Tools and Links
Forest Type Descriptions
- Land Use History of the Colorado Plateau
- World Wildlife Fund
- US Forest Service
- Species Lists – US Forest Service (Lists are within publication. See forest type code 2112 for aspen)
Ecological Forestry for Southwestern Aspen ^
Although aspen has the widest distribution of native Northern American trees (Perala 1990), in the southwest it is in decline. From 1962 to 1986 the area cover by aspen in the southwest declined by 62% (Johnson 1994). Although some researchers argue that at least in western Colorado, change in aspen cover may have increase rather than decreased (Kulakowski et al. 2004). Climate change, specifically increased temperatures and decrease precipitation, has permitted aspens to invade tree line areas previous dominated by conifers (Elliott and Baker 2004).
Aspen provides an important addition to landscape diversity in the conifer dominated mountains of the southwest. Many wildlife species use aspen forests including charismatic species such as turkey, beaver, and elk. Even the endangered willow flycatcher (Empidonax traillii) uses aspen trees. It is a shade intolerant species with a relatively short life span, generally <200 years in the southwest (Kaye et al. 2005).
Cases of aspen regeneration from seed have been documented in the western US, but asexual reproduction through root suckers is more common (Romme et al. 1997). Without disturbance, aspen can be replaced by conifers because shade tolerant conifers can become established beneath aspen canopies, capture available growing space, and prohibit aspen regeneration (Perala 1990, Kaye et al. 2005). Since aspen clones can transfer resources from mature trees to root suckers, some suckers may survive in shaded conditions. However, such subsidized regeneration is insufficient to regenerate entire stands (Perala 1990, Kaye et al. 2005). Conifers may establish at the same time as aspen in the stand, but grow more slowly initially, or seed into the stand as it develops (Kaye et al. 2005). Herbaceous and shrubby communities can also dominate sites as aspen stand decline (Mueggler 1989). Multi-aged aspen stand are possible where dieing overstory trees are replaced by root sprouts (Mueggler 1989).
While aspen is used both for pulp and lumber products, current silvicultural focus is on ensuring aspen regeneration for landscape diversity and wildlife benefits. Of particular interest to managers are aspen stands that appear to be deteriorating, with large numbers of dead trees, low densities of live trees, and few sprouts (Mueggler 1989). A decision tree is available to help managers decide when it may be appropriate to intervene to maintain aspen stands (Mueggler 1989).
If the goal is to regenerate an aspen stand, a useful heuristic is a triangle of three necessary conditions: hormonal stimulation, warm and light growing conditions, and protection from browse (Shepperd et al. 2006). Tree hormones, specifically auxin, control the growth of root sprouts (Schier et al. 1985). Mortality or even defoliation of the parent trees can reduce the flow of auxin and initiate root suckering. Once root suckers come up they require the right conditions for growth: warmth and light (Perala 1990). Aspen is a fire adapted and fire can provide the black soil and open canopy conditions that encourage sprout growth (Bartos et al. 1994). 5,000 to 210,000 suckers per acre (2,100 to 49,300 per ha) have been measured after prescribed fire (Bartos et al. 1994). Prescribed crown fire has even been suggested to regenerate aspen and conifer stand (Shepperd et al. 2006).
In stands where fuel loads or other concerns prohibit prescribed fire, removal of completing vegetation may be sufficient to permit aspen regeneration (Kilpatrick et al. 2003, Jones et al. 2005). An experiment in the Sierra Nevada Mountains showed that removal of completing conifers up to 26 in DBH (66cm) resulted in significant increase in sprout growth (Jones et al. 2005).
Even with the stimulation of fire or removal of completing vegetation, elk (Cervus elaphus) and other animal browse can overwhelm aspen regeneration (Hessl and Graumlich 2002). Elk browse can result in failure of aspen to regenerate at levels as low as 13 elk per square mile (5/km2) (Suzuki et al. 1999, White et al. 2003). High elk densities translate to >200 pellet groups/ acre (>5 pellet groups/ 100 m2) and low densities result in <40 pellet groups/ acre (White and Feller 2001). Not only does elk browse limit aspen sprout growth, but elk damage to mature stems can reduce tree health by inducing disease.
Fencing is a standard method for protection of aspen regeneration, particularly is the clone has declined in health (Shepperd et al. 2001, Shepperd et al. 2006). To prevent ungulate browse fences must be at least 7 to 8 ft (2.1 to 2.4 m) high (Kota 2005). Slash may be sufficient to reduce domestic cattle browse and allow regeneration, but elk may still be able retard growth (Rumble et al. 1996). An alternative is to use partially felled overstory trees to create a natural fence (Kota 2005). Trees are cut at 3 to 4 ft above the ground (0.9 to 1.2m) and remain partially connected to the stump. Because tree boles are supported by high stumps they provide a much greater impediment to ungulates than complete felling. A comparison of hinged tree fences to wildlife exclosures showed that although elk and deer are able to get over the hinged tree fences, however fences did reduce browse of aspen sprouts (Kota 2005). Predation and hunting also effect elk foraging behavior and hence aspen regeneration (White and Feller 2001). Success of aspen regeneration may be determined, in part, by genetics (Hessl 2002).
