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Study Methods

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Interaction of Simulated Drought and Grazing on Mixed Grass Prairie
Don Kirby, Wendi Rogers (Graduate Student), Paul Nyren, Bob Patton


Historical Perspective

Drought is defined as a 15% to 25% or greater deficit in the annual precipitation (Society for Range Management). Written evaluation of drought's effects on rangeland plant communities in the northern Great Plains began in the 1930's. All reports for the last 70 years have only examined natural droughts or drought periods with no control over environmental inputs. Therefore, each evaluation of drought has reported on a unique event that may or may not be representative of what is occurring throughout or between regions.

The best known chronicle of drought in North Dakota was maintained by Dr. Warren Whitman (Whitman et al. 1943). He noted that in 1934 and 1936, summer precipitation averaged 10.4 cm compared to 23.5 cm for the ten year period 1930-1939. In 1933, needle-and-thread and prairie junegrass occupied greater area and were more dominant in the vegetation composition than throughout the drought period (1934-1936) and did not fully recover until 1938. Sandberg's bluegrass was infrequent prior to 1934 but increased from one plant-clump per quadrat in 1933 to 17 plant-clumps per quadrat in 1938. Blue grama was the most severely affected of any of the major grass species in the 1930s drought. Its cover in 1937 was only one-third that of 1933; but by 1940, blue grama had recovered to pre-drought cover.

At the Livestock and Range Research Station near Miles City, Montana, two of the four driest summers experienced occurred between 1927 and 1934. In 1934, the summer rainfall was 8.9 cm compared to the preceding 57-year average of 23.6 cm. The area of blue grama on 55 individual m2 quadrats averaged 1542-cm2 in 1933 and decreased to 389-cm2 by 1935. Buffalo- grass experienced a decrease in average cover from 876-cm2 to 185-cm2 between 1933 and 1935. Sandberg's bluegrass increased in area from 4-cm2 in 1933 to 118-cm2 in 1935. Due to its quick growth after the last frost, maturing early in June, and going into dormancy by early summer, Sandberg's bluegrass was able to utilize snowmelt and early spring precipitation to out-compete warm season grasses for resources (Ellison and Woolfolk 1937).

Drought conditions between the fall of 1987 and summer of 1989 decreased plant basal cover on rangeland being grazed at both Dickinson and Streeter, ND locations (Kirby 1994). Additionally, herbage production in 1988 averaged 50% or less of the 9 and 11 year averages at the two locations. Kirby suggested that annual changes in species composition, plant basal cover and herbaceous production were influenced more by the amount and timing of precipitation than by stocking rate and grazing system employed.

Under grazing pressure, Biondini and Manske (1996) in western North Dakota reported that rainfall was more important than a grazing system in controlling measurable ecosystem-level variables. Total basal cover under moderate grazing, approximately 50% intensity, tends to be stable and unresponsive to stresses such as grazing or drought. The average annual net primary productivity in 1988 declined 46% with the decline continuing into 1989 on shallow and silty range sites. Cool season grasses and sedges decreased while warm season grasses and forbs increased under the climatic stress.