The explanation of 58% of the variation in Douglas-
®r site index observed in this study is quite satisfactory
as compared to values in literature. However, almost
half of the variation in site index remains unexplained.
Probably some factors are aliasing for the same key
growth factor (e.g. annual rainfall and water surplus
for actual moisture availability during the growing
season). In addition to this and to the inevitable measurement errors, main sources of unexplained
variation could be among the followings.