CHAP. III.] 



Characteristics of Trees 



47 



scientific basis. The first edition of his great work on Sylvi- 

 culture was issued in 1880, and the final expressions of his 

 matured conclusions, in the third edition of 1889, are the 

 source from which most of the following data have been 

 drawn *. 



I. Differences of Forest Trees as to Climatic 

 Requirements. 



The natural causes influencing the geographical distribution 

 of the forest trees of Europe are partly climatic, and partly de- 

 pendent on the physical conditions of soil and situation. The 

 energetic growth and development of any particular species 

 can only take place in localities that have such period of 

 vegetation, and such intensity of summer warmth, as to enable 

 the processes of assimilation and of the formation of ligneous 

 tissue to proceed satisfactorily. For the least exacting species 

 in this respect a three-monthly period of vital activity and 

 a mean summer temperature of 54 to 57 Fahr. are the 

 requisite minima 2 ; whilst on the other hand the minima of 

 the cold during the winter months also fixes their limits, so 

 that large tracts of arctic Russia and Scandinavia are destitute 

 of forest growth. The polar or northern limits of some of 

 the more common forest trees are as follows : 



And, in a similar manner, the reduction of the temperature 



1 Waldbau, 3rd edit., 1889, pp. 18-49. 



2 Weber, Die Aufgaben der Forstwirthschaft, in Lorey's Handbuch der 

 Forst-wissenschaft, 1886, vol. i. p. 13. 



