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light (for example, competitive situations) and even mature bark is capable of <br />photosynthesis, which helps to ameliorate respiration during periods of high insolation <br />(before spring leaf -out) (Pearson and Lawrence 1958). Photosynthesizing bark may help <br />aspen recover from injuries and <br />infestations (Jones and Schier 1985; Lieffers and others 2001) and may allow aspen to <br />photosynthesize at low levels during the winter giving the tree a photosynthetic "boost" <br />prior to leaf -out (Pearson and Lawrence 1958; Shepperd and others 2004). As leaf <br />chlorophyll increases during the summer, bark chlorophyll decreases causing bark to <br />become whiter (Strain 1964). <br />Although aspen does produce abundant crops of viable seed (McDonough 1979), it <br />primarily reproduces vegetatively by root suckering throughout most of its western range. <br />Occasional seedlings do establish, but seedlings require bare mineral soil and constant <br />moisture to survive (McDonough 1979). These conditions rarely occur in many of the <br />areas where aspen grows today. Aspen typically grows in genetically- identical groups <br />referred to as clones. All stems in a clone sprouted from the roots of parent trees and <br />share a common ancestor. However they do not share a common root system, as <br />connections break down from generation to generation as new trees grow new roots. <br />Most aspen stands are composed of one to several clones that may persist along a <br />continuum of successional stages, from sparsely growing individuals to apparently stable <br />pure or near -pure groves. Although clones are often separate and distinct from one <br />another, studies have demonstrated spatial intermingling where multiple clones are co- <br />located (DeByle 1964; Mitton and Grant 1980; Wyman et al. 2003; Hipkins and <br />Kitzmiller 2004). <br />Compared to conifers, aspen ramets — individual stems, or suckers, of the same genotype <br />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 <br />Winokur 1985). Occasionally aspen stands appear to perpetuate themselves with regular <br />low -level regeneration in multi -layer stable stands (Mueggler 1988; Cryer and Murray <br />1992). Aspen in the western U.S. are longer lived than elsewhere. Healthy aspen trees can <br />live over 300 years (Personal Comm., John Shaw, Forester, USDA Forest Service, Rocky <br />Mountain Research Station) and attain diameters of at least 38 inches (96.5 cm) diameter <br />at breast height (dbh), however most aspen are typically much younger and smaller. <br />Many mature stands in Colorado are currently over 120 years of age ( Shepperd 1990). <br />Tree form varies from shrubby at upper and lower forest margins to over 100 ft (30.5 m) <br />in height in prime locations with average heights of 50 to 60 ft (15 to 18 m) (Baker <br />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 />