limit represents the upper limits of the data. 

 Extrapolating beyond this upper limit gives 

 unreasonable results; predictions beyond the 

 upper limit are made by assuming the same 

 value as that at the upper diameter limit. 



Tree grade is dependent on two major fac- 

 tors: tree size (diameter), and the presence 

 of grade stoppers such as limbs, knots, decay, 

 sweep, and crook. Tree size alone is a major 

 determinant of grade — a large percentage of 

 the trees qualify for the next higher grade as 

 soon as they reach the minimum diameter. 

 For example, more than 60 percent of the 

 black cherry and yellow-poplar trees that we 

 examined qualified for grades 1, 2, or 3 when 

 they reached the minimum diameter for those 

 grades. This effect of diameter thresholds used 

 in defining grade is evident in Figures 1 to 3. 



However, species differed considerably in 

 the extent to which diameter alone deter- 

 mines grade. Only 15 to 25 percent of the 

 beech and birch qualified for the next higher 

 grade when they reached the minimum diam- 

 eter. Thus, grade stoppers are more common 



in the latter species and more often limit 

 grade after minimum diameter has been at- 

 tained. 



The importance of grade stoppers in beech 

 and birch also is evident in data for trees of 

 large size — well beyond minimum diameter 

 for grade 1 and at or near economic maturity. 

 For example, less than half of the 23-inch 

 beech and birch qualified for grade 1; but 

 more than 90 percent of the black cherry and 

 yellow-poplar qualified at that size. 



The numbers of grade stoppers in individ- 

 ual stands may vary widely due to factors 

 such as density, snow bending, ice breakage, 

 insect or disease attack, and site conditions. 

 So the grade distributions reported here repre- 

 sent average values that one might expect in 

 typical stands — and the values may be in 

 considerable error for an individual stand. 

 Thus, these estimates of grade distribution 

 are not intended to replace tree grading but 

 are to be used as a management planning tool. 



These data were collected, and will apply 

 most accurately, in second-growth cherry- 



Figure 1 — Grade distribution for black cherry/ yellow-poplar group. 



