PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



191 



since leaf drop. He found that of the climatic 

 effects, temperature was most important in areas 

 where moisture was not limiting;. During the 

 early stages of litter decay, about 40 percent of 

 the weight loss appeared to be independent of mi- 

 crobial activity. He concluded that leaching of 

 leaf solubles and the physical and chemical break- 

 down of the vegetation accounted for the early 

 weight loss. 



There has been little research on litter de- 

 composition in grasslands. Work on a fescue 

 (Festuca arundinacea Schreb.) meadow by Ma- 

 lone (30) showed that vegetation type had a 

 large influence on rate of organic matter de- 

 composition, especially when species are present 

 which produce bacteria-inhibiting compounds. 

 Fescue was found to produce such a compound, 

 and killing the fescue caused a rapid population 

 increase of soil bacteria. High and low soil tem- 

 peratures and moisture levels were found to af- 

 fect some microorganisms. Manipulation of vege- 

 tation will, therefore, have a large influence on 

 decomposition rate of organic matter, mineraliza- 

 tion processes, and nutrient cycling. 



Evaporation And Evapotranspiration 



Published results from evapotranspiration 

 (ET) studies on crop plants and intensively man- 

 aged forages are plentiful. Little information 

 is availavble on native vegetation of the arid 

 and semiarid regions (23). Blaney (4) sum- 

 marized the information for semiarid regions. 

 Based on review of the few published papers 

 and on his own field observations, he concluded 

 that under normal conditions, seasonal precipita- 

 tion of at least 30.5 cm. is necessary before water 

 will penetrate below the grass root zone. Seasonal 

 rainfall of less than 30.5 cm. is usually consumed 

 by native grassland before deep percolation oc- 

 curs. For brush and chapparal areas, at least 46 

 cm. of precipitation must fall during any one 

 year before water will move below the root zone. 



Collins (JO) reported that ET losses in the 

 United States as a whole are estimated to be 

 70 percent of the total 76 cm. of average annual 

 precipitation received. The amount varies widely 

 from region to region, ranging from 50 percent 

 of the 119 cm. received in the Appalachian region 

 to over 90 percent of the 25 to 38 cm. received 

 in some areas of the west. Without vegetative 



cover, rapid evaporation at the soil surface forms 

 a dry layer of soil which restricts upward move- 

 ment of water. This reduces the overall rate of 

 water loss. Adding vegetation allows movement 

 of soil water to the atmosphere through the plant. 

 The only limit to this system is the water avail- 

 able and the depth and distribution of the root 

 system. Potential ET is the amount of water 

 loss that will occur if at no time there is a de- 

 ficiency of water in the soil for use by the vege- 

 tation. This has been calculated to be 46 cm. 

 annually in the northern portion of the short- 

 grass prairie and 102 cm. in southern Texas (10). 

 Potential ET varies throughout the year in re- 

 sponse to the change in temperature. Seldom are 

 the values for potential ET and actual ET equal 

 in the western United States clue to the lack of 

 available soil moisture during at least a portion 

 of each season. 



Energy Balance 



The microenvironment of any organism in- 

 cludes various aspects of energy exchange. Physi- 

 ological response of the organism depends pri- 

 marily upon energy flow in terms of temperature. 

 However, all microclimatic factors — air tempera- 

 ture, relative humidity, wind speed, solar radia- 

 tion, thermal radiation — influence the organism. 

 At the same time, factors of the microclimate are 

 closely associated with and highly dependent 

 upon all factors of the microenvironment. In the 

 case of plants, light intensity and quality are 

 also important. 



A discussion of all the energy exchange func- 

 tions is not possible here. Gates (22) stressed the 

 influence of microclimate and energy exchange 

 on the individual organism. Decker (1J/.) dis- 

 cussed the energy balance of a plant cover in 

 the subhumid region of the United States, and 

 Baumgartner (3) discussed the ecological sig- 

 nificance of vertical energy distribution in plant 

 stands. Factors of importance included air tem- 

 perature, plant temperature, wind velocity, po- 

 tential evaporation, net radiation, vapor pres- 

 sure deficit, and carbon dioxide concentration. 

 The canopy can be arbitrarily divided into zones 

 or layers by consideration of these factors in a 

 vertical profile. Energy gradients exist which are 

 dependent upon canopy density and height as 



