Investigating Yield Per Acre. 459 



In humid regions this depends largely on the spread of crowns 

 and crown space occupied. But in dry regions, and on dry soils 

 in regions that may suffer from droughts during the growing sea- 

 son, the determining factor is not crown but root spread. This is 

 true of Longleaf and to a lesser extent of Shortleaf Pine in the 

 South, and of Western Yellow Pine, of the junipers and other 

 western species. 



The principal danger in the sample area method even in even- 

 aged stands lies in the probability that, trying to secure fully 

 stocked stands, in reality the plots so selected may be abnormally 

 well stocked. And when this method is attempted in dry regions, 

 the impossibility of determining the degree of stocking that con- 

 stitutes normality causes the method of small plots to break down. 

 Large plots would have an advantage over small ones in any 

 study, the results of which are intended to apply to natural stands 

 and not artificial plantations, for it is impossible under natural 

 processes to secure stands which at maturity are free from small 

 blanks and which will not fall slightly below the best yields 

 obtainable. On a 40-acre plot, to be sure, average yields could 

 be obtained. The only difficulty with large plots is the age ques- 

 tion. Evenaged stands of 40 acres may be found for certain 

 species but not for those mentioned, nor for many others. To 

 retain the advantages of the large plot and at the same time de- 

 termine the area occupied by each age class in a managed forest, 

 is the purpose of the plan here suggested. The principles made 

 use of are : 



1. Average age may be found for trees which form a rough age 

 class by determining the age of trees which have the average 

 volume for the class. 



2. The area of these rough age classes may be mapped on a 

 large plot, in considerable detail, and the total area for each age 

 class in the plot may be thus determined. 



3. Where it is impossible to do this, the relative crown space 

 occupied by trees of different age classes furnishes a means of get- 

 ting at the per cent, of the total area which the age class occupies. 



Applied to Shortleaf Pine in Arkansas, in stands where the age 

 classes were mixed in a very irregular manner, the following de- 

 tailed plan was used. 



The age classes decided on were four in number ; veteran timber, 



