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Our understanding of wood anatomy and radial growth in tree roots remains very incomplete, particularly with respect to how ecological factors affect root growth at a relatively small spatial scale, i.e., within a single root system. Here, we compared root growth with and without trampling exposure on a hiking trail. We conducted a quantitative analysis of radial growth and wood anatomical changes, including compression wood (CW) and blue rings (BRs), of two adjacent Scots pine roots in high resolution. A total of 32 cross sections from two roots sampled every 25 cm at the same distance from the respective stem were compared. The buried root (B) was completely buried and had an unexposed segment on a hiking trail. In contrast, the exposed root (E) had an exposed segment that was trampled. 1706 growth rings were analysed for the common period 1954–2015. We found that the volume of the E root in the trampling zone exceeded ten times the volume of the B root. The root surface area of the exposed sections of the E root was on average 14 times larger than that of the unexposed B root section in the trampling zone. The highest number of missing rings was found in the B root. Root sections sampled at the shortest distance from the stem showed the highest coherence in radial growth pattern, which decreased with increasing distance from the stem. BRs were recognized for the first time in tree roots. In total 25 tree rings contained BRs, and their occurrence was restricted to cross sections of the exposed root. BRs were formed over the course of 25 calendar years, i.e., in 40% of tree rings from the common period 1954–2015. Mean monthly temperatures for the years with and without BRs formation showed that colder November ( p = 0. 012) and, albeit only slightly, colder September ( p = 0.051) temperatures favoured formation of BRs in Scots pine roots. In addition, mean monthly precipitation in July ( p = 0.017) was significantly higher for BR years, suggesting an impact of moisture availability on the formation of BRs in Scots pine roots. The study highlights a high rate of growth discrepancies within a single root system. Further, altered growth of trampled roots with high proportions of BRs opens a new challenge for future dendroecological studies on tree roots.
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