SITE DETERMINATION, CLASSIFICATION, AND APPLICATION 559 



Group D will about cover the Rocky Mountains, Idaho, Utah. 

 Arizona. 



Group E will do for the spruce and balsam of Canada, for the 

 pinon and mesquite of the Southwest, for the scrub oak in the North 

 and South, for blue beech, etc. 



Group F is intended to take care of those trees — such as cotton - 

 wood on the river bottoms, loblolly pine on old fields, etc. — which make 

 an extraordinarily rapid growth during youth, but which is not main- 

 tained. As a result of this rapid growth during youth, the rotation, 

 which will be used in handling these species, will probably be set low ; 

 and it is desirable that the age, which is used in the determinations of 

 site, is set near the rotation. For these reasons, the age of determina- 

 tion is set, not at lUO years, as in all other groups, but at forty years, 

 lliis figure is based upon such meager information as is at hand, but 

 it is believed to be approximately correct. 



In all other cases the age of determination is placed at 100 years. 

 This figure is determined upon not only because this is standard in 

 Europe but also because most trees are holding pretty steadily to their 

 height growth up to this age, as was found by comparing and examin- 

 ing the growth of trees from different parts of the country. 



In order to determine the applicability of this system of grouping, 

 the heights of western yellow pine trees at 100 years were obtained 

 from many regions throughout the West and divided according to the 

 system proposed. The figures were obtained from many sources, and 

 since the quality of site upon which the trees grew was seldom men- 

 tioned in the tables, it was considered that they grew on sites which 

 were better than the average of the region. The figures are, therefore, 

 referenced to Site Quality II : 



Height of Western Yellow Pine Trees in Different Regions. Age, 100 years. 

 Region Height in Feet 



Central Oregon 43 



Crook County, Oregon 48 



Prescott National Forest, Arizona 53 



Apache National Forest, Arizona 54 



Black Hills, South Dakota 56 



Whitman National Forest, Oregon 58 



Sisters, Oregon ' . 59 



Montana 66 



Klamath, Oregon 69 



Wasco County, Oregon 70 



Union County, Oregon 79 



Stevens County, Washington 100 



Butte County, California 1 14 



