Aspen Woodlands – State Wildlife Action Plan

Example of the Aspen Woodlands Strategy Habitat, within Fremont-Winema National Forest, Oregon — The fall color of Aspen Woodlands near Rocky Point, within the Fremont-Winema National Forest. Photo Credit: USFS

Aspen (Populus tremuloides) woodlands are woodland and/or forest communities dominated by aspen trees with a forb, grass, and/or shrub understory. Aspen woodlands also occur within conifer forests.

Ecoregions

Aspen woodlands are a Strategy Habitat in the Northern Basin and Range, Blue Mountains, and East Cascades ecoregions.

Characteristics

In open sagebrush habitat, aspen forms woodland or forest communities, dominated by aspen trees with a forb, grass, or shrub understory. In forested mountain habitats, aspen can occur within conifer forests. Aspen primarily occur in riparian areas or in moist microsites within drier landscapes. Characteristic understory grasses include Idaho fescue, pinegrass, Great Basin wildrye, or blue wildrye, and shrubs include sagebrush, snowberry, serviceberry, and roses. Aspen habitats evolved in areas that historically experienced fire. Given sufficient moisture and light, aspen will sprout annually, and they will sprout more vigorously after fire. Without fire disturbance, aspen stands decrease in size (total acres covered) and may be lost to competition from conifer trees. Aspen do not occur in the hottest, driest portions of the Northern Basin and Range ecoregion.

Conservation Overview

Aspen woodlands are on the edge of their range in Oregon and are more common further east in the Rocky Mountains and north into Canada. As one of the few deciduous trees found in eastern Oregon, aspen woodlands are especially important in the Northern Basin and Range and Blue Mountains ecoregions. In a landscape dominated by shrubs and grasses, aspen provide significant vertical structure that is useful as nesting and roosting sites for birds and bats and as cover for wildlife. Aspen stands contribute to climate resilience by shading streams, sustaining water tables, and sequestering carbon in both woody biomass and soils. Aspen stands also generally have high invertebrate prey diversity and densities. Further, aspen woodlands provide fawning and calving habitat, security cover, and forage for mule deer and Rocky Mountain elk. Other wildlife that use aspen include black bear, porcupine, beaver, rabbit, and grouse. Tree Swallows, woodpeckers, and other birds nest in aspen cavities.

Throughout the west, there is concern over the loss of aspen habitats and the lack of aspen regeneration and recruitment in remnant stands. The Northern Basin and Range ecoregion has lost a large percentage of its aspen woodlands since the 1800s. Aspen stands often depend on natural fire and disturbance to reduce competition from conifers and stimulate the growth of sprouts from roots. Chronic overgrazing can prevent overstory recruitment, allow invasive plant species to establish, and degrade understory plant communities. Overgrazing can also cause erosion, ultimately lowering the water table, which negatively impacts aspen habitats. Many existing aspen trees are reaching the end of their natural life cycle, and without recruitment of young aspen, many stands will be lost completely.

Aspen typically do not occur in the hottest, driest portions of the Northern Basin and Range ecoregion. As the climate changes, warming temperatures and alterations to hydrologic regimes may impact aspen life cycles and the distribution of this Key Habitat.

Spotlight - Beaver Habitat and Beaver-Modified Habitat

Beavers are widely distributed across Key Habitats statewide, including Flowing Water & Riparian, Wetland, and Aspen Woodlands. Beaver habitat, or habitat for beaver, is the specific combination of water, food, cover, and space that beaver need to support their survival on the landscape through time. Beaver are semi-aquatic species that require still or slow-moving, perennial water at stable depths for cover, protection from predators, access to food resources, and food storage in the winter. Beavers are slow on land and prefer to forage within 100 feet of their water source. They need sufficient early seral stage stream buffers of deciduous and herbaceous riparian vegetation for food and foraging activities. Beavers are highly territorial and require adequate lateral and longitudinal habitat quality and stability to support their occupancy on the landscape. In rivers and stream networks, one beaver family unit (on average two adults, two sub-adults, and two kits) needs approximately 0.5 to 1.5 linear stream miles for ample space to survive, reproduce, and thrive. Beaver habitat, habitat for beaver, supports the building blocks that beaver need to create beaver-modified habitats, or habitat by beaver.

