238 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



fact that the mean annual increment culminates between the years 

 100 and 125, so that a rotation of, say, 110 to 120 years will yield, 

 in the long run, a larger number of cubic feet of solid wood than 

 either a shorter or longer rotation, both in the case of silver fir and, 

 as far as I can judge, also of Douglas fir. 



The above data, it must always be remembered, refer only to the 

 final cuttings. I have no data whatever which would enable me to 

 compare the intermediate returns (thinnings) of Douglas fir and 

 silver fir. I may also draw attention to the fact, that the numbers 

 of cubic feet given above refer to the actual volume of solid wood. 

 In order to obtain the number of cubic feet calculated from the 

 quarter girth, as is usual in this country, the numbers must through- 

 out be reduced byabout one-fourth (or, more accurately, 22 per cent.). 



I have compared the returns of the Douglas fir with those of the 

 silver fir, because we possess accurate tables giving the volume- 

 yield of the latter at difi"erent ages. It would have been more to 

 the purpose to substitute the larch for the silver fir, but un- 

 fortunately the laws of increment of the former have not as yet 

 been so minutely studied and recorded as in the case of the latter. 

 So much, however, is known, that the larch develops much more 

 rapidly than the silver fir during youth, and that it yields larger 

 returns of solid wood under a rotation of seventy-five years, and 

 perhaps even eighty years, in favourable localities ; under a higher 

 rotation the volume-yield of the silver fir is greater than that of 

 larch. Hence it may be safely said, that under a rotation of 

 seventy-five, and perhaps even eighty years, the larch will yield as 

 much solid wood as the Douglas fir whenever they are grown in 

 regular fully stocked woods, and in localities of equal quality — 

 with this difference, that the material yielded by the Douglas fir 

 will consist of a smaller number of trees per acre, with a greater 

 mean diameter per tree. 



The laws of increment of Scots pine are well known. On good 

 localities, like that of Taymount, the growing crop of a fully stocked 

 acre compares as follows with that of Douglas fir : — 



