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• 1985a). Aspen is found in most of eastern Canada and the U.S. (except the <br />Southeast), throughout the upper Midwest and Lake States, across sub-boreal <br />Canada and Alaska, in the Rocky Mountains from Canada through the U.S. and <br />into northern Mexico, and in mountain ranges paralleling the west coast from <br />Alaska through British Columbia, Washington, Oregon, California, and Mexico's <br />northern Baja California (Preston 1976). The species is most abundant in <br />Canada's central provinces and the U.S. states of Colorado and Utah (Jones <br />1985a; Lieffers and others 2001). In much of the western U.S., aspen is a mid- <br />elevation shade-intolerant species which is a relatively minor component of more <br />widespread conifer forests. <br />Aspen is an important tree species throughout the western United States. One <br />of the few broad-leaved hardwood trees in many western forests, it is a valuable <br />ecological component of many landscapes, occurring in pure forests as well as <br />growing in association with many conifer and other hardwood species. Aspen <br />provide desirable scenic value, the diversity of plants growing under aspen supply <br />critical wildlife habitat, valuable grazing resources, protect soils from erosion, and <br />help maintain water quality. These features make aspen a crucial component of <br />many Western landscapes. <br />At the continental scale, aspen has several physiological characteristics that <br />permit it to attain great geographic amplitude. Lieffers and others (2001) outline the <br />following important adaptive traits of aspen: 1) among the wide ranging genus <br />Populus spp. (cottonwoods, poplars, aspen) aspen seems to have a very high <br />• stress tolerance. Usually high stress tolerance is associated with slow growing <br />species and those with a limited reproduction strategy; 2) aspen appear to rely on <br />vegetative reproduction via root suckering more than other Populus species. These <br />authors assert that the passing of extensive root systems between generations <br />enhances tolerance to absorb climate stress (DesRochers and Lieffers 2001); 3) <br />Aspen also has the ability to adapt leaf size to xeric and mesic conditions (that is, <br />smaller leaves for drier sites). Aspen's smaller leaf size could keep the leaf surface <br />slightly cooler allowing earlier shut down of stomata, thus tempering water stress <br />during drought; 4) aspen seem to tolerate cold temperature and short growing <br />seasons better than most hardwoods (Pearson and Lawrence 1958); 5) leaf <br />fluttering may be an adaptive advantage in cooling leaf surfaces of many Populus <br />species and, 6) aspen appear to have a higher photosynthesis capability than <br />other Populus spp. which is comparable to that of high yield poplar hybrids. Aspen <br />photosynthesizes well in low light (for example, competitive situations) and even <br />mature bark is capable of photosynthesis, which helps to ameliorate respiration <br />during periods of high insolation (before spring leaf-out) (Pearson and Lawrence <br />1958). Photosynthesizing bark may help aspen recover from injuries and <br />infestations (Jones and Schier 1985; Lieffers and others 2001) and may allow <br />aspen to photosynthesize at low levels during the winter giving the tree a <br />photosynthetic "boost" prior to leaf-out (Pearson and Lawrence 1958; Shepperd <br />and others 2004). As leaf chlorophyll increases during the summer, bark <br />chlorophyll decreases causing bark to become whiter (Strain 1964). <br />• Although aspen does produce abundant crops of viable seed (McDonough <br />1979), it primarily reproduces vegetatively by root suckering throughout most of its <br />