Paper No. 17 



EFFECTS OF GRAZING ON MICROENVIRONMENT OF UNITED STATES 



RANGELANDS 



By Russell J. Lorenz 1 



Abstract 



Some factors of the microenvironment have 

 received considerable attention in relation to 

 grazing in the United States. Change in species 

 composition, invasion by undesirable species, 

 mulch accumulation, and soil moisture status 

 have been studied in relation to grazing season 

 and grazing intensity. Generally, the adverse 

 effects increase with grazing intensity, but differ- 

 ences in response are often not as great as are 

 differences between no grazing and grazing. In 

 many respects, interaction of the moisture regime 

 or of energy and temperature factors with canopy 

 height and density or with mulch accumulation 

 appear to be similar for rangeland and for the 

 more thoroughly studied cultivated crops. The 

 effects of grazing on factors affecting infiltration 

 and runoff are fairly well documented, but the 

 effect of grazing on factors affecting evaporation, 

 evapotranspiration, the energy balance, and nu- 

 trient cycling have been studied very little. Com- 

 prehensive information on the effect of grazing 

 on all microenvironmental factors for a specific 

 situation are lacking. 



Additional key words: Species composition, 

 mulch, infiltration, runoff, evapotranspiration. 

 microclimate. 



Introduction 



Grazing is usually considered in terms of its 

 effects on the vegetation as evaluated by species 

 change or by change in plant productivity or 

 vigor. Information concerning the effect of graz- 

 ing on microenvironment is either lacking or re- 

 st ricted to a few easilv evaluated factors. Dis- 



1 Research Agronomist, Plant Science Research Divi- 

 sion, Agricultural Research Service, U.S. Department of 

 Agriculture, Mandan, N. Dak. 



186 



cussion of these factors becomes rather involved, 

 because it is the environment, both micro and 

 macro, which largely determines the character of 

 the vegetation of an area. Alteration of any 

 environmental factor by grazing, or by other 

 means, will be reflected by change in other en- 

 vironmental factors as well as by vegetational re- 

 sponse. The cycle of cause and effect becomes 

 endless. 



It is obvious that grazing of any kind or in- 

 tensity will affect the microenvironment, and that 

 each situation is unique. A thorough understand- 

 ing of all physical and biotic factors is necessary 

 for proper evaluation of the impact of grazing 

 on a particular ecosystem. The grazing animal 

 can best convert the stored energy and nutrients 

 of much of the land area of western United 

 States to dietary protein. We must first evaluate 

 what is known about the effect of grazing on the 

 system, and then work toward understanding 

 what is not known, so that efficiency of this con- 

 version can be increased. 



The most obvious contribution of the grazing 

 animal to Che ecosystem is of a mechanical na- 

 ture — change in height and density of the canopy, 

 trampling, and deposition of excreta. Defoliation 

 is the major direct animal influence. Selectivity 

 by animals for plant species or for plant parts, 

 with changing season or with changing grazing 

 pressure, complicates study of defoliation and of 

 the final influence of grazing. The effects of graz- 

 ing intensity on change in species composition, 

 plant density, and ground cover have been re- 

 ported for a variety of situations. The reasons for 

 the change and the overall effect on the micro- 

 environment have not been fully investigated. 



Microenvironmental parameters include the mi- 

 croclimatic factors, soil and its related factors, 

 and living organisms and their related factors. 

 Much of the microclimatic data and some of the 



