20 BULLETIN 799, U. S. DEPARTMENT OF AGRICULTURE. 
and, of course, infection continues to take place in all stands after the 
50-year period is reached, and no doubt at an increased rate. 
The other consideration of importance to the forester is the period 
in the age of the stand at which the net increment of sound material 
passes its maximum. This period is more or less indefinite and diffi- 
cult to express in actual age figures. It depends primarily upon 
what the forester considers the dividing line between a stand having 
sufficient sound material to pay a profit for logging and one having 
so much rot that logging would not be profitable or the profit a small 
one, all factors considered. From the viewpoint of annual incre- 
ment the forester has determined that a rotation of 100 to 120 years 
for western white pine gives a maximum yield. 1 If this holds true, 
then it also appears that this will mean the cutting of the stand when 
the average rot percentage for the bottom sites is 1.7 and for the 
slope sites 0.5. Glancing at Table I it is seen that for both types 
of site an appreciable increase in the average annual increase in 
rot volume is recorded between the 101 to 120 and the 121 to 160 
age classes. 
In the sporophore summaries (Table VI) an apparent maximum 
point is reached in the 121 to 160 age class in respect to the number 
of sporophore-bearing trees and to the number of sporphores pro- 
duced. The 101 to 120 age class would, therefore, represent the age 
class having, on the average, a stage of development of the rot below 
the maximum stage for sporophore production. These facts point 
to the 101 to 120 age class as a possible felling age from the patho- 
logical point of view, and it remains to discover whether an average 
rot percentage of 1.7 on the bottom sites and 0.5 per cent in the slope 
sites conforms to the most economical logging or whether a higher 
rot or cull percentage is possible without a sacrifice in the returns 
on the operation. The next higher age class (121 to 160) records 
a rot percentage of 3.1 for the bottom sites and 2.3 for the slope sites. 
In the relation of age to injuries the data have shown that with 
increased age comes a greater degree of injury. This is evident for 
both types of site in all the age classes excepting the 161 to 200, 
which drops slightly below the preceding age class in degree of in- 
jury. It is so well understood that increased age brings cumulative 
risks of greater injury that a discussion seems unnecessary. 
A steady increase in the number of trees with injuries to which 
infection was traced is noted with increase in age for both the bot- 
tom and slope types of site. 
It is readily seen that age up to a certain limit has a definite re- 
lation to the sporophore production of a stand. If rot increases with 
1 Mason, D. T. Management of western white pine. In Proc. Soe. Amer. Foresters, 
v. 9, no. 1, p. 61. 1914. 
