262 



PLANT SOCIOLOGY 



Research on the dependence of the competitive power of plants 

 (germinating capacity and growth of seedlings) upon altitude has been 

 carried on by Hofmann (1921), in the United States with Pseudotsuga 

 taxifolia. He found that the germinating capacity and growth of the 

 seedlings decreased regularly with increasing altitude (Table 30). 



Table 30. — Influence of Altitude of the Parent Trees upon Germination 



IN Pseudotsuga taxifolia 



Altitude of parent 

 trees, feet 



Germination of 

 seed, per cent 



Growth of one-year 

 seedlings, inches 



500 

 2,600 

 3,000 

 3,800 



1.6 

 1.6 

 1.5 

 1.3 



If the climate tends toward oceanic conditions, as in the case of 

 isolated mountain summits, most (but not all) of the height limits are 

 lowered. Plateaus which favor continentality, on the other hand, 

 usually show a rise of height limits. 



Plateaus. — The elevation of masses, that is, the mean rise of a very 

 considerable region of the earth's surface, has a decided reaction upon 

 all temperature conditions. On high plateaus, and in mountains 

 which rise from plateaus, the isothermic lines are high. Thus the 

 isothermic lines are higher on the Colorado Rocky Mountains which 

 rise from plains with altitudes of 1,200 to 1,500 m. than they would be 

 if the mountains rose from plains at sea level. This is due to increased 

 irradiation and dynamic heating and to reduced cooling of the high 

 surfaces. This favorable heat relation is the cause of the rise of any 

 plant limits in the heart of mountains where the mean elevation of 

 surrounding areas is great. Christ (1879) speaks therefore of the high 

 surface of Orisons as the "Swiss Tibet" and calls attention to the high 

 timber line in the Engadine and the Miinster valleys. Next to the 

 Engadine, in the Alps, the Monte Rosa region shows a considerable 

 rise of the isothermic surfaces. Detailed maps of the isotherms and 

 tree limits in Switzerland clearly show their correspondence to the 

 distribution of mass elevation. The altitudinal distribution of the 

 most important alpine plant communities also agrees with this scheme, 

 as far as it is known. For example, while the maximal occurrence of 

 the Curvuletum in the center of the Rhetic mountains is at 3,000 to 

 3,100 m. and in the central Valais at 3,100 to 3,200 m., it occurs in 

 central Grisons between 2,800 to 2,900 m. and even between 2,600 to 

 2,700 m. in northern Grisons. 



