New Viezv Points in Silviculture. 215 



EUROPE 



(South coast, islands of west coast of middle Europe) 



68° — 72°; 50 — 60%; 2 — 4in. ; 61° — 66°; Dec; Feb.; 23°. 



Quercus suber, etc., Q. Ilex, Larus nohilis. Arbutus Unedo, Buxus, Cera- 

 tonia, Olea, Cupressus fastigiata, Pinus canariensis, pinea, maritima, 

 aleppensis, Chamaer ops-palms. 



That distribution of the forest depends in a most remarkable 

 degree upon humidity is clearly seen from the fact that all large 

 forest regions of the old and new world are confined to the 

 oceans and seas. Thus the European forest region owes its 

 existence to the Atlantic ocean ; the eastern Asiatic forest region 

 to the Pacific ocean ; the Indian to the Indian ocean ; and the 

 Eastern and Pacific forests of North America to the Atlantic 

 and to the Pacific oceans. 



There is no doubt that temperature and especially air hu- 

 midity play a most important part in distribution of these for- 

 ests, yet it is not always possible with the aid of the climatic fac- 

 tor alone to explain in every case the natural treeless condition 

 of certain regions. The soil conditions must also be taken into 

 account. As an example may be cited our prairies or the Russian 

 steppes. Thus a large part of our prairies and of the Russian 

 steppes as far as the climate is concerned could support forest 

 growth, yet no forest grows there or if it does it is only on 

 definite situations. Under identical climatic conditions in one 

 place the forest artificially established perishes in spite of all 

 human care, while in another place it thrives. That the presence 

 of large quantities of soluble salts in the soil may be the cause 

 of the treeless condition of a large portion of our prairies, may 

 be inferred from Hilgard's studies. In California, according to 

 Hilgard, at a precipitation of 500 millimeters, salts of sodium 

 are washed out by the ground waters, while at a precipitation of 

 less than 500 millimeters these sodium salts accumulate to a dif- 

 ferent depth of the soil and become injurious to tree growth. 



Mayr himself admits that the area of prairies goes outside of 

 the region of deficient precipitation (1.95 inches) and low air 

 humidity (below 50 per cent). He, however, ascribes this to the 

 prairie fires, which pushed the forest backward. This very often 

 repeated argument for the treeless condition of our prairies and 

 the Russian steppes was really never based on accurate obser- 

 vations, while the presence of soluble salts in the soil as the 



