Ecological dynamics
The potential native plant community is dominated by a multi age stand of quaking aspen (Populus tremuloides). Willows (Salix spp.), chokecherry (Prunus virginiana) and golden currant (Ribes aureum) are typically common. Basin wildrye (Leymus cinereus), blue wildrye (Elymus glaucus), mountain brome (Bromus carinatus), Idaho fescue (Festuca idahoensis) and a variety of forbs are present. Sedges (Carex spp.) are common along perennial stream margins. Wood’s rose (Rosa woodsii), wax currant (Ribes cereum), snowberry (Symphoricarpos spp.) and mountain big sagebrush (Artemisia tridentata spp. vaseyana) are present. Ponderosa pine (Pinus ponderosa) and/or Douglas-fir (Pseudotsuga menziesii) are minor. Vegetative composition of the community is approximately 25 percent grasses, 5 percent forbs and 70 percent shrubs and trees. Approximate ground cover is 80 to 120 percent (basal and crown).
Range in Characteristics:
Healthy quaking aspen stands are uniform with varying age classes. The understory composition is influenced by the stand age, canopy closure and the connectivity, slope and extent of the active floodplain. Understory shrub diversity and density increases significantly in open shrub-sapling and open sapling-pole stands. Basin wildrye increases along outer site edges and open areas. Under a closed older canopy understory shrub diversity and production decreases dramatically. Willows and sedges increase along well connected perennial stream with high water tables. In areas adjacent to forestland snowberry, willows, rose, elk sedge (Carex geyeri) and pinegrass (Calamagrostis rubescens) increase. On flatter deposition areas the site typically grades into a sedge/tufted hairgrass wet meadow or meadow site. Adjacent fans and drier slopes are often dominated by an aspen upland site having an increase in mountain snowberry and wax currant. Basin wildrye dominates adjacent dry swale, fan and bottomland sites.
Quaking Aspen Dynamics:
Quaking aspen occurs on this site as the dominant plant species in a stable plant community. The aspen community is multi aged with trees in varies stages of development well represented. Reference phases are shrub/grass stand initiation, sapling-pole, mature aspen and decadent aspen.
Individual aspen are short-lived and rarely survive for more than 150 years. Aspen, however, are noted for their ability to regenerate vegetatively by suckers arising along their long lateral roots. Root sprouting results in many genetically identical trees (ramets), in aggregate called a “clone”. All the trees in a clone have identical characteristics and share a common root structure, however, a stand can be made up of several clones.
When aspen trees die or light becomes available in aspen openings, chemical signals from the tree to the root stimulate new sprouts to start growing. Through this cycle of regrowth, an aspen clone can lives much longer than an individual tree. Aspen clone survival can be hundreds of years old (5,000 to 10,000 year old clone ages have been estimated in some areas).
Aspen trees are dioecious, with male and female flowers normally borne on separate trees. Sexual reproduction may occur, yet in comparison to vegetative reproduction, reproduction by seed is less commonly observed in the Western US except following fire on adequately moist seedbeds. Few aspen seedlings survive in nature due to the short time seed is viable, lack of moisture during seed dispersal, poor seedbed conditions, fungi and adverse day/night temperatures.
Historically, periodic disturbance events have been important to the maintenance of healthy aspen stands. These include drought, windthrow, wildfire, insect outbreaks, and disease (e.g. stem cankers and root pathogens), that would remove portions of the stand, with advanced ramet age increasing the likelihood of damage from some disturbance agents.
Wildfire has been an important disturbance in these aspen stands historically, stimulating vegetative reproduction, promoting stand heterogeneity and controlling the invasion of coniferous species. While the high fuel moisture content of aspen stands may render them somewhat resistant to fires for much of the year, fire frequency and intensity is influenced by the surrounding vegetation matrix within which the stand is found. For this site, surrounding vegetation types often include mountain big sagebrush, dry ponderosa pine woodlands and dry mixed conifer forests, all of which would have historically been subject to frequent low to replacement severity fires (Landfire fire regime groups 1 and 2, Landfire 2007). Without fire, coniferous species, readily invade and can dominate this site over time, outcompeting the less shade tolerant aspen for light and depleting soil moisture. Research has demonstrated that Western juniper (Juniperus occidentalis) expansion into similar sites in southeast Oregon has been widespread, coinciding with the suppression of fire in these areas beginning around the turn of the 20th century, in addition to associated grazing and climate factors (Wall et al. 2001).
