20 



COMPOSITE TYPE OX THE APACHE NATIONAL EOEEST. 



was much faster than that of the other two species in early life. 

 Also, judging from the slight increase in the periodic annual growth 

 of spruce between 100 and 150 years, it is near the point of culmina- 

 tion. It is seen that the maximum periodic annual growth of yellow 

 pine and Douglas fir is reached at approximately the same time, at 

 260 years. For the last 60 years, however, this has been increasing 

 very slowly. In spruce the maximum periodic annual growth does 

 not occur until some time after 140 years. It. too. is increasing 

 slowly, and from the indications its maximum will be reached at an 

 age not to exceed ISO years. 



Fig. 4.— Increment growth of western yellow pine, Douglas fir. and blue spruce. 



At the age of 320 years hi Douglas fir and at 360 years in yellow 

 pine, the mean annual increment does not yet equal the periodic 

 annual increment. In spruce there is a great difference between the 

 two at 150 years. As may be seen in Figure 4. the curves represent- 

 ing the periodic annual increment cross the curves representing the 

 mean annual increment at the age of 377 years in western yellow pine, 

 at 383 years in Douglas fir. and at 240 years in blue spruce. In 

 each case the mean annual growth reaches its maximum at this point. 

 It may also be seen that after this time the periodic annual increment 

 decreases more rapidly than the mean annual increment. 



