RESUMEN
An understanding of the structural organisation and chemistry of the cell walls in woody tissues is crucial from the perspective of plant mechanical strength, water transportability, as well as subsequent commercial utilisation of the wood. Poplar trees (Populus sp.), grown on two reclamation substrates ("Humus" and "Sand") under the extreme soil conditions of an external coal mining spoil heap of the lignite mine in Belchatów (Central Poland), were examined. Conventional parameters - tree-ring width (TRW) and wood density (WD) resolved annually (years 2008-2017) were corroborated by a novel approach of Raman spectroscopic analysis. Annually resolved Raman spectroscopic data representing the lignin-to-cellulose ratio (Li/Ce) enabled to estimate trends of lignification. The above traits were obtained for the three poplar genotypes: H-275, Grandis, and Androscoggin to assess the suitability of their plantation on the reclaimed heap. Our results show a significant effect of genotype on TRW, WD, and the Raman Li/Ce, while the effect of the soil substrate was less pronounced. The highest Li/Ce was identified in the H-275 genotype grown on a substrate with hummus. H-275 also showed higher TRW values compared to the other genotypes. WD was significantly higher in Grandis and Androscoggin genotypes grown on the "Sand" substrate. Associations between tree-ring parameters and climatic variables (temperature and precipitation) were mostly low and not statistically significant. Our findings from individual tree rings indicate that the genotype is the crucial factor influencing the lignification of poplar trees grown on post-mining lands.