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<br />data pertinent to that area are given by Weaver (1980). Each of the three <br />sites studied is a distinctive plant community but with comparable physical <br />relief and soil conditions among the three. One is dominated by Agropyron <br />smithii, Bouteloua gracilis, and Stipa comata.(therefore abbreviated A+B+S) on <br />a nearly flat bench 27 m above the Tongue River. The soil of this typical <br />range community ,is a fine-loamy and mixed soil, Chanta loam, and is <br />characterized as an Aridic Haploboroll developed on Tongue River alluvium over <br />an interbedded Eocene sandstone-coal shale formation. A thorough description <br />of this site appears in Haglund (1982). The second site is inhabited nearly <br />exclusively by Bouteloua gracilis on an alluvial terrace, about 4 m above the <br />Tongue River. The soil there is a well-drained, fine-loamy, frigid calcareous <br />Ustic Torriorthent, a Havre variant loam. The third community is a virtually <br />pure stand of Agropyron smithii on Yellowstone River alluvium which has <br />developed to a well-drained~ fine textured "Borollic -Camborthid in the Kobalr <br />silty.clay loam type. The latter two 'communities are described more completely <br />by Weaver et 'ale (1981). <br /> <br />Methods <br /> <br />Portions of each of the three sites were irrigated to experimentally <br />control soil water. The soil water treatments used"were applied by four <br />sprinklers to 15-m square randomly assigned plots: control (CTL), ambient <br />precipitation only; spring wet (SWT), ambient precipitation plus irrigation <br />additions of about 25mm weekly; frequently enough to keep soH moisture'stress <br />above -2 bars until around July 1 of"each growing season; and wet (WET); <br />ambient. precipitation plus irrigation additions of 25-50 mm weekly; -frequently <br />enough to keep soil moisture stress'aDove -2 bars throughout the growing <br />season. Each treatment was twice replicated. <br /> <br />In this 1979 experiment the CTL plots were essentially depleted of watler <br />(< -20 bars by mid-June at 10 and 25 cm depths) ~.These soil moisture tension <br />measurements were, in all cases, taken weekly from three or four sets of <br />plaster blocks per plot at 10, 25, and 75 cm depths (Bouyousous, 1961). Both <br />the Agropyron and Bouteloua communities were in the third yearof-consiste:nt <br />experimental treatment, whereas the A+B+S site had been irrigated first in <br />spring 1978 rather than spring 1977. An additional experimental factor was <br />composed on half of each A+B+S grassland's plots; mowing to 10 cm height, 1Nith <br />removal of clippings, occurred from June 22-24, 1979. Further details of <br />experimental design are stated in Weaver et al. (1980) for the-Agropyron and <br />Bouteloua sites and in Haglund (982) for the A+B+S grassland. <br /> <br />The dependent variable was suction debris weight taken while B-Vac suction <br />sampling for arthropods. A 3-horsepower motor coupled to a blower -generated a <br />suction over 0.1 m2 circular area. The material was collected in organdy nets <br />which let only a small portion of the particles through. Ten 30-second sa:mples <br />per treatment were taken by placing the apparatus on the ground surface between <br />0630-1000 hrs on the dates indicated in Tables 1 through 3. Sample intervals <br />were regular (2-5 m) although the particular interval was randomly chosen on <br />each sample date. <br /> <br />Material accumulated in the organdy sacks was transferred to tarred paper <br />sacks and then weighed. Mineral ash content was measured on composite samples <br /> <br />62 <br />