11,500 ft.*^^ In the Andropogon scoparius grassland on the Hemp- 

 stead Plains, Long Island, 71 species were recorded, ^^ while in 

 western North Dakota, where a similar community type is usually 

 restricted to north-facing slopes, the average number per stand 

 was only 50.^°^ Similarly, many alpine communities in central 

 Europe have more species than those in Scandinavia. The mere 

 listing of species without data on relative abundance, however, 

 may be misleading, for some species often occur in different pro- 

 portions in various communities, particularly in the Arctic and 

 Subarctic. Even though the number of species per stand is as low 

 as 12 to 20, the number of kinds of communities that can develop 

 is enormous. 



Stratification 



Stratification, or layering, is the occurrence of organisms, or 

 their parts, at different levels in a stand. Stratification is readily 

 seen above ground, but is present also in the unseen root systems 

 and rhizomes. Stratification usually occurs because life-forms 

 such as trees, shrubs, herbs, and mosses differ in their require- 

 ments and amplitudes, and therefore grow at various levels which 

 differ in light intensity, temperature, moisture conditions, organic 

 content of the soil, and other factors (Figures 2-1 and 3-2). 



The number of strata above ground vary according to the kind 

 of community. In early stages of succession usually only one 

 stratum is present, comprising such plants as lichens, mosses, or 

 annual herbs; but as succession proceeds additional strata appear, 

 so that in a mature grassland there may be three strata, the two 

 highest comprising grasses and forbs, and the lowest short herbs, 

 mosses, lichens, and occasionally algae. Five to seven strata may 

 be found in forests: two or three of trees, one or two of shrubs, an 

 herb or field layer, and a ground or moss-lichen layer (Figures 

 1-2 and 1-16). Occasional trees that project above the general 

 canopy are called emergent or supercanopy trees. The chief 

 layer in savannas is the field layer of herbs or sometimes low 

 shrubs; the scattered trees or tall shrubs hardly form a layer. 



Stratification is well developed in tropical rain forests. For ex- 

 ample, in Nigeria the top tree layer at 120 to 150 ft consists of 



82 • THe ContmiaMmity 



