344 PLANT SOCIOLOGY 



is seen in a whole series of associations of the Alps which are repeated in 

 the Tatras with slightly differing variations. However, as long as the 

 characteristic combination of species of an association suffers no impor- 

 tant change in the different parts of its range and the geographic 

 variants are distinguished only by local peculiarities, they are best 

 grouped as one association. 



The local races or variants are the subunits of which the association 

 is composed and upon which structural, ecologic, and syngenetic 

 investigations are made. In tabulation, therefore, the individual races 

 are put separately so that the varying floristic differences appear at 

 once. 



The relation of the distribution of three variants of the Xerobrome- 

 tum of the upper Rhine region to precipitation is shown on page 114. 

 Allorge recognizes three geographic variants of the widely distributed 

 Rhynchospora alba association: an Atlantic, an Atlantic montane, and 

 a central European. 



In tracing out the geographic variants of forest communities, only 

 well-developed, mature stands are to be considered, not mere stages of 

 development or bits of woodland greatly disturbed by man. 



Semi-cultural communities, like permanent pastures, which are 

 everywhere similarly treated by man, have closely similar combinations 

 of species over wide stretches of territory. For example, the Arrhena- 

 therion elatioris of central, southern, and western Europe may be 

 divided into several associations and variants of almost identical 

 physiognomy. The dominant grasses and a number of characteristic 

 companions are everywhere the same, from the Carpathians to 

 northern Spain. 



Division According to Altitude. — Vertically as well as horizontally 

 many associations show constant, though often slight, floristic modifi- 

 cations. Frequently these are due only to impoverishment, the 

 dropping out of some characteristic or companion species, as in the 

 high alpine Curvuletum as compared with that of the alpine pastures. 

 The same phenomenon is seen in the Mesobrometum of northwestern 

 Germany as one goes northwestward (Tiixen, 1930) and in many 

 associations toward the limits of their range. 



In general, if no important changes in the combination of species 

 can be shown, these modifications may best be listed as altitudinal 

 sub-associations or facies (Hohenglieder). Beger (1922) distinguishes 

 the normally developed subalpine spruce forest (Piceetum myrtille- 

 tosum) from the Piceetum normale of lower levels as a sub-association. 

 The extensive land-forming stands of Carex inflata and Equisetum 

 limosum (with a few other species) of the shores of our subalpine lakes 



