THE WESTERN YELLOW PINE MISTI.ETOE. 8 



wind movement, and great daily ranges of atmospheric temperature. 

 The amount of sunshine is unusually high. The mean annual pre- 

 cipitation in the western yellow pine forest amounts to between 20 

 and 25 inches, which occurs in two well-defined periods during July 

 and August in the form of thundershowers and from November to 

 April in the form of snow. The period from about April 15 to July 

 15 is characterized by desiccating southwest winds and excessive 

 evaporation, which, together with the small amount of precipitation, 

 produce conditions adverse to most types of vegetation. The high 

 rate of evaporation, low atmospheric humidity, and high wind 

 velocity very often cause excessive transpiration from plants. The 

 climatic and soil conditions are such that nowhere within the western 

 yellow pine type of the Southwest does this species make rapid 

 growth. 



The influence of such climatological conditions is generally con- 

 sidered unfavorable to the best development of the host and favorable 

 to the distribution and growth of the parasitic mistletoe. Weir ^ 

 has emphasized the fact that the ecological relationships of the 

 mistletoes, although they are parasitic, are similar to those of other 

 cholorphyllaceous plants in that they also respond to light, gravity, 

 and certain chemical stimuli, while the marked variations in tempera- 

 ture to which the host readily responds are only slightly effective on 

 the parasite. The low atmospheric humidity has a very slight 

 influence on the xerophytic mistletoe, while the great amount of 

 sunshine is favorable to the parasite. The unusually high winds of 

 the Southwest frequently cause rather serious losses to western 

 yellow pine stands in exposed situations through windfall, and the 

 wind is known to aid in the dissemination of the mistletoe seed from 

 tree to tree. 



It follows that, where a virulent parasite responds favorably to its 

 habitat, the host will rapidly deteriorate on the more unfavorable 

 sites. From numerous observations made in various parts of the 

 Southwest, it is apparent that a rather definite relation exists 

 between the unfavorableness of the site and the degree of mistletoe 

 infection. The percentage of infection and the resulting mortality 

 of the host is usually higher on exposed dry ridges and south slopes 

 than on the more favorable sites. This condition is very apparent 

 on the Santa Fe National Forest in northern New Mexico. 



THE WESTERN YELLOW PINE MISTLETOE. 



The family Loranthaceae has two genera in the United States, 

 Phoradendron and Razoumofskya. The species of Phoradendron 

 are parasitic on both hardwoods and conifers, but mainly on hard- 



2 Weir, James R. The Larch Mistletoe: Some Economic Considerations of Its Injurious Effects. U. S. 

 Dept. of Agri. Bui. 317, pp. 5-10, 1916. 



