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Range Managem Ent September, 1960 Journal of Volume 13, Number 5 RANGE MANAGEM ENT site, an opening in the ponderosa Effect of Mima-Type Microrelief on pine forest, was dominated by big sagebrush (Artemisia triden- Herbage Production of Five Seeded tutu) before seeding. Soil parent material is sandstone. Grasses in Western Colorado1 The sagebrush was removed WILLIAM J. McGINNIES with a brushland plow in 1949. One-tenth-acre p 1 o t s w e r e Range Conservationist, Crops Research Division, Agri- seeded by the Rocky Mountain cultural Research Service, U. S. Department of Agri- Forest and Range Experiment . culture, Fort Collins, Colorado Station, U. S. Forest Service, in 1949, 1950, 1952, and 1953 to in- Mima-type microrelief h a s the mounds vary in height from termediate wheatgrass (Agro- been commonly observed in the less than a foot to several feet pyron intermedium), c r e s t e d treeless areas of the western and in diameter from 15 to 25 United States. This microrelief feet; the diameter is approxi- wheatgrass (A. desertorum), is characterized by low mounds mately equal to the territory of smooth brome (Bromus iner- or soil pimples, commonly called a pocket gopher. The mounds mis), Russian wildrye (E Zymus “Mima mounds” after the Mima may be closely or widely spaced. junceus), and big bluegrass (Pea Prairie in western Washington, The presence of Mima-type umplu). Some light grazing by where they were described by microrelief in Colorado has been cattle and horses had been per- Dalquest and Scheffer, (1942). reported by Scheffer, (1958). mitted in recent years, but the Some contend that the Mima Dalquest and Scheffer (1942) re- area was protected during the ported that in Washington her- mounds are a result of physical first few years following plant- actions of ice wedges and erosion bage production on top of Mima ing of the grasses. (Newcombe, 1952; Ritchie, 1953) mounds was greater than be- while others feel that the tween the mounds in spite of The mounds on the study area mounds are built largely by the pocket gopher disturbance on fit closely the description of and activity of fossorial rodents such the mounds. The writer observed criteria for Mima mounds given as pocket gophers or prairie dogs the same pattern of higher yields by Dalquest and Scheffer ( 1942) (Arkeley and Brown, 1954). on the mounds in a range seed- and Arkley and Brown (1954) Present evidence tends to favor ing study area on the Uncom- and are presumed to be the re- the formation of the mounds by pahgre Plateau, in western Colo- sult of past activity by pocket rado. rodents in Colorado and adjacent gophers. The characteristic mi- areas. The subsoil in mounded In 1959 a study was made to crorelief of the Mima mounds areas usually contains a cemen- determine the extent of the was not destroyed by the plow- ted hardpan, bedrock, gravel effect of Mima mounds on the ing, and they retain an appear- bed, or water table, which limits variability of herbage produc- root growth and the activity of tion of five seeded grasses. This ance similar to nearby Mima fossorial rodents. In Colorado, study is reported herein. mounds that were not plowed. The mounds are 12 to 24 inches Description of Area high, 15 to 25 feet in diameter, IContribution from the Crops Re- and 30 to 60 feet apart (mound search Division, Agricultural Re- The study area is on 25 Mesa, search Service, U. S. Department of a part of the Uncompahgre center to mound center). A sand- Agriculture in cooperation with the Plateau, 25 miles southwest of stone bedrock underlies the en- Rocky Mountain Forest and Range Delta, Colorado (Figure 1). tire area; the bedrock is 12 to 18 Experiment Station, Forest Service, Elevation is about 8000 feet, and inches below the intermound U. S. Department of Agriculture, and the Colorado AgricuZturaZ Experi- the average annual precipitation areas and 30 to 36 inches below ment Station. is estimated to be 17 inches. The the top of the mounds. 