Forest site productivity: a review of the evolution of dendrometric concepts for even-aged stands

Forest site productivity is the production that can be realized at a certain site with a given genotype and a specified management regime. Site productivity depends both on natural factors inherent to the site and on management-related factors. This review of the evolution of site assessment highlights three tenets of forest site productivity: the height–age site index, Eichhorn's rule and the thinning response hypothesis. These tenets rely on the hypotheses that height growth correlates well with stand volume growth, that total volume production of a given tree species at a given stand height should be identical for all site classes and that stand volume growth is independent of thinning practice for a wide range of thinning grades. The maturation of long-term field experiments has provided for the revision of these hypotheses, and contributed to an understanding of situations where they do not hold. This led to the introduction of the concept of yield level, the stand volume growth per unit of height growth. The use of the yield level theory for estimating site productivity has facilitated the development of a three-dimensional model of the relationship between stem number, quadratic mean diameter and stand basal area. Given this model, a stand density index based on the combination of stem number and quadratic mean diameter provides an indication of the yield level, which may be used to adjust height-age–based estimates of site productivity.