ot oe ee ge oe 
2 BULLETIN 360, U. S. DEPARTMENT OF AGRICULTURE. 
GENERAL NATURE OF THE MISTLETOE INJURY. 
The general nature of the injury to forest growth by these para- 
sites principally consists sooner or later in a localization and gradual 
reduction of the assimilatory leaf surface of the host. As will be 
shown, this is caused by various burl and broom formations on the 
trunks and branches. The reduction of the leaf surface causes a 
falling off of the annual increment. During the progress of a study 
on the larch mistletoe in the Whitman National Forest, Oreg., in the 
summer of 1913, many data on the retardation of growth of its host 
by this parasite were assembled. More recently, in the lodgepole and 
yellow pine belt of eastern Washington and northern Idaho, the study 
was continued on these species, and at frequent intervals on the larch 
and Douglas fir in the Missoula region of Montana. The methed of 
investigation was as follows: Borings from heavily infected (burled 
and broomed) and uninfected trees were taken with a Mattison 
increment borer at 44 feet from the ground, at which point the 
trees were calipered. With practice the eccentricity of growth due to 
slope, unequal crown development, injuries, etc., may be very skill- 
fully judged, so that it is possible to strike the pith of trees within 
the range of the borer with a fair degree of accuracy. In order to 
determine as nearly as possible the average radius, in the more doubt- 
ful cases three borings were taken. On steep slopes the eccentricity of 
trees may be more accurately judged than on flat land, through the 
knowledge that more rapid growth takes place on the downhill side 
of the tree. Height was computed with the Klaussner height meas- 
urer. Trees of the same species were selected as near as possible from 
the same type of stand and of the same general age class and the same 
soil conditions. Only dominant trees free from serious wounds and 
other possible causes of deterioration were recorded. Finding that 
the effects of the mistletoe on the increment of the host could be read 
from the last 40 years’ growth of the age classes and conditions of 
infection selected, Table I was prepared. 
TABLE I.—The retardation. of growth of forest trees caused by mistletoe, for 40 
years, 1874 to 1913, inclusive. 
Average. 
Basis 
Pe num- 
Host and condition. Me of Diameter} Total 
trees). |Ageclass.| Height. | breast annual 
high. growth. 
Pinus contorta: Years. Feet. Inches. Inches. 
Aa USYeICeTO LP Areas Spe kU! SN Nea) ie eee peer et 50 65 35. 2 6.3 0.93 
Uminftectedt-teey ve Se he See he sk Shs a eee 50 60 48.5 7.8 2.93 
Pinus ponderosa: 
infec toda nat ee see ee epithe ch 50 100 49.5 18.2 | 1.54 
Wmintectede 4 eee e ee ces ar 4 Se oe on. ee eee 50 100 Cilia” PAB. Sho 
Larix occidentalis: 
Anfectedsie, Las ts eee cee eee ed age Je repens 80 144 63.0 ES 1.28 
Wimin fected ear ee ee a a hed a ee 80 144 115.0 19.5 2.154 
Pseudotsuga taxifolia 
Trifecta aa. ee cee eee ee se a8. a | 40 97 62.0 yess" 2.175 
Uninfected iss. 5c ssc ee eeen sane ese Seen eee 40 97 73.0 22.2 3.28 
