28 BULLETIN 275, U. S. DEPARTMENT OF AGRICULTURE. 



to be employed and secondarily to gain reliable material to be used 

 in a chain of similar studies on different types, from which conserva- 

 tive interpretation may derive final conclusions. They are given 

 here as examples only, as illustrations of the general considerations 

 presented in this bulletin. Since making these studies, others on a 

 larger scale and on different types in different parts of the range of 

 white fir were carried out with improved methods during the summer 

 of 1913. 



A short review of our present knowledge of the pathology of white 

 fir will be of help in the discussion and interpretation of the data 

 presented in this paper. 



White fir (Abies concolor (Gord.) Parry) can not be called a decadent 

 species; on the contrary, it is very aggressive and possesses remarkable 

 elasticity and resistance to injurious influences. Its tolerance to 

 shade is well known. Not only does it survive dense shading to a 

 high age but it responds readily to light and then reaches considerable 

 diameter and height in perfect health, provided it has not been seri- 

 ously wounded and infected. The oldest tree examined in this 

 study was 258 years old; it was badly suppressed and but 67 feet 

 high with a diameter breast high of 17.4 inches. In the trees tallied 

 between the ages of 130 and 140 years, the diameter breast high 

 varied from 8.8 to 30.7 inches and the height from 47.5 feet to 125 

 feet. 



In speaking of tolerance which allows suppressed trees to reach a 

 high age under the most unfavorable conditions and with a minimum 

 of annual growth, the behavior of the species with relation to light 

 is generally meant; but this toning down of the life functions of the 

 tree, visibly expressed in growth, very often has nothing to do with 

 lack of light. A tree may become suppressed to the lower limits of 

 its tolerance by any agent severely attacking any of its vital organs. 

 Of these, the organs of the crown are both most easily accessible and 

 most sensitive; hence, the heavy damage resulting from a serious 

 attack by insects devouring and killing the foliage or by leaf-inhabit- 

 ing fungi. White fir is subject to a disease of the foliage caused by 

 LopJiodermium nervisequium, which often kills all needles except 

 those of the current year's growth. The loss in foliage surface, and 

 therefore in photosynthetic capacity, may suppress a white fir just 

 as lack of light would. Witches' -brooms caused by Peridermium 

 elatinum on white fir are not common. They are very rarely of such 

 development on the tree that they should be classed as an injurious 

 factor to be reckoned with. Incidentally, it should be mentioned that 

 the swellings and cankers so commonly connected with Peridermium 

 elatinum on Abies pectinata in Europe are unknown in Abies concolor. 

 On the other hand, very similar swellings and cankers caused on white 

 fir by Razoumofskya abietina Englm. are extremely common. The 



