THE WESTERN YELLOW PINE MISTLETOE. aa: 
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 Loranthacee 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. Bul. 317, pp. 5-10, 1916. 
