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western range. Occasional seedlings do establish, but seedlings require bare mineral <br />soil and constant moisture to survive (McDonough 1979). These conditions rarely occur <br />in many of the areas where aspen grows today. Aspen typically grows in genetically- <br />identical groups referred to as clones. All stems in a clone sprouted from the roots of <br />parent trees and share a common ancestor. However they do not share a common root <br />system, as connections break down from generation to generation as new trees grow <br />new roots. <br />Most aspen stands are composed of one to several clones that may persist along <br />a continuum of successional stages, from sparsely growing individuals to apparently <br />stable pure or near-pure groves. Although clones are often separate and distinct from <br />one another, studies have demonstrated spatial intermingling where multiple clones are <br />co-located (DeByle 1964; Milton and Grant 1980; Wyman and others 2003; Hipkins and <br />Kitzmiller 2004). <br />Compared to conifers, aspen ramets -individual stems, or suckers, of the same <br />genotype from a parent root system -are relatively short lived. This is due to succession <br />(replacement of aspen by more shade tolerant species) and/or a typical onslaught of <br />mortality related to stem decays and diseases from ages 80 to 100 years (Baker 1925; <br />Hinds 1985; Potter 1998; Rogers 2002). Aspen thrive where somewhat regular and <br />frequent disturbance promotes regeneration (DeByle and Winokur 1985). Occasionally <br />aspen stands appear to perpetuate themselves with regular low-level regeneration in <br />multi-layer stable stands (Mueggler 1988; Cryer and Murray 1992). Aspen in the <br />western U.S. are longer lived than elsewhere. Healthy aspen trees can live over 300 <br />years (Personal Comm., John Shaw, Forester, USDA Forest Service, Rocky Mountain <br />Research Station) and can attain diameters up to 38 inches (96.5 cm) diameter at <br />breast height (dbh), however most aspen are typically much younger and smaller. Many <br />mature stands in Colorado are currently over 120 years of age (Shepperd 1990). Tree <br />form varies from shrubby at upper and lower forest margins to over 100 ft (30.5 m) in <br />height in prime locations with average heights of 50 to 60 ft (15 to 18 m) (Baker 1925). <br />Vegetative regeneration of aspen requires the interruption of the auxin/ cytokynin <br />hormone balance between roots and shoots to stimulate root buds to begin growing <br />(Schier et al. 1985). This hormonal imbalance can result from any disturbance that <br />3 <br />