SITE DETERMINATION, CLASSIFICATION, AND APPLICATION 561 



In the example given, trees on Site I are 135 feet tall when 100 

 years old. They therefore fall into Group B. Trees on Site II are 

 then, according to the site classification, 105 feet tall at 100 years ; on 

 Site III they are 85 feet tall, and on Site IV they are 65 feet tall. 

 These figures thus determine the location of the curves at the 100- 

 year point. 



It is believed that it may safely be assumed that on the average the 

 diameter varies with the age of the tree. All diameter limit planning 

 presupposes this. So on these curves the diameters are written in under 

 the ages to which they correspond. 



At present, in most of the Forest Service timber survey work, 

 trees are tallied not only by diameters but also by their merchantable 

 height. As a result, by the use of a constant figure of addition, the 

 total height may be obtained within an error of a very few feet. Or 

 the estimator may occasionally determine not only the merchantable 

 length of the tree but also the total height. It is evident that the 

 estimator is continually obtaining the data necessary for the determina- 



