Deterioration of Grassland from Stability to Denudation with Decrease in Soil Moisture

Abstract

1. Four groups of prairies of known previous history, extending from southwestern Iowa to western Kansas, were studied in 1939. 2. The Iowa group (precipitation about 32 inches) had been scarcely affected by the great drought. At one Nebraska-Kansas group, bluestem grasses (Andropogon spp.) still prevailed; at the other they had died and been replaced by Agropyron smithii. Bouteloua gracilis and Buchloe dactyloides were the chief grasses of the western Kansas prairies (precipitation about 22 inches). 3. The twelve prairies in the four groups all occurred on deep, fertile, silt-loam soils; water content was determined monthly to a depth of 6 feet. 4. The bluestem prairies of Iowa, where rainfall was plentiful, had a continuous supply of available water at all depths; those of Nebraska and Kansas had only a low supply after early spring and were repeatedly threatened with drought. In the wheat-grass group, available deep-soil moisture was the exception, and repeated exhaustion of surface soil moisture occurred. Water was available only in the surface soil at the short-grass stations and at only two or three periods. 5. Slightly higher temperatures, 50-100 per cent more evaporation, and two to three times as much wind movement occurred in western Kansas as in eastern Nebraska. 6. No previous deterioration of vegetation had occurred in Iowa. The ground layer had been destroyed and the basal area was only one-half to two-thirds normal in the drought-depleted bluestem prairies westward. Drought and dust had destroyed most of the former plant cover of the third group of prairies, which were now dominated by an open growth of western wheat grass which permitted a continuous pattern of bare soil. In western Kansas the 85 per cent basal cover of short grasses had been reduced by continued drought, burial by dust, and injury by grasshoppers to 10-15 per cent. The remaining soil was bare. 7. Grasses grew normally in Iowa, reaching a foliage level of 18 inches; they dried after midsummer and failed to flower in eastern Nebraska. Wheat grass dried very early and burned readily in July. The short grasses in west-central Kansas were dormant during most of the summer but grew to a height of 3 inches when revived by late summer showers. 8. In Iowa, fifteen species of grasses and sedges occurred at the sampling stations, but nineteen at the western, bluestem station-group. There were but nine at the wheat-grass stations and only four in the short-grass areas. 9. Native forbs similarly decreased from sixty-five to thirty-three species, then to twenty-one, and finally to eleven. 10. Conditions of the central area of the mid-continental grassland reveal in general changes that have been wrought elsewhere from east to west by continued drought.