LARCH MISTLETOE. 5 



crown and degree of vigor shown by these trees late in life prove 

 conclusively that the ragged, suppressed condition of their neighbors 

 is not due wholly to unfavorable climatic or soil conditions, but to 

 the effect of the mistletoe upon them. 



On some north slopes where the larch is crowded by lodgepole pine 

 and white fir it becomes suppressed for a time very early in life, as 

 indicated by the zone of suppression in the older trees. Those trees 

 finally escaping by their more rapid growth from the influence of 

 their neighbors usually become infected by mistletoe when the crown 

 spreads out to the light and air above. The opportunities for the 

 mistletoe to attack suppressed trees with crowns overtopped by other 

 species not subject to its ravages are not as great as when the trees 

 are standing more in the open. This is due in part to the other 

 species protecting the larch from seed falling on it, and in part to 

 the fact that permanent tissue incapable of being penetrated by the 

 primary sinker is more rapidly developed in the case of suppressed 

 individuals. New growth is of short duration and fewer vulnerable 

 points of easy infection exist. If the infection of the suppressed 

 trees does occur and the infection succeeds for a time, the mistletoe 

 plant may itself become suppressed, partly from a poor nutrient rela- 

 tionship with its host and partly through lack of light, and eventually 

 may die without producing new infections higher up. The signs of 

 old infections are frequently noted in the area of the zone of sup- 

 pression in trees that have afterwards escaped from the crowding of 

 their neighbors. If such trees again become infected later in life, 

 they may attain a fair merchantable size before the influence of the 

 parasite is made manifest. 



PHYSICAL AND CLIMATIC FEATURES OF THE FOREST REGION. 



The later geological history of the Blue Mountains, in which the 

 Whitman National Forest is located, is one of a great basaltic uplift 

 surrounding but not submerging the older granitic formations. The 

 several high and low laterally arranged ridges are composed in the 

 main of granitic rocks, forming a type of soil upon which the yellow 

 pine usually becomes the climax species. Other soil characters in- 

 duced by local variations of climate, slope, and type of ground cover 

 influence the distribution of the forest trees of this region to a 

 marked degree, and indirectly that of the mistletoe. 



Summarizing the chief climatic characteristics of the region, com- 

 piled from the reports of the United States Weather Bureau, they 

 are (1) scanty rainfall, (2) wide range of temperatures, (3) low 

 absolute humidity, (4) rapid evaporation, and (5) an abundance of 

 sunshine. The influence of such climatic conditions may be con- 

 sidered in general as unfavorable in a few localities to the best 



