The definition of a reference age, where the achieved
dominant height is defined as the site index has been frequently
discussed. There are also many examples with regard to selecting of the reference age for C. lusitanica. For instance
Pukkala and Pohjonen (1993) and Teshome and Petty (2000)
considered a reference age of 30 and 15 years, respectively, for
Ethiopia. Ngugi et al. (2000) considered a reference age of 15
years for Kenya, and Berrill (2004) considered a reference age
of 30 years for New Zealand. According to Teshome and Petty
(2000) too low an index age will give severely biased results as
the stands under consideration have not passed the juvenile
stage and too high an index age will lead to extrapolation of
most the observed variation of dominant height with age. It is
therefore deemed to be necessary to choose a reference age near
the average age to generate the most accurate prediction. The
reference age of 15 years for C. lusitanica growing at Munesa
Shashemene falls into the range of ages for all our permanent
plots.