132 THE STUDY OF PLANT COMMUNITIES • Chapter VI 



It is, therefore, adaptable to continuous operation with a record- 

 ing device. 



Such equipment is not always available to the field ecologist, 

 but, even so, some form of measurement is far more dependable 

 than an estimate. Good approximations of light intensity may be 

 obtained with photographic light meters even though they are not 

 calibrated in foot candles. Useful values are obtainable by expos- 

 ing black and white bulb atmometers in pairs. When one pair is 

 exposed in the open and differences from pairs in near-by habitats 

 are expressed as percentages of the value in full sunlight, the re- 

 sults may be quite as satisfactory as with more elaborate equip- 

 ment. Since the atmometers would be operating continuously, 

 they might even be more meaningful in terms of the vegetation. 



Light Variations.— The biologically important variations of light 

 are those in intensity and quality. These occur periodically, re- 

 curring seasonally and daily to a degree that is determined by 

 latitude 140 as discussed under the general heading of insolation. 

 Of course, altitude modifies the regional variations, and topog- 

 raphy results in more localized variation through the effects of 

 angle of slope and direction of exposure. Since the principles were 

 previously discussed (p. 124), it should be sufficient here to pre- 

 sent an illustration of how slope and exposure affect light in the 

 southern Appalachian Mountains. 41 



Variation in quality of light is not so obvious as variation of 

 intensity. Quality, however, is variable, largely because of the 

 same factors that modify intensity, for the amount of absorption 

 and diffusion by the atmosphere determines what wave lengths 

 reach the earth. Clouds, fog, smoke, dust, or atmospheric moisture 

 alone increase diffusion and absorption, and, as a consequence, dry 

 regions receive more light than humid ones, and open country 

 receives more light than smoky cities. The greater the diffusion, 

 the higher the percentage of red light and the lower the percent- 

 age of blue reaching the earth. 



A local variation of far greater general ecological importance is 

 that produced by vegetation of one stratum upon that of a lesser 

 stratum beneath it. Because plants growing in the shade of others 

 receive only the light that is not absorbed or reflected, they must 

 be adapted to functioning with reduced light intensity (often re- 