Aspen provides quality browse for many ungulates including deer, elk and cattle. Excessive grazing can have profoundly detrimental effects on aspen regeneration and stand viability by suppressing regenerating saplings (among other impacts). Aspen stands may severely impacted by high utilization rates by native ungulate (in particular elk in some regions) as well as by livestock. If the condition of the site deteriorates as a result of over grazing by livestock and/or deer/elk use a decline in aspen reproduction occurs. Tall shrubs, willow and chokecherry become hedged with severe utilization to browse height and limited reproduction. Shorter shrubs and the herbaceous understory is severely affected with the replacement of currants, snowberry, basin wildrye, Idaho fescue, blue wildrye, sedge and palatable forbs by Kentucky bluegrass (Poa pratensis), low palatability forbs (such as Veratrum viride) and annuals. With continued heavy use bare ground increases, erosion accelerates and site productivity decreases. Canada thistle (Cirsium arvense), houndstongue (Cynoglossum officinale) and white top (Cardaria draba) invade. With continued heavy use bare ground increases, channels incise, becoming deeper and wider in the process and erosion accelerates. Water tables are lowered as the adjoining channels incise. The site becomes moisture limited shifting composition toward drought adapted species and further decreasing productivity. Under prolonged heavy ungulate use aspen clone reproduction is eliminated and stands slowly become decadent, potentially transitioning the site to a shrub or juniper dominated community. Alterations to water withdraws, creation of impoundments, removal of beaver and modifications to stream channels may have similar effects on stream morphology and changes to water tables.
Severe drought events have been linked to episodes of widespread aspen mortality in North America and these types of events are expected to increase under a changing climate (Worrall et al. 2012). While the potential impacts of climate change and severe drought on this site are unknown, aspen stands in other marginal habitats in Oregon have demonstrated physiological sensitivity to drought related climate parameters (Neary et al. 2021) and it is possibly that interactions of changing climate with other stressors will render stands in the region more vulnerable to decline (Dwire et al. 2018).
State 1
Historical Reference State
This state represents the pristine historical reference conditions with no exotic species present. Healthy aspen stands are on this site type are heterogeneous with vigorous trees in all natural life stages from young saplings to mature. Stands will cycle through several development phases from shrub/grass communities through mature stands, with many phases often occurring on a single site. These dynamics are driven by an intact historical disturbance regime with periodic events acting to maintain a dynamic equilibrium with adjacent stream morphology and vegetative composition; helping to reduce conifer invasion; remove diseased and decadent mature trees and stimulate sucker reproduction. The resilience and resistance of the site is bolstered by positive feedbacks between aspen production and the formation of deep, mollic epipedons with high organic matter, nutrient content and water holding capacity; and negative feedbacks between stand maturity and disturbance frequency/magnitude.
Community 1.1
Reference Plant Community: Mature Forest Phase
The reference native plant community is dominated by a multi age stand of quaking aspen. Willows, chokecherry and golden currant are typically common. Basin wildrye, blue wildrye, mountain brome, Idaho fescue and a variety of forbs are present. Sedges are common along perennial stream margins. Wood’s rose, wax currant, snowberry and mountain big sagebrush are present. Understory composition will decline as canopy closure progresses. Ponderosa pine and/or Douglas-fir are minor. Vegetative composition of the community is approximately 25 percent grasses, 5 percent forbs and 70 percent shrubs and trees. Approximate ground cover is 80 to 120 percent (basal and crown).
Table 7. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Tree |
1121 |
1681 |
2242 |
| Grass/Grasslike |
560 |
841 |
1121 |
| Shrub/Vine |
448 |
673 |
897 |
| Forb |
112 |
168 |
224 |
| Total |
2241 |
3363 |
4484 |
Community 1.2
Shrub/Grass - Stand Initiation Phase
In this phase, the stand is dominated by herbaceous species and sprouting shrubs. Young sapling aspen may be regenerating, conifers are absent. High populations of native ungulates such as deer and elk and subsequent heavy browsing of regenerating aspen, may maintain the site in this phase. Additionally, frequent fire intervals driven by abnormal climate conditions may maintain the site in this phase.
Community 1.3
Young Forest Community Phase
In this phase the stand is dominated by regenerating and recruiting aspen from sapling to pole size. Initial closed conditions and very high, uniform stem densities will give way to open stand conditions, lower stem densities and multi-layered stand structure as self-thinning, overstory recruitment and further regeneration takes place. Shrub and grass composition will decrease relative to community 1.2. Some conifers may be present at low levels.
