124 THE STUDY OF PLANT COMMUNITIES • Chapter VI 



The air near the earth's surface is warmed by absorption of 

 insolation and reradiated heat from the earth. With increasing alti- 

 tude the atmosphere becomes less dense and also contains less 

 moisture and other heat-absorbing substances. Consequently, tem- 

 peratures decline with altitude. Even the warm air rising from the 

 earth is cooled by its expansion. Latitudinal temperature zones are, 

 therefore, further disrupted by mountains where increasing alti- 

 tude produces the same differences as increasing latitude. This is 

 particularly noticeable on high mountains where, because of the 

 combined effects of temperature and moisture, one may see zones 

 of vegetation altitudinally arranged, which at lower altitudes are 

 latitudinallv distributed over hundreds of miles. 



Just as latitudinal temperature zones are irregular, so are the alti- 

 tudinal zones not perfect. Cold air drainage has been discussed, 

 (p. 98.) It results in low night temperature in the valleys when 

 tablelands and upper slopes are much warmer. 129 The areas may be 

 distinctively marked by the vegetation they support. In moun- 

 tainous country, orchards are frequently grown successfully at 

 much higher altitudes on slopes than in valleys. 81 Slope and expo- 

 sure disrupt mountain temperature zones even more. Since the 

 maximum effectiveness of insolation comes only when it strikes 

 a surface at right angles, the greater the variation from a ninety- 

 degree angle, the less radiant energy will strike a unit area. In the 

 Northern Hemisphere, therefore, a south-facing slope receives 

 more insolation per unit area than a flat surface, and a north-facing 

 slope receives less (see Fig. 67). Thus the same temperature con- 

 ditions found on a tableland may occur at a higher altitude on a 

 near-by south-facing slope and at a lower altitude on a north slope. 

 The distribution of vegetation being correlated with temperature 

 and the consequent moisture differences, a particular community 

 will be found above its ordinary altitudinal range on south slopes 

 and below it on north slopes, and the extent of this irregularity in 

 zonation is affected both by the angle of the slope and its exposure. 

 In Wyoming, Douglas fir from the montane zone may come down 

 to 7,500 feet on north-facing slopes while mountain mahogany 

 from the lower woodland zone may be found extending upward to 

 better than 8,500 feet on south-facing slopes. In general, a vegeta- 

 tion zone extends higher on the south side of a mountain than on 

 the north side. 



