Origin of Biomes and Succession 255 



of these conditions is one of the "relatively uniform areas" which 

 are usually called communities. In Fig. 112 each station represents 

 a different community. 



The whole spectrum of Fig. 112 exemplifies the concept of biomes 

 as landscape aspect areas and as ecological units. This series of 

 stations contains radically different vegetational types: (1) the 

 extremely dense evergreen coniferous forest dominated by hemlock 

 (Tsuga), (2) the relatively open evergreen forest dominated by 

 the two species of Pinus, and (3) the deciduous forest with its 

 dense summer shade when the trees are in leaf and sparse winter 

 shade when the trees are without leaves. In each of these three 

 types the dominants superimpose different ecological conditions 

 over the general climatic and edaphic characteristics of the area 

 and between each type a corresponding difference exists in the 

 ground biota and other subdominant elements of the community. 

 Each of these major types is a hiome. As Fig. 112 shows also, the 

 distinctive biomes merge gradually from one into another, the 

 intermediate areas being called ecotones. Although in mountainous 

 regions a small area frequently contains several biomes in close 

 proximity, in more level country each biome tends to be an exten- 

 sive area. Thus, as a rule, the more level the country, the fewer 

 and more extensive are the biomes, and, by the same token, the 

 wider are the ecotone areas. 



Adjoining biomes may be much more sharply separated than is 

 the case of the biomes just described from the Great Smoky Moun- 

 tains. Sharp separation is illustrated by the biomes of the Kaibab 

 Plateau in Arizona. Rasmussen (1941) described the piiion pine- 

 juniper biome as occurring between about 5,500 to 6,500 feet eleva- 

 tion, the Rocky Mountain yellow pine biome between about 6,500 

 to 8,200 feet, and the fir-spruce biome between 8,200 to the sum- 

 mit, 9,200 feet. A chart of the dominant species (Fig. 113) em- 

 phasizes the relatively small overlap between the three biomes. 



The biomes described by Whittaker and Rasmussen are each 

 adapted to different climatic conditions involving temperature, hu- 

 miditv, and rainfall. The correlation of these with altitude is due 

 to the rapid changes of the climatic factors with changes in eleva- 

 tion. In some cases edaphic factors may be the agents separating 

 biomes. An excellent example from the Sonoran desert of Arizona 

 is described by Yang and Lowe ( 1956 ) . The two distinctive biomes 

 in the area occur under an identical macroclimate, but the palo 

 verde-sahuaro (Cercidium-Cereus) biome grows in the lighter and 

 more rocky soils on the upper parts of the slopes, and the creosote 