Community 1.4
Overmature, Decadent Phase
In this phase the stand is dominated by large, mature aspen with a relatively even stand structure (~100 years). Closed canopy conditions decrease understory diversity and favor shade tolerant grasses, forbs and shrubs. Understory aspen regeneration is uncommon due to lack of light at lower heights but may occur in localized patches as decadent overstory trees succumb to various disturbance agents. Conifers may be increasing in this state and the site risks a transition to a conifer dominated state with a prolonged lack of large scale disturbance.
Pathway P1.1b
Community 1.1 to 1.2
Large-scale disturbance such as fungal disease, insect outbreak, high severity wildfire or extreme drought event leads to mortality of aspen trees
Pathway P1.1a
Community 1.1 to 1.4
Extended time elapses in the absence of disturbance allowing the forest to become over mature and conifer expansion to occur.
Pathway P1.2a
Community 1.2 to 1.3
Extended time without widespread disturbance.
Pathway P1.3a
Community 1.3 to 1.1
Extended time without widespread disturbance.
Pathway P1.3b
Community 1.3 to 1.2
Large scale disturbance such as fungal disease, insect outbreak, high severity wildfire or extreme drought event leads to mortality of aspen trees.
Pathway P1.4a
Community 1.4 to 1.2
Large scale disturbance such as fungal disease, insect outbreak, high severity wildfire or extreme drought event leads to mortality of aspen trees.
State 2
Current Potential State
This state is similar to the historical reference state yet with the introduction of non-native plant species and an increased presence of western juniper. Kentucky bluegrass is one of the most common and persistent invading herbaceous plants with others including cheatgrass (Bromus tectorum), Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare), houndstongue (Cynoglossum officinale), Whitetop and St. Johnswort (Hypericum perforatum) also common. Ecological process and function have not been altered fundamentally by this low level of invasion, yet resistance and resilience to disturbance are decreased. Vegetated communities include all historical functional and structural groups, yet composition and richness may be reduced. This state is common due to widespread invasion of Kentucky bluegrass and expansion of western juniper in the Western US.
Community 2.1
Current Potential Mature Forest Community
The reference native plant community is dominated by a multi age stand of quaking aspen. Understory regeneration of aspen will slow as canopy closure progresses, yet periodic disturbance will remove mature aspen, create gaps and allow for localized pulses of regeneration to occur. Kentucky bluegrass (and potentially other exotic species) is a common associate with herbaceous plants listed in the reference plant communities.
Community 2.2
Current Potential Shrub/Grass - Stand Initiation Phase
In this phase, the stand is dominated by herbaceous species and sprouting shrubs, Kentucky bluegrass (and potentially other exotic species) is common. Young sapling aspen may be regenerating from root suckers or seed given favorable conditions, seedling conifers may be present in low numbers. High populations of native ungulates such as deer and elk and subsequent heavy browsing of regenerating aspen, may maintain the site in this phase. Additionally, frequent fire intervals driven by abnormal climate conditions or adjacency with rangelands dominated by invasive annual grasses may maintain the site in this phase.
Community 2.3
Current Potential Young Forest Community Phase
In this phase the stand is dominated by regenerating and recruiting aspen from sapling to pole size. Initial closed conditions and very high, uniform stem densities will give way to open stand conditions, lower stem densities and multi-layered stand structure as self-thinning, overstory recruitment and further regeneration takes place. Shrub and grass composition will decrease relative to community 1.2, exotic species are likely present. Conifers may be present at moderate density.
Community 2.4
Current Potential Overmature, Decadent Phase
In this phase the stand is dominated by large, mature aspen with a relatively even stand structure. Closed canopy conditions decrease understory diversity and favor shade tolerant grasses, forbs and shrubs, with exotic species present. Understory aspen regeneration is uncommon due to lack of light at lower heights but may occur in localized patches as decadent overstory trees succumb to various disturbance agents. Conifers may be increasing in this state toward codominance with mature aspen and the site risks a transition to a conifer dominated state with a prolonged lack of large scale disturbance.
Pathway P2.1b
Community 2.1 to 2.2
Large scale disturbance such as fungal disease, insect outbreak, high severity wildfire or extreme drought event leads to mortality of aspen trees.
Pathway P2.1a
Community 2.1 to 2.4
Extended time elapses in the absence of disturbance allowing the forest to become over mature and conifer expansion to occur.
Pathway P2.2a
Community 2.2 to 2.3
Extended time without widespread disturbance.
Pathway P2.3a
Community 2.3 to 2.1
Extended time without widespread disturbance.
Pathway P2.3b
Community 2.3 to 2.2
Large scale disturbance such as fungal disease, insect outbreak, high severity wildfire or extreme drought event leads to mortality of aspen trees.
