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provinces and the U.S. states of Colorado and Utah (Jones 1985a; Lieffers and others 2001). In much of the western U.S., aspen is amid-elevation shade-intolerant species which is a relatively minor component of more widespread conifer forests. Aspen is an important tree species throughout the western United States. One of the few broad-leaved hardwood trees in many western forests, it is a valuable ecological component of many landscapes, occurring in pure forests as well as growing in association with many conifer and other hardwood species. Aspen provide desirable scenic value, the diversity of plants gro~roing under aspen supply critical wildlife habitat, valuable grazing resources, protect soils from erosion, and help maintain water quality. These features make aspen a crucial component of many Western landscapes. At the continental scale, aspen has seven~l physiological characteristics that permit it to attain great geographic amplitude. Lieffers and others (2001) outline the following important adaptive traits of aspen: 1) among the wide ranging genus Populus spp. (cottonwoods, poplars, aspen) aspen seems to have a very high stress tolerance. Usually high stress tolerance is associated with slow growing species and those with a limited reproduction strategy; 2) aspen appear to rely on vegetative reproduction via root suckering more than other F'opu/us species. These authors assert that the passing of extensive root systems between generations enhances tolerance to absorb climate stress (DesRochers and Lieffers 2001); 3) Aspen also has the ability to adapt leaf size to xeric and mesic conditions (that is, smaller leaves for drier sites). Aspen's smaller leaf size could keep the leaf surface slightly cooler allowing earlier shut down of stomata, thus tempering water stress during drought; 4) aspen seem to tolerate cold temperature and short growing seasons better than most hardwoods (Pearson and Lawrence 1958); 5) IeaFfluttering may be an adaptive advantage in cooling leaf surfaces of many Populus species and, 6) aspen appear to have a higher photosynthesis capability than other Populus spp. which is comparable to that of high yield poplar hybrids. Aspen photosynthesizes well in low light (for example, competitive situations) and even matures bark is capable of photosynthesis, which helps to ameliorate respiration during periods of high insolation (before spring leaf-out) (Pearson and Lawrence 1958}. Photosynthesizing bark may help aspen recover from injuries and infestations (Jones and Schier 1985; Lieffers and others 2001) and may allow aspen to photosynthesize a4 low levels during the winter giving the tree a photosynthetic "boost" prior to leaf-out (;Pearson and Lawrence 1958; Shepperd and others 2004). As leaf chlorophyll increases during the summer, bark chlorophyll decreases causing bark to become whiter (Strain 1964). Although aspen does produce abundant crops of viable seed (McDonough 1979), it primarily reproduces vegetatively by root suckering throughout most of its 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