Annual and Intra-Annual Variation in Lignin Content and Composition in Juvenile Pinus pinaster Ait. Wood

This study investigated chemical variations in softwood juvenile wood, focusing on intra-ring variation between earlywood (EW) and latewood (LW) components along the radii. While no radial trends in lignin content and hydroxyphenyl/guaiacyl (H/G) ratio were found, the variation between EW and LW within a single growth ring was highly significant. Analytical pyrolysis demonstrated that earlywood contained, on average, 2.4% more lignin than latewood. Surprisingly, EW exhibited a lower H/G ratio (0.036) compared to LW (0.041), challenging the typical correlation between high lignin content and high H/G ratios. Principal component analysis (PCA) of the pyrolysis products confirmed distinct structural differences in lignin between EW and LW, reflecting their disparate functional roles—transport and mechanical support, respectively. Overall, analytical pyrolysis was highly effective for assessing the significant intra-ring variation in both lignin content and structural composition.