Quantitative analysis of ring growth in spruce roots and its application towards a more precise dating

Missing and wedging rings are common features of tree growth. They occur more frequently in roots than in stems and were reported for various species and sites. These rather frequent irregularities in roots make dendrochronological analysis and cross-dating of roots more challenging. The goal of this study was to present a compiled method for a quantitative analysis of ring-growth irregularities. The analysis was conducted on ten spruce (Picea abies L. Karst) roots taken from the Gorce Mountains (Southern Poland). A four-step cross-dating of these root samples was applied. Three to six cross-sections were analysed within each root and cross-dated with a corresponding stem and site chronology. All ring-growth analyses were conducted on micro sections. Finally, the dating method was evaluated using three control indicators. The study revealed that wedging rings occurring in both, cross-sectional and longitudinal profiles were observed in 17.3% of the rings analysed. The application of a combined zig-zag segment tracing and serial sectioning allowed to significantly reduce, compared to previous methods, the amount of undetected missing rings and revealed them as cross-sectional or longitudinal wedging rings. Thanks to the application of control indicators the irregularities occurring in rings of roots were quantified and compared with different environmental factors such as droughts, air pollution, insect outbreaks and geomorphological processes. Significant positive correlation between root age and the number of radial growth irregularities in roots was demonstrated. A detailed investigation of multiple cross-sections per root enabled to trace all types of ring irregularities in the roots and substantially reduced cross-dating subjectivity.