THE WESTERN YELLOW FIFE MISTLETOE. 



17 



The data presented in the preceding pages show that, although 

 minor fluctuations are exhibited, the rate of growth of the host 

 decreases with the amount of mistletoe infection; or, in other words, 

 is dependent on the degree of parasitism. Possible exceptions may 

 be cases of very light infection which appear to act as stimulants for 

 the host tree. These results are really to be expected when the 

 effects of mistletoe parasitism are fully understood. 



Table 11. — The effect of mistletoe infection on the rate of growth of western yellow pine 

 on two intensive sample plots aggregating 24 acres on the Tusayan National Forest. 



Degree of infection. 



Average 



diameter 



growth, 



1914-1919. 



Periodic 



annual 



increment, 



1914-1919. 



Periodic 



annual 



increment, 



1914-1919.1 



Total 

 volume of 

 class, 1919. 



Basis 

 (number 

 of trees, 

 4 inches 

 and over 

 d.b.h.). 



. . . . 



Inches. 

 0.70 



.88 



Cubicfeet. 

 4.56 

 5.65 

 2.23 



Per cent. 

 2.54 

 2.79 

 1.83 



Cubicfeet. 



1,636.2 



553.9 



254.5 



359 





98 



XXX 



.49 



114 



1 On cubic-foot basis. 



Note. — Since there were only 13 trees classed as moderately infected, they were omitted because of 

 the small basis. 



EFFECT ON LIMBS. 



The first visible effect of the infection of a limb by mistletoe is the 

 formation of a fusiform swelhng (PI. II) about the center of infection 

 similar to those occasionally found on stems. This hypertrophy is 

 the first stage of a later abnormal growth of branches from the en- 

 larged portion, which results in the formation of witches' brooms. 

 (PI. Ill, Fig. 1.) When the stimulus of the parasite has once become 

 manifest through the formation of a broom, the tendency toward 

 abnormal branching continues, although the aerial parts of the 

 mistletoe have may died. 



The comparative diameter growth or accretion of 54 limbs on 18 

 standing black jacks is averaged by infection classes in Table 12. 

 Measurements were taken at 6 inches from the bole of the tree and 

 at 6-inch intervals beyond this point. The measurements taken at 

 6 to 18 inches from the bole of the tree were averaged and are pre- 

 sented in the table to facilitate a clearer and more comprehensive in- 

 terpretation of the significant data. Limb measurements were also 

 taken on the felled trees to check the comparative growth of the 

 healthy and mistletoe-infected limbs of the standing trees. From 

 these data, which are summarized in Table 13, it is apparent that the 

 limbs had been infected at least 25 years prior to the time at which 

 they were analyzed. The data presented in both tables show that, 

 for the limbs examined, the rate of growth in diameter of limbs 

 infected with mistletoe is greater than that of uninfected limbs. It is 

 3600°— 22— Bull. 1112 3 



