THE WESTERN YELLOW PINE MISTLETOE. 5 



mistletoe infeotion; XX, medium mistletoe infection; XXX, heavy- 

 mistletoe infection. 



In order to check any material effect that the different crown 

 classes might have on the rate of growth the trees were also classified, 

 according to their dominance or relative position in the crown canopy 

 o.f the group, into the following crown classes: 



Isolated. — ^Trees growing in the open which do not form a contig- 

 uous part of the regular group canopy. 



Dominant. — Trees with crowns extending above the general level 

 of the group canopy and receiving full light from above and partly 

 from the side; larger than the average trees in the group, and with 

 crowns well developed but possibly somewhat crowded. 



Codominant. — Trees with crowns forming the general level of the 

 group canopy and receiving full light from above, but comparatively 

 little from the sides; usually with medium-sized crowns more or less 

 crowded on the sides. 



Intermediate. — Trees with crowns below, but still extending into 

 the general level of the group canopy, receiving a little direct light 

 from above but none from the sides, usually with small crowns 

 considerably crowded on the sides. 



Suppressed. — Trees vfith crowns below the general level of the 

 OToup canopy and receiving no direct light either from above or 

 from the sides; usually with small, poorly developed crowns. 



There are two well-recognized forms of Pinus ponderosa, "black 

 jack" and "yellow pine," based on age, rate of growth, and the 

 resulting color of the bark. The term "black jack" applies to the 

 young, vigorous trees under 125 to 150 years old which are charac- 

 terized by a dark, almost black, or dark brown, narrow-furrowed 

 bark. The "yellow pine" form comprises the older trees, which are 

 characterized by a yellowish or reddish brown, widely furrowed bark. 

 There is a marked difference in the form and volume of black jacks 

 and yellow pines. The average black jack has a greater taper, a 

 more rapid rate of growth, and approximately 10 per cent smaller 

 cubic volume than the average yellow pine of the same diameter 

 and height, which were the chief reasons for the segregation of black 

 jacks and yellow pines. 



The mean diameter growth or accretion of the standing black jacks 

 for the 5-year period from 1910 to 1915, grouped according to degree 

 of mistletoe infection, is shown in Table 1. The diameters were 

 averaged by mfection classes for each year mentioned, and the 

 difference was computed as the growth durmg the 5-year period or 

 the periodic accretion. The average growth per year during the 

 period considered is termed the periodic annual accretion. The data 

 show an almost consistent decrease in the diameter growth of black 

 jack with the degree of mistletoe infection. The slight inconsistency 

 exhibited in the rate of diameter growth of the o and x infection 



