To derive this equation, the yield curves were plotted for each level of site 

 index. Then a series of lines was drawn, passing through the origin of coordinates 

 and tangent to each yield curve. From the point of tangency the following was read: 

 (a) the age of culmination of mean annual increment, and (b) the yield at that age for 

 each level of site index. The mean annual increment at its culmination age for each 

 site index level was obtained by dividing (a) into (b) . The relationship between site 

 index and yield capability was approximated by the least squares fitting of a suitable 

 curve form to the series of points plotted from site index and yield capability. This 

 is shown in figure 7. 



ENGELMANN SPRUCE 



Site Index 



An equation with which site index can be estimated directly from measurements of 

 tree age and tree height is given by Brickell (2). This equation is: 

 1 1 



S = H + 2 b.X 



where 



S = site index at a 50-year base age 

 H = total tree height 

 A = total tree age 



and 



bi = 



0. 



10717283X102 



Xi = 



(In A - In 50) 



b2 = 



0. 



,46314777X10-2 



X2 = 



[(lO^O/A^) - 32] 



b3 = 



0. 



,74471147 ■ , 



X3 = 



H[(10'*/a2) - 4] 



b, = 



-0. 



,26413763X105 



X, = 



-2 5 -2 5 

 H (A • - 50 ) 



bs = 



-0. 



,42819823X10-1 



X5 = 



H (In A - In 50)2 



be = 



-0. 



,47812062X10-2 



H = 



h2[(10'+/a2) - 4] 



by = 



0, 



,49254336X10-5 



X7 = 



h2[(1010/a5) - 32] 



b8 = 



0, 



,21975906X10-^ 



Xs = 



h3[(101°/a5) - 32] 



b9 = 



5. 



,1675949 



Xg = 



H3[A-2-75_ 50-2. 75j 



bio= 



-0, 



.14349139X10-7 



Xl = 



H'+[ (100/A) - 2] 



bii = 



-9, 



,481014 



Xll = 



H^A"'-'- 50-'^-^]. 



The standard error of estimate (Syx) for this equation is 0.69 foot of site index units, 

 An example of how this equation might be programed in FORTRAN IV (IBM 360) is shown in 

 figure 8. 



13 



