268 



PLANT SOCIOLOGY 



limited to the south, southeast, and southwest slopes of valleys, hills, 

 and hummocks in west-central and northern Europe. Conversely, in 

 southern Europe, even in southern France, the central European forest 

 vegetation is strictly confined to shady north exposures. Classic 

 examples of this are seen in the Montagne de Saint-Baume near 

 Marseilles and on the Pic St. Loup, north of Montpellier. 



The work of Pessin (1925) and of Ochsner (1928) illustrated the role 

 played by exposure in the colonization of tree trunks by epiphytic 

 communities. Pessin determined the differences in evaporation 



between northwest, northeast, and south 

 exposures at various heights, on the trunk 

 of Quercus stellata (Fig. 136). Ochsner 

 examined the epiphytes at various points 

 of the compass on the trunk of Populus 

 nigra on the Swiss lowlands, with the 

 resultant graphic view given in Fig. 137. 



The foliose lichens are localized almost 

 entirely on the shady north side; the 

 Leucodon type of mosses, on the sunny 

 south side; while Orthotrichum and the 

 liverwort Radula occupy the rainy west 

 side. 



Slopes of the Soil. — The slope of the soil 

 surface affects vegetation directly as well 

 as indirectly. 



The indirect effect is due to the influence 

 upon the water supply of the soil and the 

 shifting of the angle of incidence of the sun's 

 rays, modifying the intensity of insolation. With continued clear 

 weather a horizontal plane in Davos at 1,500 m. would receive 141,992 

 gram calories, while one at right angles to the sun would receive 255,061 

 gram calories (Dorno). 



Steepness of slope accelerates the circulation of soil water, which is 

 an advantage to vegetation in humid climates but a disadvantage in 

 arid ones. According to Piwowar (1903), the angle of inclination, 

 however, is not conditioned by grain size but by the fracture and tex- 

 ture of the rook. The more massive, jagged, coarsely grained the 

 steeper the rubble will lie. Thus granite slopes reach an angle of 43 

 deg. ; slate, only 30 deg. 



The steeper the slope the more speedily the superficial water is 

 carried away and the more readily the rubble and the soil itself slide 

 down. 



o 

 o 



I 



10 



t.3 3 9.1 13m, 



Heighf 



Fig. 136. — Differences in 

 evaporation at various heights 

 and in various exposures on 

 the trunk of Quercus stellata. 

 {After Pessin.) 



