Increment cores from the Finnish National Forest Inventory as a source of information for studying intra-annual wood formation

We tested, whether increment cores, routinely sampled and measured as a part of the Finnish National Forest Inventory (NFI), could be used for studying the progress of wood formation during growing season. Widths of incomplete current-year annual rings of the NFI cores were compared to respective measurements from small wood samples (microcores), taken from the same trees. Results of the two methods were rather similar. However, statistically significant differences between the two methods were found for trees, which had been cored near the time of the onset of radial growth. This was caused by cases, where current-year wood formation was observed from NFI cores, while no current-year tracheids were found from the microcores. The accuracy of the microcoring method was also tested by having a subset of the microcores measured by three persons. The results were consistent in the majority of cases. The results thus suggest that the NFI increment data do have potential for analyzing the timing and rate of wood formation during the growing season.