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3 <br />precipitation in the high northern great plains of the Powder River Basir. <br />(see index map Fig. 1) and adjacent surface mine coal regions of Montana is <br />about 13 inches per annum but about 30 percent is in the form of snow and <br />most years receive less than mean precipitation. Since the region is one <br />of low humidity and high winds, 60 to 80 percent of the snow sublimates di- <br />rectly while 20 percent or more of the potentially effective groiaing season <br />precipitation is intercepted and evaporated on ground surface or in the <br />plants, resulting in critically reduced and highly variable effective <br />precipitation. <br />The Northern Plains areas of hbntana and Wyoming are characterized by a <br />short 4-month growing season from late A9ay to late September and sparce <br />short-grass prairie and shrub vegetation. Almost universal overgrazing in <br />the past century has reduced potential vegetation below its natural level <br />and favored a disturbed high plains arid land mosaic of plant communities <br />visually dominated by shrubs such as sage and fourwing saltbrush. Grasses <br />dominate mostly on some subirrigated alluvial bottomlands with open pine- <br />juniper woodlands found in higher areas and bedrock 'breaks' where reduced <br />grassland competition and available dew r mni~t}y~e storage allow conifer <br />establishment. <br />----~ <br />Because of aridity and potential evaporation far in excess of precipitation <br />with well-drained soils and deep water tables, native plants have extensive, <br />largely shallow, widespreading root systems Wltll a majority of their total <br />