MISTLETOE INJURY TO CONIFERS. 



29 



girdled area and initiates an abnormal swelling (fig. 26). The swell- 

 ing continues to increase in size and weight, likewise all members of 

 the crown, so that eventually the slender stem below can no longer 

 support the overdeveloped crown and is broken down by the wind. 

 A specimen in the laboratory shows the number of rings of the stem 

 at the girdled area at the time it was cut to be eight, with a diameter 

 of 1 inch. The swelling just above and within the same internode 

 showed 15 rings, with a diameter of 3 inches. The same phenomenon 

 is sometimes produced in yel- 

 low pine by Peridermium fila- 

 mentosum Pk. When it is re- 

 called that the cambium and 

 the outer wood of the girdled 

 area are actually dead, the 

 length of time the crown con- 

 tinues alive is really remark- 

 able. 



In point of general insect at- 

 tack it has been noted that the 

 beginning of an infestation 

 may start with trees badly 

 suppressed by mistletoe. The 

 fact that trees heavily sup- 

 pressed by mistletoe have a 

 weak flow of sap causes them 

 to be first selected by certain 

 forest-tree insects. For this 

 reason mistletoe areas form 

 centers from which infesta- 

 tions may spread. Again, nu- 

 merous infestations may start 

 simultaneously over a wide 

 territory, owing to the weak- 

 ening of the trees by these par- 

 asites instead of from a few 

 detached areas, as is often the 

 case. This has been found par- 

 ticularly true in the case of yellow pine and the red turpentine beetle 

 mentioned above. In all regions of heavy mistletoe infection of the 

 Douglas fir, Dendroctonus pseudotsuga Hopk. is usually very abun- 

 dant. This was the rule in the Whitman National Forest, Oreg., and 

 though the numerous dead trees of this species in the forest were 

 undoubtedly the result of an immediate attack by the beetles, their 

 work was hastened, it seemed, by the serious mistletoe suppression 

 which was exhibited by most of the dead trees. During the season 



PIG. 26. A young yellow pine, showing com- 

 plete girdling of the stem by a combined at- 

 tack of mistletoe and insects. The cambium 

 is destroyed, but the crown remains alive and 

 continues to elaborate food materials, which 

 are stored just above the girdled area. 



