A STUDY OF HEAET-EOT IN" WESTERN HEMLOCK. 23 



higli rate of decay is general in western-hemlock and grand-fir stands 

 and especially so in low, poorly drained bottoms. The hemlock and 

 also the grand fir reacts more favorably in the sites where the factors 

 concerned in the growth of the tree are less favorable to the advance 

 of the attacking fungus. 



The relation which such factors as vigor, crown rating, size, and 

 -volume bear to the inception and progress of decay has never been 

 thoroughly demonstrated. A diflEicult part of this task lies in ob- 

 taining comparable figures to indicate the rating for lack of vigor, 

 the crown rating, the degree of infection, and the degree of injury. 

 By standardizing these factors on numerical data, a basis for com- 

 parison has been secured. In the first part of Table III are arranged 

 the averages of 120 trees, taken from the various plats worked as 

 s:rowing on the river-bottom site. In this table the total height 

 represents a measurement from the ground to the tip of the tree. 

 The crown size is given as the product of the height of the crown by 

 the width. The crown rating and lack of vigor are determined from 

 the field sheets, according to the standard outlines given. Simi- 

 larly, the total volume and the volume of rot determine the rot per- 

 centage. In the c'olumn ''Lack of vigor," the figures represent the 

 relative numerical values, determined in the following manner: All 

 trees (18) of the 101 to 160 age class were grouped together (second 

 part of Table III). Three of these trees had a lack of vigor repre- 

 sented by 00, 10 trees by 000, and 5 trees by 0000. 3(00) equals 

 6(0), 10(000) equals 30(0), and 5(0000) equals 20(0), which gives a 

 total of 56(0). This figure divided by 18 gives 3.1(0), which is indi- 

 cated by 3.1. This gives a numerical basis for the relative values 

 •of lack of vigor in plotting the graphs. A similar method was applied 

 to the data in the column under crown rating, and the results secured 

 were used in the plotting of the graphs (figs. 11 and 12). This method 

 was also used in the "Average degree of injury" column in Table IV 

 and in plotting points for the graphs in figures 11 and 12. 



The process was applied to the trees of the various plats which were 

 growing upon the southwestern-slope sites, and the resulting data 

 are arranged in the second part of Table III. Pathological graphs 

 were then constructed from each of the two parts of Table III, using 

 all the factors concerned and arranging the units in such a manner 

 as to secure the least confusion in following the individual graphs. 

 In the graph in figure 11, constructed from the figures relating to 

 river-bottom plats in Table III, the first points to be noted are that 

 the diameter, height growth, crown size, total volume, and volume 

 of rot all increase with the increase in age. This is found to be true 

 also in figure 12 (with the exception of crown size), which graphically 

 expresses the data given for plats of the southwestern-slope type in 

 Table III. -In searching for those factors most prominent in their 



