OROGRAPHIC FACTORS 



263 



Exposure. — ^The course of the altitudinal limits of many plants and 

 plant communities, with their irregular variations, is chiefly a function 

 of inclination toward the sky. The graphic presentation of the 

 melting of snow (Fig. 130) shows the advantage of south as compared 

 with north exposure. A. P. de Candolle (1856) and Sendtner (1854) 

 have recognized this. Sendtner devoted much time and patience to the 

 determination of altitudinal Umits in various exposures. In southern 

 Bavaria he found the highest limits almost constantly on the south- 

 west; the lowest, on north and northeast slopes, with a maximal 

 difference of 213 m. for Picea excelsa and of 140 m. for Alnus viridis. 



Fig. 131. — Timber lines on varioua exposures in the Lower Engadine Dolonaites, 

 Switzerland: showing the climax regions of the Pinus-Larix forests {A), of the Caricion 

 curvulae (S), and of the crustose lichen communities (C). 



The differences in the upward limits of the tree line in the east- 

 central Swiss Alps are much smaller. In fact, as shown in Fig. 131, 

 the upper boundary of the stone pine on a north exposure lies even 

 higher than on any other exposure. 



From this fact it follows that vegetation reacts variously to expo- 

 sure, so that a correction of all altitude limits to a mean exposure 

 cannot be made. Heliophytes and light-loving communities reach 

 their highest limits on southern exposures, while shade plants such as 

 Pinus cemhra and shade-loving communities such as the Empetreto- 

 Vaccinietum, Polytrichetum sexangularis reach higher limits on north, 

 northeast, and northwest exposures. 



