348 PLANT SOCIOLOGY 



northern Alps includes in places an Alnus viridis horizon. The beech 

 zone of the Auvergne terminates upward with a narrow Sorhus-Betula 

 horizon. 



Bounding the Zones of Vegetation. — The distinguishing of altitude 

 zones in mountains was one of the first activities in plant geography. 

 It was attempted by Von Haller (1742) and Giraud-Soulavie (1783). 

 But Sendtner in his description of the vegetation of southern Bavaria 

 gave a stricter standard for the determination of zones. According 

 to him, in order to determine a "plant region" (altitudinal zone), the 

 altitudinal limits of every species of the area must be precisely given. 

 The boundaries of the zones are to be set "where the greatest differences 

 in the character of the vegetation come in, that is, where the largest 

 number of new species appear and others disappear." This compre- 

 hensive method has met with scant approval. It is only apparently 

 precise and is beset with many sources of error. It is simpler and safer 

 to determine the boundaries of the plant communities. The bounda- 

 ries of most of the species agree with the boundaries of the association. 



After determining the upper and lower boundaries of all the asso- 

 ciations in the southern Cevennes, three vegetative zones have been 

 distinguished there (1915): (1) evergreen oaks up to 600 m.; (2) white 

 oak up to 1,000 m.; and (3) the beech zone up to the summits. This 

 distribution of zones corresponds to that of the climax regions (Fig. 

 161). 



When the climax communities are relatively undisturbed in com- 

 position and distribution, as in the Alps and in the Carpathians, the 

 altitudinal zones may be divided exactly as are the climax communities. 

 The zones are then named after the climax communities. 



General names for levels, like colline, montane, subalpine, alpine, 

 must be used with caution. They are best combined with a vegetation 

 name, as subalpine spruce-wood zone, alpine grassland zone. But even 

 this cannot replace an exact definition of each zone. 



Zones of vegetation are an expression of local conditions of climate 

 — a fact that is generally overlooked. It is therefore often of doubtful 

 value to try to point out parallels between the vegetations of widely 

 separated mountains, as of the Alps and the Scandinavian mountains. 

 Within the Alps there are great differences in the distribution of zones 

 in the eastern, western, southern, northern, and central regions. The 

 boundaries of the zones do not change in definite and corresponding 

 directions, nor can it be said that one zone replaces another exactly. 



Inversion of Zonation. — An inversion of the zones of vegetation is 

 found occasionally in narrow canyons or on very steep shaded slopes. 

 The upper reaches of such places receive more light and heat than the 