Beaver-modified habitat, or habitat by beaver, are the specific conditions beaver create when they alter their terrestrial and aquatic habitat to improve their fitness and survival. Habitat modifications include denning, damming and ponding water, creating canals or side-channels, importing woody and vegetative materials into flowing water and wetlands, and changing the structure of riparian vegetative communities. This suite of habitat modifications and their cumulative effects can provide benefits such as increased complexity and connectivity of Key Habitats and habitat, structure, and refugia for SGCN. Nevertheless, beaver activity can also result in flooding, loss of vegetation, economic loss on working lands, and conflict with private landowners. Actions focused on beaver habitat and beaver-modified habitats should also include efforts to mitigate negative impacts and reduce potential conflicts.

Habitat limitations for beaver, such as declining surface water availability, altered floodplain disturbance regimes, conversion and loss of wet meadow and wetland habitats, and altered riparian vegetation communities, are also primary limiting factors for many SGCN.

Limiting Factors and Recommended Approaches

Limiting Factor: Altered Fire Regimes

Aspen stands often depend on natural fire and disturbance to reduce competition from conifers and stimulate the growth of sprouts from roots. Fire suppression has resulted in conifer encroachment and lack of reproduction in aspen communities.

Recommended Approach

Carefully reintroduce natural fire regimes using site-appropriate prescriptions, accounting for the area size and vegetation characteristics that affect resiliency and resistance to disturbance. Prescribed fire has been successful with regenerating aspen groves by increasing sprouting. Use mechanical treatment methods (e.g., masticating, cutting for firewood) to control encroaching conifers. Apply treatments appropriately with respect to season, size, and location. Pursue landscape level treatments, working to restore connectivity of aspen communities. The inclusion of mechanical ground disturbance to stimulate the growth of sprouts from root structures may be one approach to offsetting the lack of fire, but the results of this type of treatment are less predictable.

Limiting Factor: Overgrazing

Overgrazing has limited aspen recruitment through direct consumption or trampling of sprouts and indirect effects such as limiting water availability. When conditions are over-grazed, aspen may sprout but not fully grow into trees. Heavy cattle and ungulate pressure can also impact the soil, herbaceous layer, and recruitment. The direct consumption of aspen and terminal buds tends to be the greatest when sites are used by multiple species such as cattle, sheep, deer, and elk.

Recommended Approach

Limit over-grazing. Use fencing and exclosures to encourage reproduction at high priority sites until trees exceed browse height. Grove protection may be necessary for up to 10 years if elk are also present. Implement grazing plans to maintain aspen health, such as limiting grazing during spring and summer.

Limiting Factor: Invasive Species

Invasive plants, introduction of non-native pasture grasses, and historical overgrazing have altered the understory of many aspen stands. Invasive plants may also limit aspen suckering by crowding out and overtopping young sprouts. Junipers have reduced soil moisture in many rangeland aspen groves, increasing the presence of more drought-tolerant upland plants. Prolonged intensive grazing by livestock can lead to increased noxious weeds if grasses and sedges are overutilized. Stewardship actions intended to help aspen (e.g., conifer removal, fire) often stimulate noxious weeds, which thrive in disturbed and open areas.

Recommended Approach

Emphasize prevention, risk assessment, early detection, and quick control to prevent new invasive species from becoming fully established. Control invasive plants using site-appropriate herbicides and methods. Reintroduce native bunchgrasses and flowering plants at priority restoration sites. Minimize soil disturbance in high priority areas to prevent the establishment of invasive plants.

Limiting Factor: Drought

Persistent drought is already occurring in some regions, and climate change is increasing the frequency and severity of extreme weather events, including heatwaves and droughts. Climate models predict that more frequent, longer, and more severe regional drought conditions will increase as summer precipitation continues to decrease, exacerbating wildfire risk and reducing water availability. Drought has been shown to increase aspen mortality and reduce recruitment, which could lead to long-term declines in aspen habitat. The interactive effects of increased drought, chronic grazing, and fire suppression are an increasing concern in Oregon.

Recommended Approach

Implement monitoring to detect changes in regeneration, growth, and mortality in drought prone areas. Manage grazing to reduce effects on the water table. Reduce or eliminate encroaching vegetation that competes with aspen for water. Actions that reduce the impacts of other stressors will also improve aspen resilience to drought.

Resources for more information

Land Manager’s Guide to Aspen Management in Oregon

US Forest service Guide on Managing Aspen

Guide to Quaking Aspen Ecology and Management (2017)