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8 <br />and/or water tables are raised as through manipulation of shallow aquifers <br />or reduction of vegetative cover which had previously maintained seasonal <br />consumptive use of s}iallow groundwaters. This problem is most readily <br />apparent when native vegetation is converted to wheat and the fields are <br />allowed to lie fallow during some seasons or parts of seasons when native <br />vegetation would have maintained some evapotranspirative demand. <br />Thus, where grazing or farming efforts have occurred to the point of re- <br />duction of native bunch-grass species and replacement by shallow-rooting <br />shrubs and annuals, salt-tolerant species are common and net decreased site <br />productivity is probable. The question one must ask is why salt-affected <br />soils are not universal in this region given that substrate units con= <br />taming salts are almost ubiquitous? The answer probably lies in the <br />geologic and climatic history of the region. Although precipitation is <br />today much less than evaporation, such has not always been the case. The <br />easily disequilibrated surface salt content today suggests strongly that <br />today's climatic conditions are not those of the times of formation of <br />much of the soil and its associated biotope. Either greater past aridity, <br />and thus lower water tables, ~~increased precipitatio___n=to _evaporation <br />ratio and thus net downward migration salts must have occurred in the area <br />of the Ft. I3nion formation as salts which can today move upward do not do <br />so until disturbed. <br />One factor affecting the plants of much of the Dbntana and Wyoming high <br />plains is thus saline soil conditions and these are associated largely <br />