2 BULLETIN 1112, U. S. DEPARTMENT OF AGRICULTURE. 
serious enemies of the western yellow pine, and it has confronted the 
_ forester from the beginning of silvicultural practice in the Southwest. 3 
Especially on southern exposures and near the lower limit of the type, # F 
areas may be found on which fully 75 to 90 per cent of all the trees — 
above 6 inches in diameter are infected with mistletoe. 
In marking western yellow pine, there is often a question as to 
the advisability of cutting trees of various ages and degrees of mis- 
tletoe infection, or of allowing them to remain as seed trees and form 
a part of the future stand which will provide the basis for a subsequent 
cut. This problem involves not only a study of the effect of mistletoe 
on the rate of growth and the longevity of its host, but also of the 
effect of the parasitism on the amount and periodicity of seed pro- 
duction. 
PHYSICAL AND CLIMATIC FEATURES OF THE REGION. 
The areas in which the intensive studies were conducted lie within 
the Coconino and Tusayan National Forests on the Colorado Plateau, 
near the San Francisco Mountains in northern Arizona. The general 
topography of the plateau is rolling and consists of alternating flats 
comparatively free from rock and of rather low ridges which are very 
rocky. The underlying rock of the plateau consists principally of 
Coconino sandstone and Kaibab limestone overlaid by a lava flow of 
basalt and acidic volcanic rocks, mainly dacite, andesite, rhyolite, 
and latite, which constitute the main outcrops and surface rocks of 
the region. The soil varies widely from a fine sandy loam to a red- 
dish friable clay, often containing an admixture of gravel and large 
volcanic bowlders. In the greater part of the region northeast of the 
San Francisco Mountains the soil is covered with black volcanic 
cinders to depths varying from one-half inch to about 18 inches. 
Mistletoe infection is rare on these cinder areas and where found at 
all is light. The level mesas and lower slopes of the plateau, ranging 
from approximately 6,500 to 8,500 feet in elevation, are covered with 
a forest, of which approximately 95 per cent is western yellow pine, 
constituting the most extensive forest of this species in North 
America. 
The climate of that portion of the Colorado Plateau which is 
covered with a western yellow pine forest is indicated by the climato- 
logical data which have been secured at the Fort Valley Forest 
Experiment Station. Pearson’s! studies in this locality, together 
with the records of the United States Weather Bureau throughout 
the region, have shown that the climate is characterized by marked 
seasonal variations in precipitation, atmospheric humidity, and 
1 Pearson, G. A. A Meteorological Study of Parks and Timbered Areas in the Western Yellow Pine 
Forests of Arizona and New Mexico. Monthly Weather Review, vol. 41: 1615-1629, 1914. 
