THE WESTERN YELLOW PINE MISTLETOE. 5 
~ mistletoe infection; 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 
of the group, into the following crown classes: 
Tsolated.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 partl 
from the side; larger than the average trees in the group, and wit 
_ crowns well developed but possibly somewhat crowded. 
Codominant.—Trees with crowns forming the general level of the 
oup canopy and receiving full light from above, but comparatively 
ittle 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 hight 
from above but none from the sides, usually with small crowns 
considerably crowded on the sides. | 
Suppressed.—Trees with crowns below the general level of the 
group canopy and receiving no direct’ light either from above or 
rom 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 infection classes for each year mentioned, and the 
‘difference was computed as the growth during the 5-year period or-. 
the periodic accretion. The average growth per year during the 
or 
. 
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 2 infection 
