Disentangling intra-annual Pinus sylvestris growth responses to hydro-climatic conditions: Insights from quantitative wood anatomy in peatlands

Peatlands are vital ecosystems that regulate global carbon storage and hydrology, driven by waterlogged conditions that foster organic matter accumulation. However, disentangling the interactions between climate, hydrology, and tree growth in peatlands remains challenging. To investigate these relationships, we developed a 117-year ring width (RW) chronology (1902-2019) and complementary intra-annual quantitative wood anatomical (QWA) records for Pinus sylvestris from the Mycklemossen peatland, Sweden. Using QWA and principal component analysis, radial cell lumen diameter (D_rad) and cell wall thickness (CWT_rad) emerged as key indicators to understand links between climate, hydrology, and tree growth, due to their functional roles in water transport and structural stability. Compared to RW, QWA parameters demonstrated significantly higher sensitivity to hydro-climatic variability, revealing distinct intra-seasonal growth patterns. Earlywood D_rad correlated positively with prior growing-season temperatures (r = 0.52, March-August) and winter water table levels (WTL; r = 0.41, February-March), while latewood D_rad correlated negatively with autumn temperatures (r = -0.49, September-October) and WTL (r = -0.44, October-March). Latewood CWT_rad exhibited strong positive correlations with maximum temperatures (r = 0.66, March-May) and winter WTL (r = 0.50, November-March). Categorization of WTL and temperature into discrete classes revealed that both excessively shallow and deep WTL negatively impact xylem traits, while temperature showed linear effects, with high temperatures enhancing earlywood cell expansion and latewood cell wall thickening. These findings demonstrate the superior precision of QWA in capturing peatland tree growth responses to local (WTL) and regional (temperature) hydro-climatic variability, thereby enhancing our understanding of peatland ecosystem resilience to climate fluctuations and supporting conservation efforts in the face of global environmental change.