26 BULLETIN 722, U. S. DEPARTMENT OF AGRICULTURE. 



ill degree of injury with an increase in the degree of infection and with 

 increased age. With increase m age the cumulative chances for 

 infection due to injuries of all kinds, and especially to branch stubs, 

 increase appreciably, and it is but natural that the older trees bearing 

 many injuries and a liigh degree of injury should show an equally 

 high degree of infection. 



The two types of stand compared on the basis of the amoimt of 

 injmy are found to vary but little in the general relation between 

 the degree of infection and the degree of injury. In both types the 

 groups of higher rot percentage also show a higher degree of injury, 

 and similarly the groups of lower rot percentage show a smaller degree 

 of injury. In both sites the infections traced to branch stubs bore 

 the largest percentage and the frost cracks the smallest. Broken 

 tops in both instances came second in importance. 



In the southwestern-slope type (Table IV), the percentage attrib- 

 uted to broken tops was larger than in the river-bottom type (Table 

 IV), due to the more exposed location and to the older stand. Infec- 

 tions traced tO branch stubs in the slope type equaled 72.8 per cent 

 of the total, while in the river-bottom type it equaled 91.5 per cent. 

 It would appear that in spite of the younger age class the river- 

 bottom type developed more infection-producing branch stubs than 

 the other. A reason for this may be found in the fact that the 

 crowded, suppressed condition of the river-bottom stand was much 

 more favorable to the infection of branch stubs than the other more 

 open type of stand. The high proportion of branch-stub infections 

 to injury mfections can be partly explained by the fact that the 

 trees of the river-bottom type, being younger, had fewer injuries. 



In the slope type the largest amount of injury was found in the 

 oldest age class (201 years and older), and in the river-bottom type 

 it was also found in the older age class (101 to 160 years). In the 

 slope type 10 per cent of the trees were uninfected, and in the river- 

 bottom type a much smaller percentage (3) was uninfected, showing 

 by this comparison a more favorable environment for the attacking 

 fungus in the river-bottom sites. 



The slope type exhibited more frost cracks, broken tops, and mis- 

 cellaneous injuries to the stand than the river-bottom type, which is 

 due partly to the older age and partly to the more exposed situation. 

 The wind plays an important part in both the formation of frost 

 cracks and in the broken-top condition of many of the trees. It was 

 particularly interesting to note that most of the oldest and largest 

 frost cracks were fomid to have formed in the hollows between the 

 root spurs. This seems to be more general in the slope type, where 

 the exposure to high winds and the height of the t^ ees (in connection 

 with low temperature) appears to play an important part in their 



