186 THE STUDY OF PLANT COMMUNITIES • Chapter VII 



out. Normally, however, such areas of any extent are rare, and 

 slopes, bluffs, and ridges with different exposures, lowlands, drain- 

 age lines, and depressions are present. 



Such irregularities in topography produce light, temperature, 

 and moisture conditions that differ greatly between north and 

 south slopes or ridges and depressions. The effect of exposure on 

 these individual factors having been previously discussed (p. 124, 

 132), it is necessary here only to emphasize that vegetation on 

 slopes is the resultant of interaction of light, temperature, and 

 moisture differences. South-facing slopes receive more light, have 

 higher temperatures, and are drier than the average site in the 

 area, while north-facing slopes receive less light, are cooler and 

 moister than the average. Of course, these differences vary with 

 degree and extent of slope, but, in general, the environment of 

 north and south slopes differs sufficiently to maintain distinctive 

 vegetative types. 



Apart from the interaction of the factors mentioned above, 

 slopes affect runoff and the amount of soil water and, likewise, the 

 possibility of erosion. 



Since water always moves toward depressions, they are invari- 

 ably moister than uplands and usually support distinctive vegeta- 

 tion. If topography is immature, as in the northeastern United 

 States, drainage is relatively poor and depressions contain ponds 

 or lakes supporting aquatic vegetation. Some lakes fill with sedi- 

 ment, marl, and organic materials to form bogs, which likewise 

 have their characteristic species. With more mature topography, 

 depressions are connected by streams, which make drainage far 

 more effective. Even so, the streams are usually bordered by flood 

 plains supporting vegetation requiring more favorable moisture 

 conditions than obtain upon the uplands. 



The greatest differences in vegetation associated with local 

 variations in topography can usually be correlated with moisture, 

 either in respect to an excess or to a deficiency. If the latter, 

 adaptations that facilitate absorption or restrict transpiration are 

 likely to characterize the plants. In a region where moisture is 

 rarely a critical factor, slope and exposure produce scarcely no- 

 ticeable differences in vegetation. This occurs only under condi- 

 tions where a combination of fog, clouds, or rain maintains a hu- 



