264 PLANT SOCIOLOGY 



It is well known that the differences in insolation and light absorp- 

 tion between south and north slopes in the mountains are very con- 

 siderable, much greater than in the plains. The total light in a south 

 exposure reaches 1.6 to 2.3 times higher values than in a north exposure 

 (Riibel, 1908). The sharp distinction of the vegetation between the 

 north and south slope is therefore not only due to temperature but due 

 also to this distribution of light (Fig. 132). 



In the higher chains of the Central Alps turfs of Festuca varia, F. 

 spadicea, F. alpestris are restricted almost exclusively to hot, dry, or 

 rocky south, southeast, and southwest slopes. On north exposures 

 tall herbaceous communities of Adenostijles tomentosa, dwarf heaths, 

 and clumps of Luzula spadicea take their place. The altitudinal 

 distribution of these plant communities cannot profitably be compared, 



/IB H 



Fig. 132. — Differences in insolation; C-D receives tlie strongest radiation; D-E the 



least; EFG lies in the shade. (After De Martonne.) 



because they react so differently to the exposure of their habitats. 

 The enormous advantages of south, southeast, and southwest slopes 

 to organic life as a whole are very apparent. In the eastern Swiss Alps 

 vascular plants ascend 400 to 500 m. higher on south than on north 

 exposures. 



The mean climatic boundary lines lie lower on south exposures and 

 higher on north exposures than the theoretical orographic limits seem 

 to require. Occasionally, some notable deviations are observed. 

 Thus in the upper Engadine the orographic limit of alpine meadow on 

 south exposures is above 2,900 m.; on north exposures it is at about 

 2,600 m. The Cat-ex curvula association ascends to 3,050 m. on the 

 south side of the Piz Sesvenna but stops at 2,650 m. on the north side. 

 The summit flora of the highest elevations crowds together in the 

 protected niches of the south side, in search of warmth. 



Likewise the highest tilled lands, orchards and vineyards, lie on the 

 south slopes with the highest winter retreats of civilization. Shaded 

 slopes in the mountains are gladly left to the forests. 



In dry regions, even in southern Europe, the shaded side affords 

 better conditions for forest growth and regeneration, and it bears a 

 rich, varied mesophytic flora, which in turn reacts upon humus forma- 

 tion and the acidity of the soil. 



