Climate and water-table levels regulate peat accumulation rates across Europe

PLOS One | https://doi.org/10.1371/journal.pone.0327422 July 23, 2025 4 / 16 aPAR: p = 0.0101; maximum aPAR: p = 0.0025; average aPAR: p = 0.0004). In contrast, relationships between aPAR and other climatic variables are ambiguous (Table S8 in S1 File, Fig 3, S2,5 & 6 in S1 File). We find that the highest aPAR values are found in the Scandinavia and Baltics region (S3) and that there is no discernible difference in aPAR between notable, continent-wide climate phases of the late Holocene, including the Little Ice Age (~1500– ~1850 cal. yr. CE), Medi- eval Warm Period (~950– ~1250 cal. yr. CE) and Roman Warm Period (~1– ~400 cal. yr. CE) (S4). We observe a weak negative relationship between average aPAR and average WTD (Theil-Sen beta = −0.31) (Fig 3). Spearman’s rank correlation confirms a weakly significant relationship (p = 0.0854) (S2). aPAR is generally lower under deeper water tables (higher WTD), although a degree of non-linearity is suggested through the presence of the highest aPAR in the 5–10 cm WTD range (Fig 3). It is notable that none of our sites has high average aPAR when WTD is deeper than ~11 cm, suggesting a hydrological threshold. To explore this relationship in greater detail we re-examined the data from the 1-cm thick layers. Fig 4 shows there is a clear distribution of accumulation rates in relation to WTD – the highest box median values of AR occur when the reconstructed WTD is in the 5–10 cm range, which is in alignment with the site average-based analysis (Fig 3). Application of a Loess Smoothing Model indicates an optimal WTD range between 7 and

Fig 1. Location of study sites. Site descriptions are provided in Table 1. https://doi.org/10.1371/journal.pone.0327422.g001