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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 sunlight <br />and warmer soil temperatures to allow the new suckers to thrive (Navratil 1991, Doucet <br />1989). Full sunlight to the forest floor best meets these requirements. However, young <br />aspen suckers are susceptible to competition from other understory plants and herbivory <br />from browsing ungulates, even if abundant suckers are present. <br />Having access to a well developed parental root system gives aspen sprouts a great <br />advantage over other plants. The parent roots supply carbohydrates and access water deep <br />in the soil profile allowing sprouts to grow rapidly, out - compete other vegetation, and <br />withstand frequent droughty conditions in the West. <br />Re- establishing aspen on surface -mined lands is therefore problematic, since the parent <br />root systems are destroyed when topsoil is removed. Planting aspen in a non - irrigated <br />location in a Colorado study was not successful (Shepperd and Mata 2005). <br />Transplanting greenhouse or nursery-grown aspen seedlings into the field has similar <br />problems to those of natural seedlings, indicating that the small root mass of transplanted <br />seedlings is insufficient to absorb enough moisture to maintain the seedlings during <br />periods of summer drought in the wild. <br />In contrast, transplanting sapling -sized aspen in irrigated urban landscapes has not been a <br />problem, because the abundant supplies of water in lawns and landscape beds enable the <br />transplants to thrive. Although aspen is somewhat tolerant of drought conditions (Lieffers <br />et al. 2001), irrigation could benefit growth and survival of planted aspen stock, because <br />moisture stress negatively affects aspen response to nutrient uptake (van den Driessche et <br />al. 2003). Water deficit stress also reduces stomatal conductance, root hydraulic <br />conductivity, and shoot leaf water potential in aspen (Siemens and Zwiazek 2003). <br />Irrigation has been shown to increase growth of hybrid poplar, a closely related species <br />(Hansen 1988; Strong and Hansen 1991). <br />Therefore, it seems reasonable to conclude that supplemental irrigation of aspen planted <br />on reclaimed surface -mined lands could increase initial survival and allow trees to grow <br />sufficient root systems to ultimately survive without additional water and establish new <br />self - regenerating clones on mined lands. Testing this hypothesis, gaining additional <br />knowledge about different planting methods, and documenting factors that potentially <br />limit the re- establishment of aspen is crucial to re- establishing aspen on surface -mined <br />lands in the and west. This research was a collaborative three -year effort, with 2005 and <br />2006 funding from Seneca Coal Company; and 2007 funding from Seneca Coal <br />