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