition from State 3 to State 4 (Xerified Shrubland State to Seeded Grassland State)
If managers are dissatisfied with the levels of productivity and/or the dominance of undesirable and noxious weeds present in State 3, the location is suitable, and finances are available, they could till and re-seed with a suite of montane grasses that would not only increase forage but may help to stabilize streambanks as well.

R3a: Restoration Pathway from State 3 to State 2 (Xerified Shrubland State to Willow Shrub Carr/ Diminished Understory State)
It may be possible for this site to recover to a willow-dominated system where beaver populations and activity have been restored. The ponding caused by construction of beaver dams helps raise the water table, creating a less favorable environment for the upland species that moved in and allows the original mesic species to re-occupy the site.

Seeded Grassland State

A seeded grassland state is possible if the site is tilled and re-seeded to increase forage for livestock and to stabilize walls of streambanks. Levels of grazing will have to be controlled (4.2a) or the initially pure grassland (4.1) will quickly be re-invaded by rabbitbrush, sagebrush, willow, or other mesic shrubs (4.2), along with the noxious understory forbs such as burdock, cocklebur, horehound, houndstongue, stickseed, and a variety of thistles. Heavy continuous season long grazing would deplete the seeded grasses, giving an advantage to shrubs and other invasive species (4.1a).

Community Phase 4.1 pure grassland
This plant community is dominated by a suite of seeded grass species used to increase forage production for livestock and stabilize streambanks.

Community Pathway 4.1a:
Heavy continuous season long grazing will deplete the seeded grasses, allowing shrubs and other invasive species to re-establish.

Community Phase 4.2 shrub & weed invaded grassland
This plant community is a product of heavy grazing on seeded grass species. Seeded grasses are diminished and an encroachment of woody species including willow, sagebrush, or rabbitbrush has occurred.

Community Pathway 4.2a:
Moderation of grazing is required to sustain a purely grassland phase.