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leaf-out (Pearson and Lawrence 1958; Shepperd and others 2004). As leaf chlorophyll increases <br />during the summer, bark chlorophyll decreases causing bark to become whiter (Strain 1964). <br />Although aspen does produce abundant crops of viable seed (McDonough 1979), it primarily <br />reproduces vegetatively by root suckering throughout most of its western range. Occasional <br />seedlings do establish, but seedlings require bare mineral soil and constant moisture to survive <br />(McDonough 1979). These conditions rarely occur in many of the areas where aspen grows <br />today. Aspen typically grows in genetically-identical groups referred to as clones. All stems in a <br />clone sprouted from the roots of parent trees and share a common ancestor. However they do not <br />share a common root system, as connections break down from generation to generation as new <br />trees grow new roots. <br />Most aspen stands are composed of one to several clones that may persist along a continuum of <br />successional stages, from sparsely growing individuals to apparently stable pure or near-pure <br />groves. Although clones are often separate and distinct from one another, studies have <br />demonstrated spatial intermingling where multiple clones are co-located (DeByle 1964; Mitton <br />and Grant 1980; Wyman et al. 2003; Hipkins and Kitzmiller 2004). <br />Compared to conifers, aspen ramets - individual stems, or suckers, of the same genotype from a <br />parent root system - are relatively short lived. This is due to succession (replacement of aspen by <br />more shade tolerant species) and/or a typical onslaught of mortality related to stem decays and <br />diseases from ages 80 to 100 years (Baker 1925; Hinds 1985; Potter 1998; Rogers 2002). Aspen <br />thrive where somewhat regular and frequent disturbance promotes regeneration (DeByle and <br />Winokur 1985). Occasionally aspen stands appear to perpetuate themselves with regular low- <br />level regeneration in multi-layer stable stands (Mueggler 1988; Cryer and Murray 1992). Aspen <br />in the western U.S. are longer lived than elsewhere. Healthy aspen trees can live over 300 years <br />(Personal Comm., John Shaw, Forester, USDA Forest Service, Rocky Mountain Research <br />Station) and attain diameters of at least 38 inches (96.5 cm) diameter at breast height (dbh), <br />however most aspen are typically much younger and smaller. Many mature stands in Colorado <br />are currently over 120 years of age (Shepperd 1990). Tree form varies from shrubby at upper and <br />lower forest margins to over 100 ft (30.5 m) in height in prime locations with average heights of <br />50 to 60 ft (15 to 18 m) (Baker 1925). <br />Vegetative regeneration of aspen requires the interruption of the auxin/ cytokynin hormone <br />balance between roots and shoots to stimulate root buds to begin growing (Schier et al. 1985). <br />This hormonal imbalance can result from any disturbance that interrupts the flow of auxin from <br />photosynthesizing leaves to a tree's roots. This can result from disturbances that kill the parent <br />trees outright, such as a fire, disease, and timber harvest, or from disturbances that only <br />temporarily defoliate the parent tree, such as a late frost, defoliating insect attack, or light <br />herbicide application. Severing lateral roots from parent trees can also initiate suckering, as <br />would occur when fire, burrowing animals, or other factors kill portions of a lateral root. The <br />sucker initiating process has been referred to as interruption of apical dominance (Schier et al. <br />1985.). <br />In any case, the initiation of bud growth must also be accompanied by sufficient sunlight and <br />warmer soil temperatures to allow the new suckers to thrive (Navratil 1991, Doucet 1989). Full