PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



187 



soils data have been gathered without the grazer 

 because the uncontrollable influences of grazing 

 complicate the study of these factors. However, 

 there is considerable information available on 

 the effects of various biotic factors on micro- 

 climate. The known influence of grazing on these 

 biotic factors allows inference to possible effects 

 of grazing on the microclimate. Plants them- 

 selves are a part of the microenvironment, there- 

 fore, any change in growth form, size or density, 

 or in species composition of the community, in- 

 duced directly or indirectly by grazing, has an 

 effect on the overall microenvironment. Change in 

 species composition produced by grazing has been 

 described for many rangelands; however, only 

 unique instances will be cited here when they 

 serve to illustrate specific effects of grazing on 

 the microenvironment. 



Through the ages, the grazing animal has in- 

 fluenced the evolution of United States range- 

 lands. In a summary of the history of grazing, 

 Heady (25) considered grazing to be a force in 

 natural selection long before man replaced wild 

 with domestic animals. Plants and animals in cen- 

 tral North America evolved in a grassland-grazing 

 animal context. With the shift from wild to do- 

 mestic animals, grazing intensity and selectivity 

 changed the grasslands somewhat, but Heady 

 concluded that this replacement, as such, did not 

 destroy the grasslands. Most grasslands of the 

 Great Plains survived change in grazers in spite 

 of considerable mismanagement. However, deter- 

 ioration of rangeland, accompanied by increased 

 rate of soil erosion and increased sediment load, 

 is often associated with immigration of livestock 

 and ranchers. Grazing by domestic animals has 

 been blamed for this situation, and in many 

 cases, rightly so. It is easy to see the end result 

 of poor grazing management, but to understand 

 the reasons why certain changes occurred, and 

 to develop methods for correcting old errors 

 while avoiding new ones is far more difficult. 

 Reed and Peterson (4-5) found extreme weather 

 fluctuations, rather than grazing intensity, to be 

 the major influence on mixed prairie range. How- 

 ever, the rate of vegetation change produced 

 by weather fluctuations was influenced by grazing 

 intensity. 



Crocker (12) discussed the soil and ecological 

 problems arising from chronic overgrazing. He 



defined chronic overgrazing as an intensity of 

 grazing which modifies plant cover so that pro- 

 ductive capacity is seriously impaired. He em- 

 phasized the importance of assessing direct effects 

 on the vegetation as well as those indirectly af- 

 fecting production through soils effects. 



Buffiington and Herbel (7) attribute the con- 

 version of the desert-grassland climax of southern 

 New Mexico to a desert-shrub climax, dominated 

 by mesquite (Prosopis julifora [Swartz] D.C.) 

 to grazing by domestic animals. They eliminated 

 climatic change or fire as possible causes for the 

 change, but found close correlation with begin- 

 ning of the grazing industry. Islands of mesquite 

 supplied seed which was disseminated through 

 the digestive tract of the animals. In this case, 

 grazing set off a series of events which resulted 

 in conversion to a desert-shrub type in a rela- 

 tively short time. 



Some grasslands, such as the bunchgrass types 

 of the Pacific Northwest and California, are very 

 sensitive to grazing. It has been suggested (51) 

 that low grazing pressure, due to lack of large 

 herds of grazing wild animals in these areas, al- 

 lowed the more sensitive bunchgrasses to survive. 

 Introduction of domestic animals increased the 

 grazing pressure which resulted in loss of the 

 perennial bunchgrass in many areas. 



Some aspects of the microenvironment have 

 received considerable attention; others have been 

 studied very little, although indirect inferences 

 have been drawn. All microenvironmental fac- 

 tors are closely related, thus the following dis- 

 cussion is arbitrarily divided into categories. 



Moisture Regime 



No one environmental factor influences plants 

 more than does water. Management practices can 

 be employed which increase the efficiency of water 

 use, but shortage of available water often re- 

 mains as a limiting factor. Grazing has a definite 

 effect on factors which contribute to the moisture 

 regime — interception and redistribution of pre- 

 cipitation by the plant canopy; infiltration and 

 runoff as influenced by standing vegetation, litter 

 accumulation and trampling. Evaporation and 

 evapotranspiration (ET) continually reduce tin- 

 reservoir of available water. The most active 

 region of the water regime extends from a few 

 cm. in the soil to the top of the vegetation canopy. 



