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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



well as upon interaction among factors within 

 each zone. 



Robertson (45) studied the spectral energy of 

 light available to plants under several sky con- 

 ditions, including bright sunshine, skylight, twi- 

 light, light from cloudy skies, and light within 

 canopies of varying densities. The proportion of 

 red and far-red energy increased as haziness in- 

 creased or as solar elevation decreased. Trans- 

 mission through a crop canopy resulted in a high 

 proportion of far-red energj-. He suggested that 

 the spectral composition of light be considered as 

 an environmental factor, particularly as it relates 

 to species growing under an overstory of other 

 species. 



Decker (13) compared net radiation over a 

 short-grass turf, an alfalfa field, and a corn plot. 

 Net radiation represents the energy available for 

 thermal and biological processes. Therefore, dif- 

 ferences between canopies of different height are 

 important to the entire energy exchange of the 

 community. During daylight hours he found 

 higher net radiation values for the taller vege- 

 tation types, an indication that more heat energy 

 was available in the corn and alfalfa canopies 

 than in the short-grass canopy. If the heat sup- 

 plied to the soil and atmosphere were the same 

 for all three covers, then the extra energy would 

 have to be used in evaporational processes. Thus. 

 ET would be greater for the taller crops than 

 for the shorter ones. 



In a study of the energy budget within a 

 cornfield. Brown and Covey (6) found an ex- 

 ponential relationship with leaf area approxi- 

 mating extinction of net radiation within the 

 crop. The largest leaf-air temperature differ- 

 ences were found to be near, but not at, the top 

 of the crop. They found that 46 percent of the 

 net radiation was used for transpiration in this 

 corn crop, 13 percent for soil evaporation, 32 per- 

 cent for sensible heat flux and 6 percent for soil 

 heat flux. Thus. 16 percent of the net radiation 

 reached the ground under the dense crop. 



The effect of vegetative cover and mulch on sur- 

 face temperatures and soil temperatures was re- 

 ported by Hopkins (26). With an air tempera- 

 ture of 32° C. maximum surface temperatures 

 were recorded on a west-facing grazed hillside, 

 which had less mulch than any other area in his 

 study. In an adjacent exclosure, surface tempera- 



ture under 10 cm. of mulch was 29° C. At the 

 same time, temperatures in the upland short-grass 

 were 37 and 32 C. on the grazed and ungrazed 

 sites, respectively. The ungrazed short-grass had 

 5,357 kg. mulch/ha. while the moderately grazed 

 had 3,127 kg. 



Vegetation Effects 



Plant competition for various components of 

 the microenvironment is greatly affected by graz- 

 ing, particularly if grazing is of a selective na- 

 ture. Competition for soil moisture has received 

 most attention and is perhaps the most import- 

 ant factor in much of the United States. In spe- 

 cific instances, competition for light is also of 

 major importance. Reproduction of most spe- 

 cies is highly dependent upon these factors. This 

 is substantiated by many reports of impaired 

 reproduction of certain species under heavy 

 grazing or with no grazing. Production of seed 

 and suitable conditions for germination and 

 plant establishment are necessary. Smith (49) 

 described various disturbances which affected 

 seed germination of annual plants in the Sierra 

 Nevada foothills of California. Herbage remov- 

 al, fire and soil disturbances significantly re- 

 duced the number of germinating seedlings of 

 Bromus mollis L., Bromus tectorum L., and 

 Trifolium microcephalum Hook. Burning or soil 

 disturbance caused the largest reduction in the 

 number of grass seedlings. However, the soil dis- 

 turbance was perhaps more severe than would be 

 encountered under most grazing conditions. 



Penfound (35) studied the effects of protec- 

 tion from grazing of tall-grass prairie and of re- 

 vegetated cropland in Oklahoma. He compared 

 protected prairie, grazed prairie, protected crop- 

 land and grazed cropland. The protected prairie 

 changed from midgrass through tall-grass prairie 

 to tall-grass prairie with many woody species. 

 He predicted that if protection had continued, 

 this area would have been taken over by woody 

 plants. The grazed prairie vegetation remained 

 nearly constant in the midgrass stage. Protected 

 cropland vegetation changed rapidly from an- 

 nuals to forb-short grass to short grass-mid- 

 grass-tall grass; however, when cropland was 

 grazed the successional changes were from an- 

 nuals to short jjrass-midffrass to midgrass. In 



