Hypothesizing that the shape of the height growth curve may
depend on site factors, not sufficiently represented by site index
(Splechtna, 2001), we tested if there are significant differences
between the coefficients k in Eq. (4), if the site index systems
are calculated separately for the three growth districts, Gambo,
Shashemene and Munesa (Fig. 1). Comparing the sum of
squared residuals around the individual site index systems with that around the common site index system for all three growth
districts together, resulted in an F-test for improvement of the
accuracy of the height growth predictions if individual site
index systems are used