2008-03-12_REPORT - C1982057 (2)

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• western range. Occasional seedlings do establish, but seedlings require bare mineral soil and constant moisture to survive (McDonough 1979). These conditions rarely occur in many of the areas where aspen grows today. Aspen typically grows in genetically-identical groups referred to as clones. All stems in a clone sprouted from the roots of parent trees and share a common ancestor. However they do not share a common root system, as connections break down from generation to generation as new trees grow new roots. Most aspen stands are composed of one to several clones that may persist along a continuum of successional stages, from sparsely growing individuals to apparently stable pure or near-pure groves. Although clones are often separate and distinct from one another, studies have demonstrated spatial intermingling where multiple clones are co-located (DeByle 1964; Mitton and Grant 1980; Wyman et al. 2003; Hipkins and Kitzmiller 2004). Compared to conifers, aspen ramets -individual stems, or suckers, of the same genotype from a parent root system -are relatively short lived. This is due to succession (replacement of aspen by more shade tolerant species) and/or a typical onslaught of mortality related to stem decays and diseases from ages 80 to 100 years (Baker 1925; Hinds 1985; Potter 1998; Rogers 2002). Aspen thrive where somewhat regular and frequent disturbance promotes regeneration (DeByle and Winokur 1985). Occasionally aspen stands appear to perpetuate themselves with regular low-level regeneration in multi-layer stable stands (Mueggler 1988; Cryer and Murray 1992). Aspen in the western U.S. are longer lived than elsewhere. • Healthy aspen trees can live over 300 years (Personal Comm., John Shaw, Forester, USDA Forest Service, Rocky Mountain Research Station) and attain diameters of at least 38 inches (96.5 cm) diameter at breast height (dbh), however most aspen are typically much younger and smaller. Many mature stands in Colorado are currently over 120 years of age (Shepperd 1990). Tree form varies from shrubby at upper and lower forest margins to over 100 ft (30.5 m) in height in prime locations with average heights of 50 to 60 ft (15 to 18 m) (Baker 1925). Vegetative regeneration of aspen requires the interruption of the auxin/ cytokynin hormone balance between roots and shoots to stimulate root buds to begin growing (Schier et al. 1985). This hormonal imbalance can result from any disturbance that interrupts the flow of auxin from photosynthesizing leaves to a tree's roots. This can result from disturbances that kill the parent trees outright, such as a fire, disease, and timber harvest, or from disturbances that only temporarily defoliate the parent tree, such as a late frost, defoliating insect attack, or light herbicide application. Severing lateral roots from parent trees can also initiate suckering, as would occur when fire, burrowing animals, or other factors kill portions of a lateral root. The sucker initiating process has been referred to as interruption of apical dominance (Schier et al. 1985.). In any case, the initiation of bud growth must also be accompanied by sufficient sunlight and warmer soil temperatures to allow the new suckers to thrive (Navratil 1991, Doucet 1989). Full sunlight to the forest floor best meets these requirements. However, young aspen suckers are susceptible to competition from other . understory plants and herbivory from browsing ungulates, even if abundant suckers are present.