231 232 McGINNIES reasons for the consistently higher yields on tops of the mounds are not entirely clear, but the differences are apparent- ly a result of differences in stand quality, soil fertility, and soil volume above the bed rock. These are discussed separately below. Stand quality (as indicated by stand ratings) was better on top of the mounds, where the aver- age rating was 8.0 (out of a possi- ble 10) as compared with an average of 5.1 between the mounds. Stand ratings on the mounds were better than inter- FIGURE 1. General view of study area on Uncompahgre Plateau in western Colorado, mound ratings by 29 percent for showing typical Mima-type microrelief. Russian wildrye, 43 percent for intermediate wheatgrass, 45 per- Methods Results and Conclusions cent for crested wheatgrass, 67 Only those few mounds and Great differences in the abili- percent for big bluegrass, and adjacent intermound areas that ties of these five cool-season 183 percent for smooth brome were entirely within a seeded species to produce herbage on (Table 1). Whether the higher plot were sampled. Because the mound and intermound areas ratings on the Mima mounds are seeded plots were long and nar- were observed (Table 1). Air-dry a result of better initial es- row, very few mounds fit this herbage yields from the tops of tablishment of the seedings or a requirement. Of the 10 mounds the mounds were greater than result of greater drouth mortal- sampled, 3 had been planted to the yields from between the ity between the mounds could intermediate wheatgrass, 3 to mounds, by an average of 94 per- not be determined. However, the crested wheatgrass, 2 to smooth cent (intermediate wheatgrass), plants on the mounds were gen- brome, and 1 each to Russian 180 percent (crested wheatgrass), erally more robust despite their wildrye and big bluegrass. Her- 323 percent (smooth brome), 358 higher density, but variation be- bage samples were clipped from percent (Russian wildrye), and tween species was great. For ex- one 9.6-sq. ft. plot on top of each 542 percent (big bluegrass). The ample, while Russian wildrye mound and one 9.6-sq. ft. plot on Average herbage yields and stand ratings of five grasses on fop the adjacent intermound area in Table 1. of and between Mima mounds in western Colorado. 1959. late July, 1959. The plots were systematically picked as being Location No. of Air dry Stand Soil or- representative of the two sam- of mounds herbage rat- ganic pling sites. The herbage samples Species samples studied yield ings matter were air-dried and weighed to (lb/A) (percent) determine herbage y i e 1 d. A Mound 3 1030 10.0 4.9 numerical stand rating (0 = no stand, 10 = best possible stand) Intermediate wheatgrass i Intermound 3 530 7.0 2.6 was made on both the mound Mound 3 1100 7.7 4.5 and the intermound area just Intermound 3 393 5.3 2.5 prior to clipping. Crested wheatgrass 1 Soil samples of the surface 6 Mound 2 635 8.5 4.0 inches were taken from the plots Smooth brome i Intermound 2 150 3.0 3.0 after the herbage was clipped. Mound 1 550 9.0 5.1 These samples were analyzed by Intermound 1 120 7.0 3.4 Colorado State University for Russian wildrye 1 %-and 15-atmosphere mois- Mound 1 770 5.0 5.6 ture percentages, soil texture, Big bluegrass IIntermound 1 120 3.0 3.1 pH, soluble salts, organic matter, kound 817 8.0 4.8 phosphate (P205), and potassium (K20). Average of all species 1Intermound 263 5.1 2.9 MIMA-TYPE MICRORELIEF 233 had only a ‘29-percent better wilting point. The soil was more most no seed heads (Figure 2). stand on the mound, it had a 358- friable and less compact on top Even in favorable years, seed percent higher yield on the of the mounds because of the heads are rare in the intermound mound, but intermediate wheat- higher organic-matter content, areas. grass had a stand 43 percent and the better structure may Discussion better on the mound while the have had some influence on in- The type of mounds mentioned yield was only 94 percent great- creasing water infiltration and in this study are probably much er. Thus, differences in stand yields. However, because of the more common than is generally quality alone cannot account for underlying sandstone bedrock, realized, although in most cases the differences in yield. the volume of soil available to they are not so well developed the plants growing between the Soil pH averaged 6.4 and and obvious as the mounds re- mounds is only about half of soluble soil salts averaged .06 ported here. However, it can be that available to plants growing percent, but differences between predicted that the effect of the on the mounds. Since the avail- the mound-top and intermound Mima mounds on herbage pro- able water holding capacity per v a 1 u e s were not significant. duction will be similar and will unit of soil volume is the same Phosphorus and potassium aver- differ only in the degree of for both soils, with twice the aged 70 and 259 pounds per acre effect. The Mima-type micro- volume of soil in the mounds, on top of the mounds and 58 and relief will frequently show up the plants on the mounds would 150 pounds per acre between the from the air or on aerial photo- have access to twice the amount mounds.
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