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r~ <br />u <br />lz <br />These in turn hold more soil moisture, thus supporting better vegetative <br />cover, and bring about a soil aggradational phase coupled with conditions <br />favoring weathering of soil minerals. Potential climax vegetation would <br />probably consist of grasslands with intermixed open pine forests. <br />Today, however, grazing pressure by domestic animals coupled with attempts <br />at cropping and periodic drought such as that experienced in the 1930's <br />have rendered many high plains sites vulnerable to large scale erosion in <br />the future. I}rought comparable to the 1930's is virtually certain to <br />return. <br />Vegetative cover relies upon living soil for its support, for its nutrient <br />storage and cycling, and for biochemical manufacture of necessary substances. <br />Not only are conditions of gas exchange and moisture retention critical with- <br />in a soil column, but also vertical geochemical segregation limits assemblages <br />of plants, soil microorganisms and othe soil biota. This geochemical segregation <br />takes place only very slowly over long periods of geologic time. It is mani- <br />fest by what plant ecologists see ,as plant succession. Most such succession <br />is nothing more than biogeochemical development of a soil. This development <br />tends toward increased degrees of closure of nutrient and material c}•cling <br />loops, increased moisture retention, increased development of nutrient storage <br />sites within the soil, and increased biological and thermodynamic efficiency <br />of use of available energy flux <br />There is thus a tendency toward increased <br />plant cover and increased biomass. These tendencies are functions of bio- <br />