Effects of stem and pith eccentricity on the accuracy of basal area increment estimations

Julie Messier, Christina AM Rossi

Accurate tree growth quantification is crucial in ecology to assess tree growth. Basal area increment (BAI) is typically calculated from tree rings on increment cores, assuming trees are perfect circles with centered piths. However, trees often have pith offset and stem out-of-roundness, leading to estimation errors. Yet, we do not know how much estimation error results from these eccentricities. Using geometric principles that hold across all tree sizes, we quantified the effects of these eccentricities on BAI accuracy by comparing estimates from four calculation methods and varying core numbers (one to four) against true BAIs taken from cross-section scans.
Analysis of 109 cross-sections from 25 temperate species showed that with one core, pith eccentricity accounts for 21% of the error in BAI estimation, and stem eccentricity for 8%. Taking multiple cores, especially two-opposite cores, significantly reduces these errors, with four cores fully accounting for both eccentricities.
We recommend using multiple cores to minimize error, with two-opposite cores—taken uphill and downhill—being the most effective approach. We also provide methods for quantifying and reporting pith and stem eccentricity in the field, offering practical guidance for practitioners to calculate estimation errors based on their methods.