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Quaking aspen ( Populus tremuloides is the most widespread tree species in North <br />America (Baker 1925; Preston 1976; Lieffers and others 2001), and thought to be second <br />in worldwide range only to Eurasian aspen ( Populus tremula) (Jones 1985a). <br />Aspen is found in most of eastern Canada and the U.S. (except the Southeast), throughout <br />the upper Midwest and Lake States, across sub -boreal Canada and Alaska, in the Rocky <br />Mountains from Canada through the U.S. and into northern Mexico, and in mountain <br />ranges paralleling the west coast from Alaska through British Columbia, Washington, <br />Oregon, California, and Mexico's northern Baja California (Preston 1976). The species is <br />most abundant in Canada's central provinces and the U.S. states of Colorado and Utah <br />(Jones 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 of the few <br />broad- leaved hardwood trees in many western forests, it is a valuable ecological <br />component of many landscapes, occurring in pure forests as well as growing in <br />association with many conifer and other hardwood species. Aspen provide desirable <br />scenic value, the diversity of plants growing under aspen supply critical wildlife habitat, <br />valuable grazing resources, protect soils from erosion, and help maintain water quality. <br />These features make aspen a crucial component of many Western landscapes. At the <br />continental scale, aspen has several physiological characteristics that permit it to attain <br />great geographic amplitude. Lieffers and others (2001) outline the following important <br />adaptive traits of aspen: <br />1) Among the wide ranging genus Populus spp. (cottonwoods, poplars, aspen) aspen <br />seems to have a very high stress tolerance. Usually high stress tolerance is associated <br />with slow growing species and those with a limited reproduction strategy; <br />2) Aspen appear to rely on vegetative reproduction via root suckering more than other <br />Populus species. These authors assert that the passing of extensive root systems between <br />generations 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, <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 during <br />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 <br />species and, <br />6) Aspen appear to have a higher photosynthesis capability than other Populus spp. which <br />is comparable to that of high yield poplar hybrids. Aspen photosynthesizes well in low <br />