TREE GROWTH AND CLIMATE IN THE UNITED STATES 529 



bottom land type ; 4, white pine type ; 5, Engelmann spruce type ; 6, 

 silver pine type ; 7, lodgepole pine type ; 8, sugar pine type ; 9, western 

 yellow pine type; 10, sequoia type; 11, Douglas fir type; 12, southern 

 pine type; 13, southern hardwood types. 



Likewise it is impossible to give absolute figures for evaporation so 

 that the following list is merely a crude classification proceeding from 

 the types which have least evaporation to those that have the most. 

 The length of the growing season and general dryness of the climate 

 are the important factors here: 1, spruce type; 2, northern hardwood 

 type ; 3, Engelmann spruce type ; 4, white pine type ; 5, lodgepole pine 

 type; 6, Douglas fir type; 7, southern hardwood cove type; 8, sequoia 

 type ; 9, southern hardwood slope type ; 10, sugar pine type ; 11, western 

 yellow pine type; 12, southern bottom lands type; 13, southern pine 

 type ; 14, southern hardwood ridge type. 



The standings with reference to total precipitation, evaporation, 

 and run-ofif are mainly useful as far as they give some measure of the 

 amount of available moisture. Since there are no absolute figures for 

 evaporation and run-off and only meager data on precipitation the 

 rating according to available moisture must be rough since it is based 

 uopn relative data only. Arranged in order of the probable amount 

 of available moisture the types are as follows : 1, Douglas fir type ; 

 2, spruce type ; 3, northern hardwood type ; 4, southern bottom land 

 type ; 5, Engelmann spruce type ; 6, white pine type ; 7, silver pine 

 type; 8. southern hardwood cove type; 9, sequoia type; 10, southern 

 hardwood slope type; 11, southern pine type; 12, lodgepole pine type; 

 13, sugar pine type; 14, western yellow pine type; 15, southern hard- 

 wood ridge type. 



In spite of the meagerness of the data now extant it seems feasible 

 to draw some tentative conclusions in regard to climatic conditions 

 and their relation to diameter and height growth, density, and yield. 

 Stated briefly, these are: 



Rapid diameter and height growth require a long growing season 

 and a fair amount of available moisture. For example, the types 

 which have the largest individual specimens answer these require- 

 ments as illustrated by the sequoia type, Douglas fir type, and the 

 southern hardwood coves. 



Density is dependent upon the amount of available moisture and 

 varies directly with it. For example, the only types in which dense 

 stands occur, Douglas fir, northern spruce, Engelmann spruce type, 

 southern bottom land, and white pine type, all have a relatively large 



