Ecological dynamics
The potential native plant community is dominated by a multi age stand of quaking aspen (Populus tremuloides). Mountain snowberry (Symphoricarpos oreophilus) and wax currant (Ribes cereum) are prevalent in the understory. Idaho fescue (Festuca idahoensis), blue wildrye (Elymus glaucus), rose (Rosa spp.) and a variety of forbs are common. Mountain big sagebrush (Artemisia tridentata ssp. vaseyana), basin wildrye (Leymus cinereus), mountain brome (Bromus marginatus), bluebunch wheatgrass (Pseudoroegneria spicata), elk sedge (Carex geyeri) and pinegrass (Calamagrostis rubescens) are present. 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 presence of tall and short shrubs, forbs and grasses varies with precipitation, available subsurface moisture and aspen canopy cover. Shrubs decrease under older closed aspen canopies and increase in stand initiation and open sapling-pole stages. Tall shrubs, chokecherry (Prunus spp.) and willows (Salix spp.) increase with an increase in perennial subsurface and surface moisture. Mountain snowberry and currants consistently occur in the understory. Idaho fescue is the dominant grass with blue wildrye increasing in shade. Basin wildrye and mountain big sagebrush increase in lightly shaded areas and near stand edges. On moist swales and other sites adjacent to forestland, elk sedge and pinegrass will increase. On lower elevation and droughtier aspects, mountain big sagebrush, wax currant and Idaho fescue increase. Needlegrasses (Achnatherum spp.) and blue wildrye increase on droughtier coarse textured soils. In areas with late winter snow drifts, understory shrubs such as snowberry, rose and serviceberry (Amelanchier spp.) increase. Associated riparian and seep areas are dominated by an aspen riparian site. Basin wildrye dominates associated swale 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 various 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 and 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, particularly Western juniper (Juniperus occidentalis) on this site, readily invades and can dominate this site over time, outcompeting the less shade tolerant aspen for light and depleting soil moisture. Research has demonstrated that juniper 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). Unlike other sites providing important forage for ungulates in the region, aspen stands may be as impacted by high wildlife densities (such as elk) as by high numbers of cattle. If the condition of the site deteriorates as a result of overgrazing by livestock, deer, and elk, a decline in aspen reproduction and composition of Idaho fescue, basin wildrye, elk sedge and palatable forbs occurs. Advanced decline in stand health is most commonly seen as a complete cessation of aspen recruitment resulting in an even-aged stand of damaged and disease affected decadent aspen. The understory is severely affected with the replacement of Idaho fescue, basin wildrye and elk sedge by Kentucky bluegrass (Poa pratensis), blue wildrye, mountain brome and low palatable forbs such as false hellebore (Veratrum viride), lupines (Lupinus spp.) and asters (Aster spp.). Snowberry, currants and other palatable shrubs decrease. With continued heavy use bare ground increases, erosion accelerates and site productivity decreases. 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.
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 helping to reduce conifer invasion, remove diseased and decadent mature trees and stimulate 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. 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. Mountain snowberry and wax currant are prevalent in the understory. Idaho fescue, basin wildrye, blue wildrye, rose and a variety of forbs are common. Mountain big sagebrush, mountain brome, bluebunch wheatgrass, elk sedge and pinegrass are present. 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 6. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Tree |
897 |
1345 |
1793 |
| Shrub/Vine |
673 |
1009 |
1345 |
| Grass/Grasslike |
560 |
841 |
1121 |
| Forb |
112 |
168 |
224 |
| Total |
2242 |
3363 |
4483 |
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. 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), 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 community.
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 Dominated 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.
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.
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.
