Age estimation of Norway spruce using incomplete increment cores: Testing new and improved methods

Information on tree age is often vital for dendrochronological studies, especially when the Regional Curve Standardisation technique is used. Several linear and non-linear methods of tree age estimation using partial increment cores (without the presence of pith) were evaluated and modified to provide more accurate estimations than are currently used. To achieve the objective, core samples from 142 Norway spruce trees (Picea abies [L.] Karst.) were collected from an altitudinal gradient in the Western Carpathians. The samples in which the pith was included were then used for further analyses. Several known age estimation approaches were adjusted to combine the advantages of direct increment- and indirect age-diameter-based methods. Inverse differential forms of non-linear growth functions were tested and proposed as a new advanced approach for age estimation. The results show that most of the modified linear methods achieved a mean square error of less than 10% when the length of the partial core exceeded 90% of the stem radius and less than 20% when the length of the core was at least 60% of the stem radius. Using an appropriate differential form of the non-linear growth functions, a mean square error of less than 20% was reached, even when the core length was shorter than 60% of the radius. The results show that current linear methods for age estimation can only be used if the missing part of the core sample is very short, with only a few rings absent. In the case of a large number of missing rings, a differential form of non-linear functions should preferably be used instead.