2 BULLETIN 1112, U. S. DEPARTMENT OF AGEICULTURE. 



serious enemies of the western yellow pine, and it has confronted the 

 forester from the beginning of silvicultural practice in the Southwest. 

 Especially on southern exposures and near the lower limit of the type, 

 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 

 Fpref5ts of Arizona and New Mexico. Monthly Weather Review, vol. 41: 1615-1629, 1914. 



