THE STRUCTURAL CHARACTERISTICS OF THE COMMUNITY 41 



Pinus montana forest of the Swiss National Park, with abundant 

 Erica and wholly undisturbed by man, is an example of this (Fig. 

 21). 



The frequency graphs often show two maxima : one in the highest-, 

 the other in the low^est-frequency classes. This was proclaimed to be 

 a regular phenomenon dependent upon the composition of natural 

 vegetation, until Nordhagen (1922) showed that the apparent and 



/ jr M IV V vi VR wr II X 



0-10 lO-ZO 20-30 30-W ¥0-50 50-60 60-70 70-80 SO-SO S0-K07o 

 Fig. 21. — Frequency diagram of a floristically heterogeneous Pinus montana forest rich 

 in Erica carnea. Ten quadrats of }'2 sq. m. each. 



puzzling drop from the highest to the next lower class really results 

 from the unequal breadth of the classes, since the highest-frequency 

 class is in fact not at all of equal value to the next lower. 



The species of the highest-frequency classes are often termed "local 

 constants," a misleading expression which should be avoided. The 

 better and more correct form would be "very frequent" or "most 

 frequent" species. 



Layering. — The concept of layering goes back to Kerner (1863) and 

 Ragnar Hult (1881). The latter in his descriptions of vegetation 

 distinguished no less than seven layers and portrayed them graphically 

 along with the corresponding life forms. 



We now recognize four principal layers: 



Tree layer. 



Shrub layer. 



Herb layer {Feldschicht). 



Moss layer (ground layer). 



These principal layers may be further subdivided. An upper and 

 lower shrub layer may be distinguished in tall scrub; a lower, middle, 

 and upper tree layer in the tropical forest. The graphic chart of 

 layering according to Hult-Sernander (Fig. 22) gives along with the 

 layering (horizontal strata) also the cover of each layer of vegetation. 



The characteristic layering of plant communities is the product of a 

 long-continued process of adaptation and selection, in which light is a 

 dominant factor. The more numerous the upper layers are and the 

 more completely these occupy the available space the greater must be 

 the tolerance of the lower lavcrs toward reduced light intensity. 



