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 recentty, in the lodgepole and 

 yellow pine belt of eastern Washington and northern Idaho, the study 

 Avas continued on these species^ and at frequent intervals on the larch 

 and Douglas fir in the Missoula region of Montana. The method of 

 investigation w-as as follows: Borings from heavily infected (burled 

 and broomed) and uninfected trees were taken with a Mattison 

 increment borer at 4^ 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 w^ere 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 growtli of forest trees caused by mistletoe, for J^O 

 years, ISl.'f to 1913, inclusive. 



