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interrupts the flow of auxin from photosynthesizing leaves to a tree's roots. This can <br />result from disturbances that kill the parent trees outright, such as a fire, disease, and <br />timber harvest, or from disturbances that only temporarily defoliate the parent tree, such <br />as a late frost, defoliating insect attack, or light herbicide application. Severing lateral <br />roots from parent trees can also initiate suckering, as would occur when fire, burrowing <br />animals, or other factors kill portions of a lateral root. The sucker initiating process has <br />been referred to as interruption of apical dominance (Schier et al. 1985). <br />In any case, the initiation of bud growth must also be accompanied by sufficient <br />sunlight and warmer soil temperatures to allow the new suckers to thrive (Navratil 1991, <br />Doucet 1989). Full sunlight to the forest floor best meets these requirements. However, <br />young aspen suckers are susceptible to competition from other understory plants and <br />herbivory from browsing ungulates, even if abundant suckers are present. <br />Having access to a well developed parental root system gives aspen sprouts a <br />great advantage over other plants. The parent roots supply carbohydrates and access <br />water deep in the soil profile allowing sprouts to grow rapidly, out-compete other <br />vegetation, and withstand frequent droughty conditions in the West. <br />Re-establishing aspen on reclaimed surface-mined lands is therefore <br />problematic, since the parent root systems are destroyed when topsoil is removed. <br />Planting aspen in anon-irrigated location in a Colorado study was not successful <br />(Shepperd and Mata 2005). Transplanting greenhouse or nursery-grown aspen <br />seedlings into the field has similar problems to those of natural seedlings, indicating that <br />the small root mass of transplanted seedlings is insufficient to absorb enough moisture <br />to maintain the seedlings during periods of summer drought in the wild. <br />In contrast, transplanting sapling-sized aspen in irrigated urban landscapes has <br />not been a problem, because the abundant supplies of water in lawns and landscape <br />beds enable the transplants to thrive. Although aspen is somewhat tolerant of drought <br />conditions (Lieffers et al. 2001), irrigation could benefit growth and survival of planted <br />aspen stock, because moisture stress negatively affects aspen response to nutrient <br />uptake (van den Driessche et al. 2003). Water deficit stress also reduces stomata) <br />conductance, root hydraulic conductivity, and shoot leaf water potential in aspen <br />4 <br />