THE DISTRIBUTION OF COMMUNITIES 



347 



3. The upper littoral zone, or zone of green algae, lying between the highest 

 flood tide and lowest ebb. This is the region of Chaetomorpha, Ulothrix, Clado- 

 phora, and Ulva. 



4. The supra-littoral zone or zone of lichens, with Verrucaria maura, Rhizo- 

 carpon spp., etc., within reach of spray. 



Within these principal zones more or less distinct subzones or 

 horizons may often be distinguished. Exceptions or inversion of the 

 usual zonation deserve particular attention, since they reflect special 

 local conditions often slightly apparent. 



Altitudinal Zones. — Different levels of altitude on mountains 

 present an important special case of the beltlike arrangement of 



Fig. 175. — Altitudinal zones in the Swiss National Park. The Larix-Pinus cembra 

 zone to 2,200 m., Pinus montana prostrata zone 2,200 to 2,300 m., alpine meadow zone 

 2,300 to 2,550 m., and the foreground of Nardus grassland induced by grazing. {Photo 

 by Heller.) 



vegetation. As shown in Fig. 175, the different belts follow one 

 another on the slopes of the mountain, giving by their physiognomy a 

 quick survey of the changes of climate. Only when the slope of the 

 land is extremely gentle may the horizontal and vertical zones merge 

 into one another. The number of clearly marked altitudinal zones 

 increases with the height of the mountains and with nearness to the 

 equator. Many other factors come in, however, in opposition to this 

 general rule, as, for example, the number and vertical extent of the 

 climax communities of the region. 



Within the broader altitudinal zones secondary belts may be 

 distinguished. These are best called horizons. The spruce zone of the 



