178 Temperature 



the principal biotic formations, or biomes, that will be discussed more 



fully in Chapter 12. 



A synoptic view of the life zones of the world could be obtained 

 by chartering a plane at the equator and flying toward the north 

 pole, or toward the south pole. On such a trip, you would observe 

 that the life zone characteristic of the equatorial region is the tropical 

 rain forest. In this zone a wide variety of trees provides continuous 

 shade and humid conditions for the many species of plants and ani- 

 mals that live in and under the forest. North of the tropical rain 

 forest the flying ecologist comes to a life zone in which dry seasons 

 alternate with rainy seasons. Farther to the north the plane passes 

 over a belt of varying width of desert or grassland and thence over 

 deciduous forests of oak, hickory, beech, and maple. The colder 

 region to the north is heavily forested with spruce and other conifers 

 that form a circumpolar life zone. At the limit of the spruce forest 

 the plane crosses the tree line and reaches the tundra where dwarf 

 birches or willows, scattered grasses or other flowering plants, mosses, 

 and lichens struggle to keep alive on ground that is frozen for much 

 of the year. Eventually all land vegetation is lost to view in a land- 

 scape of universal snow and ice. If the plane trip had been made 

 from the equator southward, the same general sequence of life zones 

 would have been traversed except for the gap caused by the great 

 expanse of the Antarctic Ocean. 



13,500 ft 



North 



UINTA MOUNTAINS 



TAVAPUTS PLATEAU 



4,000 m 



3,500 



3,000 



2,500 



2,000 



1,500 

 South 



Fig. 5.20. Diagrammatic representation of the altitudinal life zones in the Uinta 

 Basin. (Graham, 1937.) 



Without an airplane you may observe the same general series of 

 life zones on a much smaller scale by climbing a mountain. If the 

 mountain is a high one and located in the tropics, as is Mt. Popo- 

 catepetl in Mexico with an altitude of 5448 m, the basal slopes may 



