New Methods in Digital Wood Anatomy: The Use of Pixel-Contrast Densitometry with Example of Angiosperm Shrubs in Southern Siberia

Simple Summary A new method, pixel-contrast densitometry, that is, measurement of the wood density based on photographs of wood cross-sections, was introduced recently for conifers. We tested it for woody plants with other types of wood structure using eight species of shrubs and small trees in Southern Siberia. Two steps were proposed to create black and white images from original photos, where cell walls are black, and empty spaces are white. In these images, the proportion of empty spaces, porosity, was measured. For all species, five measurements per ring appeared to be similar, but measurements between rings formed in different years were different. A comparison of decades of measurements with examples of three species showed the presence of common inter-annual variation in wood porosity, particularly its maximum, mean, and minimum values. These easily measured characteristics register environmental variability during their formation. Directions for further research were also discussed.Abstract This methodological study describes the adaptation of a new method in digital wood anatomy, pixel-contrast densitometry, for angiosperm species. The new method was tested on eight species of shrubs and small trees in Southern Siberia, whose wood structure varies from ring-porous to diffuse-porous, with different spatial organizations of vessels. A two-step transformation of wood cross-section photographs by smoothing and Otsu's classification algorithm was proposed to separate images into cell wall areas and empty spaces within (lumen) and between cells. Good synchronicity between measurements within the ring allowed us to create profiles of wood porosity (proportion of empty spaces) describing the growth ring structure and capturing inter-annual differences between rings. For longer-lived species, 14-32-year series from at least ten specimens were measured. Their analysis revealed that maximum (for all wood types), mean, and minimum porosity (for diffuse-porous wood) in the ring have common external signals, mostly independent of ring width, i.e., they can be used as ecological indicators. Further research directions include a comparison of this method with other approaches in densitometry, clarification of sample processing, and the extraction of ecologically meaningful data from wood structures.