Paper No. 19 



INFLUENCE OF GRAZING ON THE MICROCLIMATE OF MIXED GRASS 



PRAIRIE 



By Warren C. Whitman 2 



Abstract 



On a Northern Plains mixed grass prairie in 

 the 1970 season, grazed vegetation had 78, 13, 32, 

 and 37 percent as much standing crop, standing 

 dead material, litter, and total aboveground plant 

 material, respectively, as ungrazed vegetation. 

 Air and soil temperatures, to a depth of 120 cm., 

 were higher on the grazed site throughout the 

 season. Wind movement near the ground surface 

 on the grazed site was three times that on the 

 ungrazed site. Soil moisture to the full profile 

 depth was somewhat greater on the ungrazed 

 site until after midsummer. Atmospheric mois- 

 ture and vapor pressure deficits were only slightly 

 different over the two grazing treatments. 



Additional key words: Microenvironment, North- 

 ern Plains grassland, temperature, soil moisture, 

 humidity 



It has been generally accepted that grazing has 

 a rather profound influence on the microenviron- 

 ment of native grassland, but surprisingly little 

 factual data are available regarding the exact 

 nature of this influence. Ellison (9) stated that 

 grazing reduces both standing herbage and the 

 accumulation of mulch, with the result that evap- 

 orative losses are encouraged, and a lighter, 

 warmer and drier microenvironment is created. 



Rice (17) studied the growth and develop- 



' Results of a cooperative study between the North 

 Dakota Agricultural Experiment Station and the Grass- 

 land Biome project of the International Biological Pro- 

 gram, supported in part by National Science Foundation 

 Grant No. 13096 to Colorado State University, Fort Col- 

 lins, Colo. Published with the approval of the Director, 

 North Dakota Agricultural Experiment Station. 



2 Professor of Botany, North Dakota State University. 

 Fargo. N. Dak. 58102. 



ment of several tall grass prairie" dominants in 

 Oklahoma and recorded some microclimatic fac- 

 tors. He found that the more mesic grasses were 

 reduced in abundance by high air and soil tem- 

 peratures and high evaporation. Hopkins (13) 

 found that the mulch layer in Kansas grassland 

 reduced soil temperatures, retarded evaporation 

 from the soil surface, and increased the amount of 

 available moisture in the soil. 



Since fire removes accumulations of vegetation 

 and litter, it might be expected that burning 

 would have microenvironmental effects similar to 

 grazing. Dix and Butler (7), Aikman (7), Ro- 

 bocker and Miller (18). Ehrenreich (8), Kucera 

 and Ehrenreich (14), Hadley and Kieckhefer 

 (11), and Anderson (2) studied the influence. of 

 burning on grasslands in the tall grass region, 

 and evaluated some microenvironmental effects. 

 In general, these studies have shown that the 

 microenvironment of the burned plots was more 

 severe than that of unburned plots. Dix's 

 study (6) is the only one which deals directly 

 with the influence of burning on the Northern 

 Plains mixed grass prairie. Daubemire (5) has 

 reviewed extensively the studies of fire influences 

 on grassland, including microenvironmental ef- 

 fects. Old (15) has published a detailed study 

 of microclimate, fire, and plant production in 

 Illinois tall grass prairie. She developed an in- 

 teresting microclimatic index for the comparison 

 of the microenvironment of different burning 

 regimes. 



Whitman and Wolters (23) characterized mi- 

 croclimatic gradients in ungrazed mixed grass 

 prairie in western North Dakota. Scoby (19) ex- 

 amined the utilization of radiant energy by mixed 

 grass prairie in the same area. Whitman (22) 

 has discussed the general role of microclimate in 

 the grassland ecosystem. These studies, however, 

 do not provide an adequate evaluation of graz- 



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