114 STRUCTURE AND UNITS OF VEGETATION. 



edaphic zones in the fact that the plant reactions affect the general climate 

 but little. They are in consequence relatively permanent, and disclose their 

 successional relationship only as a result of pronounced climatic changes. 

 The zonal arrangement of climax associations and their consociations is pro- 

 duced by the gradual decrease in water and temperature from an area of excess. 

 The effect of reduced temperature is found in the direction of the poles, and 

 produces east and west zones. The effect of diminishing rainfall operates 

 from coast to interior, and is recorded in zones which run north and south. 

 The two are necessarily superimposed, and the final expression in terms of 

 structure is further complicated by the influence of mountain ranges and large 

 interior bodies of water, such as the Great Lakes. Mountain ranges may not 

 only disturb the primary climatic zones, but they also present new regions of 

 relative deficiency and excess, and consequent zonation. 



Direct evidence of the successional relation of climax zones, such as is uni- 

 versal for edaphic zones, is not abundant. There are, however, several 

 sources of conclusive proof of their essential developmental connection. The 

 most important evidence is that furnished by peat-bogs and tufa deposits, 

 which bear witness to successive climax stages due to change of climate. The 

 similarity of these to zoned climax communities of to-day leaves no doubt of 

 their zonation, which is also attested by the fact that the zones of to-day about 

 water-bodies are recorded in superimposed layers of plant remains. Further 

 evidence is afforded by the vegetation of canons. The well-known fact that 

 the local climate of the north and south exposures is very different has already 

 been dwelt upon. The result of this difference is to produce in miniature the 

 effect which a general climatic change would cause over the whole mountain- 

 slope. A change in the direction of greater heat or dryness would tend toward 

 the xerophytic preclimax of the south exposure, while the opposite change 

 would give rise to the postclimax of north exposures. Indeed, the behavior 

 of such consocies as that of Pinus ponderosa is direct proof of the develop- 

 mental nature of climax zones. At lower altitudes it forms a xerophytic 

 climax over a vast stretch of the Rocky Mountain region; at elevations 2,000 

 to 3,000 feet higher it is the subfinal stage in the development of spruce forest. 

 In other words, its spatial or zonal relation as a mountain climax to the sub- 

 alpine spruce climax indicates its precise successional relation in the develop- 

 ment of the latter. 



The bilateral zones of river valleys also furnish evidence of the potential 

 development consequent upon climatic change. This is especially true where 

 the valley lies in the direction of decreasing rainfall, as is true of the Niobrara, 

 Platte, Republican, and others. The result is not only that forest, scrub, and 

 grassland are brought into the closest zonal juxtaposition, but also that there 

 is a gradual shifting of the consocies, as the edaphic conditions of the river- 

 bottom are modified by an increasingly arid climate. The developmental 

 series previously indicated for the Otowanie Woods, namely, 'Rhus, Fraxinus, 

 and Q-uercus-Hicoria, often with other consocies also, becomes a climatic series 

 with exactly the same sequence from moist to dry conditions. Finally, the 

 broad ecotones or transition areas between climax communities are clear 

 indexes of the effect of climatic swings. They are mixed communities, and 

 correspond closely to the mixture of two contiguous stages, i. e., a mictium, 

 in the course of succession. 



