The impact of seasonal variations in inner bark width and xylem growth on irreversible annual stem diameter measurements

To better interpret continuous xylem growth measurements from dendrometers, we evaluated how the widths of developing xylem and phloem, along with seasonal trends in inner bark width (conducting phloem, non-conducting phloem and the youngest periderm), contribute to irreversible annual stem diameter changes in Pinus nigra, Fraxinus ornus and Ostrya carpinifolia. Histometric analyses were performed on microcores taken from tree stems in 2020. The seasonal patterns of xylem and phloem formation, as well as changes in inner bark width and xylem growth, varied both within and among species. Of the total annual stem radial increment, xylem and phloem increments accounted for 40.5% and 8.1% in P. nigra, 9.6% and 2.5% in F. ornus, and 17.6% and 7.1% in O. carpinifolia, respectively. The remaining portion comprised inner bark tissues, with non-conducting phloem forming the largest fraction and thus exerting a major influence on variation in inner bark width. Despite seasonal variation in inner bark width, its width at the beginning and end of the growing season was similar across all species, indicating that subtracting inner bark width from the annual stem radial increment at the end of the growing season provides a reasonable estimate of annual xylem increment. However, seasonal changes in inner bark width differed from those of xylem growth, suggesting that xylem phenology cannot be reliably assessed without considering its ontogeny at the cellular level.