Bartos, D. L., J. K. Brown, and G. D. Booth. 1994. Twelve Years Biomass Response in Aspen Communities Following Fire. Journal of Range Management 47(1):79-83.
Elliott, G. P., and W. L. Baker. 2004. Quaking Aspen (Populus Tremuloides Michx.) at Treeline: A Century of Change in the San Juan Mountains, Colorado, USA. Journal of Biogeography. 31(5):733-745.
Hessl, A. E. 2002. Aspen, Elk, and Fire: Direct and Indirect Effects of Human Institutions on Ecosystem Processes. Bioscience 52:1011-1022.
Hessl, A. E., and L. J. Graumlich. 2002. Interactive Effects of Human Activities, Herbivory and Fire on Quaking Aspen (Populus Tremuloides) Age Structures in Western Wyoming. Journal of Biogeography 29:889-902.
Johnson, M. 1994. Changes in Southwestern Forests: Stewardship Implications. Journal of Forestry 92(12):16-19.
Jones, B. E., T. H. Rickman, A. Vazquez, Y. Sado, and K. W. Tate. 2005. Removal of Encroaching Conifers to Regenerate Degraded Aspen Stands in the Sierra Nevada. Restoration Ecology 13(2):373–379.
Kaye, M. W., D. Binkley, and T. J. Stohlgren. 2005. Effects of Conifers and Elk Browsing on Quaking Aspen Forests in the Central Rocky Mountains, USA. Ecological Applications 15(4).
Kilpatrick, S., D. Claus, and D. Scott. 2003. Aspen Response to Prescribed Fire, Mechanical Treatments, and Ungulate Herbivory. Pages 93-102 in P. N. O. a. L. A. Joyce, editor. Fire, fuel treatments, and ecological restoration. RMRS-P-29. USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO.
Kota, A. 2005. Fences and on-Site Forest Materials as Ungulate Barriers to Promote Aspen Persistence in the Black Hills. MS. Utah State University, Logan, UT.
Kulakowski, D., T. T. Veblen, and S. Drinkwater. 2004. The Persistence of Quaking Aspen (Populus Tremuloides) in the Grand Mesa Area, Colorado. Ecological Applications 14(5):1603–1614.
Mueggler, W. 1989. Age Distribution and Reproduction of Intermountain Aspen Stands. Western Journal of Applied Forestry 4(2):41-45.
Perala, D. A. 1990. Quaking Aspen. in R. M. Burns and B. H. Honkala, editors. Silvics of North America. Agriculture Handbook 654, USDA Forest Service, Washington, DC.
13 Romme, W. H., M. G. Turner, R. H. Gardner, Hargrove, W.W.;, G. A. Tuskan, D. G. Despain, and R. A. Renkin. 1997. A Rare Episode of Sexual Reproduction in Aspen (Populus Tremuloides Michx.) Following the 1988 Yellowstone Fires. Natural Areas Journal 17(1):17-25.
Rumble, M. A., T. Pella, J. C. Sharps, A. V. Carter, and J. B. Parrish. 1996. Effects of Logging Slash on Aspen Regeneration in Grazed Clearcuts. The Prairie Naturalist 28(4):199-208.
Schier, G. A., J. R. Jones, and R. P. Winokur. 1985. Vegetative Regeneration. Pages 29-33 in N. V. DeByle and R. P. Winokur, editors. Aspen: Ecology and Management in the Western United States. RM-GTR-119. USDA Forest Service, Fort Collins, CO.
Shepperd, W. D., D. L. Bartos, and S. A. Mata. 2001. Above- and Below-Ground Effects of Aspen Clonal Regeneration and Succession to Conifers. Canadian Journal of Forest Research 31(5):739-745.
Shepperd, W. D., P. C. Rogers, D. Burton, and D. L. Bartos. 2006. Ecology, Biodiversity, Management, and Restoration of Aspen in the Sierra Nevada. RMRS-GTR-178, USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO.
Suzuki, K., H. Suzuki, D. Binkley, and T. J. Stohlgren. 1999. Aspen Regeneration in the Colorado Front Range: Differences at Local and Landscape Scales
Journal. Landscape Ecology 14(3):231-237.
White, C. A., and M. C. Feller. 2001. Predation Risk and Elk-Aspen Foraging Patterns. in W. D. Shepperd, D. Binkley, D. L. Bartos, T. J. Stohlgren, and L. G. Eskew, editors. Sustaining aspen in western landscapes. RMRS-P-18. USDA Forest Service, Rocky Mountain Research Station, Grand Junction, CO.
White, C. A., M. C. Feller, and S. Bayley. 2003. Predation Risk and the Functional Response of Elk-Aspen Herbivory. Forest Ecology and Management 181(1):77-79.