Anatomical Barriers to Impregnation in Hybrid Poplar: A Comparative Study of Pit Characteristics in Normal and Tension Wood

Fast-growing hardwoods like poplar often lack natural durability in outdoor use and require homogeneous impregnation with protective agents, though achieving homo-geneity remains a known challenge. Various anatomical structures influence fluid transport in wood. This study compares pit characteristics in normal wood and tension wood of a hybrid poplar genotype, including both impregnated (with an aqueos, dye-containing solution) and non-impregnated regions, to identify anatomical barriers to impregnation. Light and scanning electron microscopy revealed significant differences in pit morphology and frequency in libriform fibres between normal wood and tension wood. In non-impregnated regions, pits were often encrusted. Cross-field pits did not differ between normal wood and tension wood but showed distinct differences between impregnated and non-impregnated regions: in the latter, pits were occluded by ty-lose-forming layers. Intervessel pits differed in border and aperture size between ear-lywood and latewood in both normal wood and tension wood. Hence, fluid transport is strongly impeded by occluded cross-field pits and, to a lesser extent, by encrusted fibre pits.