ZOB 



A BIOLOGICAL JOURNAL. 



Vol. III. APRIL, 1892. No. i. 



FORMS OF TREES AS DETERMINED BY CLIMATIC 



INFLUENCES. 



BY GUSTAV EISEN. 



A traveler from the Arctics or from the high wooded mountains, 

 in any district of the world, cannot but be impressed by the differ 

 ent forms which trees and shrubs assume in the respective regions. 

 Nowhere is this difference in form more striking than between 

 the trees inhabiting the pine region of Sierra Nevada and those 

 which grow on the lower plains in the interior valleys. 



We have so constantly been accustomed to take things as they 

 ■are, without inquiring into the causes why they are so, that it seems 

 to us quite natural that the forms of trees of the high mountains 

 should be different from those of the lowlands and valleys. Still 

 this difference is so great and so very apparent that the causes which 

 operate in making up these different forms must be very great and 

 very important ones. 



In the high Sierras, for instance, in that region below the snow 

 line, where the pines and spruces dominate, we find that almost 

 every shrub and every tree resembles the other in a general way. 

 The trees are tall and erect, with a central undivided trunk from 

 which the branches slope down towards the ground. The shrubs, 

 again, are low and depressed, spreading out horizontally, form- 

 ing dishlike masses, hugging the ground instead of seeking the 

 sky. A few thousand feet further down in the region where the 

 evergreen pines and spruces have ceased, the trees as well as the 

 shrubs begin to assume a different aspect. The trees in this region 

 are not so erect, their branches are less sloping, their crowns extend 

 further, the trunks are often branching; there is, in fact, a decided 

 difference in their general form. The shrubs, again, are more erect 

 and bushy, forming often dense masses, which show little or no 

 tendency to flatten out. 



