208 



Forestry Quarterly. 



forest is not able to penetrate by the help of nature can be trans- 

 formed into forests by artificial means. 



If the precipitation exceeds 3.9 inches, dryness of the air can 

 not prevent the existence of the forest. If the land is watered 

 by artificial means or otherwise supplied with water by natural 

 means (irrigation, ground water) a forest will spring up even 

 though the air may be ever so dry during the period of growth. 



On the basis of the average temperature, air humidity, and 

 precipitation during the vegetative period as well as on the basis 

 of the annual temperature, the dates of occurrence of the first 

 and last frosts, and the absolute minimum temperature, Mayr 

 established several forest regions or zones for Europe, America, 

 and Asia. Each zone is characterized by tree species, which have 

 about the same climatic requirements. The different zones are 

 arranged below in tabular form for ready comparison. 



ZONES OF SIMILAR CLIMATE, ELEVATION AND WOODY 



SPECIES OF THE FORESTS OF NORTH AMERICA, 



EUROPE AND ASIA 



[Note: The climatic data are given in the following sequence, a? denot- 

 ing absence of information : average temperature during growing season, 

 May I to Aug. 31; Humidity; Precipitation during growing season; 

 Mean annual temperature; First, fall frost; Last, spring frost; Lowest 

 recorded temperature.] 



I. Cool Region of the Stunted and Dwarfed Trees, Limits of the Forest,— 

 The Alpine Region, — The Polar Region 



Atlantic Region 

 (Northern Canada) 



45°-48°; 80+; ?; 32°; 



NORTH AMERICA. 



Central Region 

 (Rocky Mountains) 



11,500 feet 



?; ?; —49° ?; ?; ?; ?; ?; ?; ?; 



Betula, Alnus, Salix, Pinus flexilis, Pinus 

 Juniperus, Abies bal- albicaulis, Pinus aris- 



smnea, Pinus banksi- 

 ana, Larix laricina. 



tata, Picea pungens 

 and engelmdnni 

 Larix lyalii. 



Pacific Region 

 (Sierra Nevada, 

 Alaska) 



9,900 feet — 1,650-3,960. 

 46°; 90%; 6 in.; 28°; 



?;?;-4° 



Salix, Populus, Alnus, 

 Pinus balfourinana, 

 Larix lyalii, Tsuga 

 mertensiana. 



