REPRODUCTION OF WESTERN YELLOW PINE 283 



to 29.61 feet. The latter, while not expressed in our commercial unit, 

 are theoretically more accurate than the former, since they are not sub- 

 ject to the well known inaccuracies of log scales. The increments as 

 expressed in the two units are not strictly comparable because the 

 board-measure increment considers only merchantable content, while 

 the cubic-measure increment considers the entire stem, including stump. 

 It should be noted also that cubic volumes in Table 5 include all trees 

 down to four inches in diameter, while board-measure volumes include 

 only the trees 12 inches and over. 



The increment data presented in Table 5 will appeal to the silvicul- 

 turist as something tangible and usable in his work. These are doubt- 

 less the most reliable data available for the region covered, but they 

 should be accepted with a full understanding of their limitations, which 

 are briefly outlined in the following paragraph. 



Growth data based upon a period of five years cannot be regarded 

 as representing the normal rate of growth during a rotation. Cli- 

 nial'c conditions during the period covered may be above or below 

 normal. Investigations by Douglass^ show pronounced fluctuations in 

 the width of annual rings, wiiich have been associated with periodic 

 fluctuations in precipitation. Aloreover, assuming constant climatic 

 conditions, the rate of growth in an even-aged cut-over stand would 

 vary over a period of 20 or 30 years, due to competition between the 

 trees for moisture and light. Considering competition alone, we 

 should expect a great acceleration of growth in individual trees im- 

 mediately after cutting, and a gradual falling off as the density ap- 

 proaches that of a fully stocked stand. There would also be a falling 

 off in those trees which passed the age of maximum growth rate. 

 Greenamyre found from stump analyses on a 26-year-old cutting near 

 Flagstaff that a marked increase in diameter growth took place the 

 first year after cutting and continued up to the time of the investiga- 

 tion. He estimates the period of accelerated growth at 40 years. The 

 greatest response was shown by trees 40 to 80 years old, but was evi- 

 dent in trees 200 years old. The increment of a stand as a whole 

 would, assuming a normal rate of restocking, be affected more by the 

 entrance of new trees than by the variation in growth of individuals 

 already on the ground. Therefore, while the slowing up in individuals 

 after a period of 30 or 40 years would tend to decrease the current 



'"The Climatic Factor as Illustrated in Arid America." Prof. Ellsworth Hun- 

 tington, with contributions hy Charles Schuchcrt, .'Kndrcw E. Douglass, and 

 Charles J. Kullmer. Carnegie Institution, Washington, D. C. 



