Lignin obtained in acidic conditions is a waste product in various technological processes like sulfite pulping, organosolv pulping, or bioethanol production. Knowing the structure of the lignin enables its use in high-value-added applications. In this paper, the lignin structure isolated from Pinus sylvestris L. and Populus deltoides × maximowiczii wood in acidic conditions was investigated. Two methods of lignin isolation (Klason method and a method using a sulfuric and phosphoric acid mixture) were compared. Additionally, lignin acetylation was performed. The lignin samples were analyzed using different instrumental techniques, such as size exclusion chromatography (SEC), attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR), and scanning electron microscopy (SEM). Based on the studies carried out, it was found out that the lignin isolated from pine and poplar wood in acidic conditions had a highly condensed structure. This was evidenced by the high-weight average molar mass of lignin (up to 118,700 g/mol) and the precipitates, aggregates, and agglomerates on its surface. Moreover, the characteristic signals of condensed lignin in ATR-FTIR analysis (band with wavenumber of 767 cm⁻¹) and their decrease/disappearance (band that usually occurs with a wavenumber of about 814 cm⁻¹) were observed. Lignin acetylation and analysis in the 0.5% LiCl/DMAc system have proven particularly effective in the case of the condensed poplar lignin. The beneficial effect of lignin acetylation was confirmed by SEM analysis. The high-molecular-weight condensed lignin, despite some of its problematic properties connected mainly with solubility, is a valuable substance that can be used for different applications (carbon fibers or as an additive for thermoplastic blends), which was confirmed by the studies in this paper and the findings of other scientists.

Scots pine (Pinus sylvestris L.) exhibits substantial variation among provenances in growth and wood properties. This study investigated provenance-related variation after nearly six decades of growth under common-garden conditions and examined relationships between tree growth, wood properties and climatic characteristics of provenance origins. The study was conducted in a provenance experiment established in 1966 at the Rogów Forest Experimental Station in central Poland, comprising 16 provenances and five experimental replicates. Tree height, crown length, trunk volume, diameter at breast height (DBH), wood density, dynamic and static modulus of elasticity (MOE_dyn and MOE_st), modulus of rupture (MOR) and compressive strength (CS) were analysed. Provenance significantly affected all traits except MOE_dyn, whereas experimental replicate significantly affected most traits. Correlations calculated across individual trees were generally weak, while provenance-specific relationships varied substantially in strength and direction. At the provenance level, tree height and DBH showed the strongest associations with climatic characteristics of the provenance origins, particularly altitude, mean annual temperature, precipitation, aridity and growing season length. Selected mechanical properties also showed moderate climatic associations. The results demonstrate persistent provenance-related differentiation under common environmental conditions and indicate that growth performance alone cannot reliably predict wood quality. However, the climatic associations should be considered exploratory and do not constitute direct evidence of local adaptation.