Similar importance of inter-tree and intra-tree variations in wood density observations in Central Europe

Abstract. Wood density is a crucial variable linked to mechanical, physiological, and ecological properties. In this study, we analyzed an extensive dataset of over 48,000 wood density samples collected from 2,920 trees. Our aim was to explore variations in wood density, at both inter-tree and intra-tree levels, along with the factors contributing to these variations. Inter-tree variations reveal significant differences in wood density among eight dominant species, highlighting their role in shaping wood density. As tree species exhibit specific spatial distributions associated with microhabitats, we anticipated a link between wood density distribution and microhabitat. Using a feature selection approach and random forest model, we identified six predictors, including satellite-based vegetation indexes, topographic variables, and soil sand content, capable of predicting 91 % of spatial wood density variations. The Normalized Difference Vegetation Index (NDVI) positively representing the amount of carbon within trees correlated with wood density, while the Normalized Difference Water Index (NDWI), reflecting water content, and soil sand content showed negative associations. Geomorphons and soil sand context provided insights into wood density variations and specific landforms. Lower wood density values were linked to landforms with low geomorphons (summit, ridge, or shoulder), whereas higher wood density was found in landforms with high geomorphons (valley, depression, or hollow areas). Furthermore, our study highlighted the importance of considering intra-tree variation, a facet often overlooked in previous research. Interestingly, the magnitude of intra-tree variation is comparable to, and in some species even exceeds, that of inter-tree variations. The intra-tree wood density samples display significant differences both vertically along the height and radially from the center to the bark zones of trees. These variations are influenced by tree growing strategy, living conditions, and physiological structure. In summary, our research delved into the multifaceted features of wood density, shedding light on critical aspects of this fundamental variable.