Pathway P2.4b
Community 2.4 to 2.2
Large scale disturbance such as fungal disease, insect outbreak, high severity wildfire or extreme drought event leads to mortality of aspen trees.
State 3
Conifer State
In this state, conifer succession has advanced to overtop aspen and outcompete the species for light and soil moisture. On this site, western juniper is the most likely conifer invader, yet ponderosa pine (Pinus ponderosa), or Douglas fir (Pseudotsuga menziesii) may also encroach on favorable sites. Over mature, decadent aspen may still be present and aspen reproduction may occur in patches with adequate light and moisture. However, feedbacks have been altered and aspen will likely not successfully compete with conifers to regenerate successfully, recruit into the overstory, and regain forest structure without management interventions or altered disturbance regimes. Impacts and emerging feedbacks may include alterations to soil chemistry, changes in hydrologic cycling (including increased sublimation and translocation of snowfall by conifers), and persistent increases in understory shading favoring more shade tolerant conifer reproduction (Wall et al. 2001, LaMalfa and Ryle 2008). Heavy herbivory of sapling aspen, and/or low intensity fire that leaves more fire resistant conifers intact may further accelerate the loss of aspen to conifers. Several community phases may occur within this state with the common thread of conifer dominance.
State 4
Drained State
Streambanks have become unstable from loss of vegetation and the channel degrades becoming deeper and wider in the process. Subsurface flows are affected. The water table drops and storage of water for the late season flows is reduced. Plants well adapted to a drier climatic regime increase or invade and production drops. Channel widening and incision are common in this state as unstable banks and vegetation loss create a positive feedback loop that decreases resilience to runoff events. Abandoned floodplains transition into primary terraces and primary terraces transition toward secondary terraces which become dominated by drought adapted species that do not require a connection to the water table. Many community phases may exist within this state depending on the disturbance history and stand dynamics prior to incision.
Transition T1A
State 1 to 2
Invasion of the site by non-native plant species.
Transition T2A
State 2 to 3
Time and lack of wildfire allows juniper to overtop and outcompete aspen for light and moisture. Shade intolerant aspen die off and regeneration is dramatically decreased due to closed canopy conditions. On some sites ponderosa pine and/or Douglas fir may encroach and facilitate a similar dynamic in the absence of fire.
Transition T2B
State 2 to 4
This transition may be the result of several disturbances that lower water tables beyond depths that support riparian woody vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness. These may include: alteration of streamflow by irrigation or impoundment leading to a lowering of the water table during times of year when riparian woody vegetation is dependent; removal of beaver; direct manipulation of channel morphology (namely straightening for agricultural or development purposes); removal of large woody debris or large woody debris sources, from channels or adjacent forests and significant alterations of upland watershed vegetation altering peak discharge or sediment loads.
Restoration pathway R3A
State 3 to 2
Mechanical removal of conifers may release understory aspen from canopy shading. Prescribed fire may also be used to remove conifers and stimulate aspen regeneration. However, under circumstances where advanced decline has led to an absence of aspen regeneration, the use of prescribed fire may actually lead to further damage to the clone and may accelerate complete clone mortality. Depending on the degree of encroachment and aspen loss, these interventions may transition the site to any one of the current potential community phases. Under conditions of high native ungulate or livestock use of the stand, management actions such as fencing or jackstrawing may need to follow conifer removal to allow aspen regeneration to occur unimpeded.
Context dependence. Restoration options will be highly site specific and may not be possible in many circumstances.
Transition T3A
State 3 to 4
This transition may be the result of several disturbances that lower water tables beyond depths that support riparian woody vegetation, alter sediment supply and transport leading to scouring and channel incision, or directly increase flow velocities or flashiness. These may include: alteration of streamflow by irrigation or impoundment leading to a lowering of the water table during times of year when riparian woody vegetation is dependent; removal of beaver; direct manipulation of channel morphology (namely straightening for agricultural or development purposes); removal of large woody debris or large woody debris sources, from channels or adjacent forests and significant alterations of upland watershed vegetation altering peak discharge or sediment loads.
Restoration pathway R4A
State 4 to 2
Restoration of hydrologic and biotic process and function through rehabilitation of channel and vegetation structure may be possible but will require considerable inputs, time and cost. This may require the placement of large woody debris, creation or removal of impoundments, alteration of water withdrawals, management changes to adjacent agricultural or grazing practices, or mechanical manipulation of stream channel courses among other intensive interventions.
Context dependence. Restoration options will be highly site specific and may not be possible in many circumstances.
