Aspen are medium-sized deciduous trees, often referred to as “quaking” aspen. The leaves of the aspen tree shake and shimmer in the wind and provide and accompanying sound easily identified once heard. Aspen trees usually do not live more than 150 years, though they may persist more than 200 years. It grows on many soil types, especially sandy and gravelly slopes, and is quick to pioneer disturbed sites where there is bare soil. It grows best where soils are moist and sunshine is plentiful. Aspen is intolerant of shade, and does not compete well with more shade-tolerant conifer species.
Quaking aspen are scattered across the Sierra Nevada, usually in stands of fewer than five acres and usually adjacent to streams, springs, lake shores, and meadows. Aspen is found within a wide range of elevation in the Sierra, from the lower elevations of western juniper on the east side to higher zones of fir and lodgepole pine, generally along creeks or meadows.
Aspen groves are often out-competed by conifers in the Sierra Nevada, due to extensive livestock grazing and the absence of regular fire. As a result, the health of aspen has deteriorated and estimates suggest its extent in western North America has been reduced by as much as 96%. Aspen habitat, especially when associated with riparian vegetation, is the single most species-rich avian habitat in the Sierra Nevada.
Aspen is an especially desirable component of riparian areas where it contributes to the stability of streams, provides shade, nurtures a diverse and abundant understory community, and contributes a pleasing aesthetic component. Like other riparian communities, aspen communities comprise only a small portion of the landscape but provide habitat for many species. The multilayered herbaceous vegetation and shrubs that thrive beneath aspen canopy provide nesting, denning, and foraging habitat for insects, birds, amphibians, and mammals. The fruits produced by this diverse plant life and the insects that are abundant in the moist aspen environment provide food for a wide variety of birds. Northern goshawks, owls, and other raptors rest in the upper canopy and hunt adjacent habitats and cavity-nesting songbirds make use of all layers of the canopy and brush of aspen stands.
Livestock Grazing Impacts
Livestock grazing has a direct impact on aspen trees in this forest community. Through the early sapling stage, grazing reduces aspen growth, vigor, and numbers. Historic grazing consumed vegetation around aspen stands, reducing fuel available for fire. Also, under conditions of moderate-to-heavy livestock grazing, both livestock and wildlife graze more heavily on vegetation in aspen stands, including any emerging aspen shoots. The aspen forest type produces an abundance of forage, as much as many grasslands and more than 10 times that produced under associated conifers. Cattle and sheep grazing the aspen understory has been the primary consumptive use of the aspen forest in the Sierra Nevada.
Aspen is considered to be a fire-induced successional species that will dominate a site until it is replaced by less fire-enduring and more shade-tolerant species, such as conifers. Fire reduces the overstory, stimulates shoots to sprout, and kills invading conifers growing in the aspen grove. Fire also reduces conifer encroachment, opens up the tree canopy, removes shrub cover, and stimulates sucker release. In some areas, many aspen stands are the same age, dating from a single great fire or a year of widespread fires. Fire appears to be necessary for the continued well-being of aspen on most sites. In the absence of fire, many aspen stands are replaced by grass, forbs, shrubs, or conifers. Less-frequent fire over the past century has limited the regeneration of aspen groves.
Aspen is an aggressive pioneer species. It readily colonizes burned areas and can persist even when subjected to frequent fires. In the Sierra Nevada Mountains, the extensive stands of aspen are usually attributed to repeated wildfires. It may dominate a site until replaced by less fire-enduring but more shade-tolerant conifers. The soil water tapped by conifers has contributed to the drying of meadows, reducing water available for aspen. Pine and fir trees eventually tower over the aspen stands, shading them from sunlight.
Krasnow, K.D., and S.L. Stephens. 2015. Evolving paradigms of aspen ecology and management: impacts of stand condition and fire severity on vegetation dynamics. Ecosphere 6(1):12. http://dx.doi.org/10.1890/ES14-00354.1. (4.8 MB PDF)
Otway, S. G., et.al. 2006. Predicting Ground Fire Ignition Potential in Aspen Communities, In: Andrews, P.L., et.al. 2006. Fuels Management-How to Measure Success: Conference Proceedings. Proceedings RMRS-P-41. Fort Collins, CO: U.S. Forest Service, Rocky Mountain Research Station. p. 537-546. (320KB PDF)
Shaw, J. D. 2004. Analysis of Aspen Stand Structure and Composition in the Western U.S.: Implications for Management. In: Proceedings: Canadian Institute of Forestry and Society of American Foresters Joint 2004 Annual General Meeting and Convention, Society of American Foresters. (1.04KB PDF)
Shepperd, W. D.; and S.A. Mata. 2005. Planting Aspen to Rehabilitate Riparian Areas: A Pilot Study. Research Paper, RMRS-RN-26. Fort Collins, CO: U.S. Forest Service, Rocky Mountain Research Station. 5 p. (328KB PDF)
Shepperd, W.D., et.al. 2006. Ecology, Biodiversity, Management, and Restoration of Aspen in the Sierra Nevada. Gen. Tech. Rep. RMRS-GTR-178. Fort Collins, CO, U.S. Forest Service, Rocky Mountain Research Station 122 p. (4.79MB PDF)