Optimal rotation length of even-aged spruce forests is under discussion in Estonia. Thus far no single criteria is accepted to determine the rotation length. Moreover, the rotation length is affected by a repertoire of stand-level variables. A previous estimate of optimal rotation length was assessed through simulation of stand development by static and dynamic model components. However, such results depend on the model components added to the system that predicts the development. Here we investigate the cumulative diameter growth of Norway spruce in twelve stands throughout Estonia. Our analyses are based on actual tree-ring growth data, measured and rigorously validated to generate mean radial increment for each of the studied stands. Using the tree-ring data, we are not only able to estimate the time it takes for a tree to attain certain diameter that allows regulatory felling, but also to determine the optimal rotation length based on the assumption that the observed radial growth rate is a longtime variate that is specific to a given site. The interrelationship between the diameter growth and rotation length was used when estimating the optimal rotation length for each stand, that is, the lower the diameter growth the longer the optimal rotation length. Cumulative diameter growth showed a relatively rapid increase to a phase of maximum growth increment, with a subsequent decrease towards a phase of considerably slower growth. It appeared that the regulatory felling age of spruce stands yields a near-maximum sum of diameter growth in a variety of Estonian spruce stands. In specific circumstances, such as diverging regulatory and optimal cutting age, a reduction of rotation length may be justified.

Keywords: Norway spruce, felling age, diameter growth, growth increment, saw logs.