236 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



Taking these facts into consideration, the following numbers of 

 Douglas firs per acre have been estimated : — 



By multiplying these numbers with the mean sectional area per 

 tree, the total sectional areas of all trees per acre are obtained. 



(b.) Height. — The following facts are at our disposal : (1.) A 

 Douglas fir thirty-two years old has a mean height of 60 feet ; 

 mean annual height growth = 22^ inches ; (2.) a Douglas fir fifty- 

 seven years old has a mean height of 90 feet ; mean annual height 

 growth (from thirty-second to fifty-seventh year) = 14 inches ; both 

 grown in Perthshire. (3.) Mature Douglas firs in the most favour- 

 able localities of North America are reported to reach a mean 

 height of 213 feet ; such trees are often up to 500 years old, but 

 it is not known at what age the height growth ceases ; let us 

 assume that it is very slight after the age of 250 years. With 

 these facts before us, the heights up to an age of 140 years may be 

 estimated as follows : — 



(c.) Form Figure. — The form figures for silver fir are : — 



The form figure of the Taymount plantation shows ^39 for an 

 age of thirty-two years. This is very considerably below the form 

 figure for silver fir, and it is in accordance with the facts of the 

 case. The Douglas fir is a much more tapering tree than the 



