A close-up view of wood structure and properties across a growth ring of Norway spruce (Picea abies [L] Karst.)

A growth ring of an adult Norway spruce (Picea abies L] Karst.) was analyzed to a high resolution at the single cell level with respect to structural and mechanical changes during the growth period. For this purpose structural characterization was performed by means of light microscopy, scanning electron microscopy and wide angle X-ray diffraction for investigating the geometry of cells, their cell wall fractions and cellulose microfibril angles (MFA). The mechanical properties were determined in microtensile tests on individual tracheids which had been taken from sequentially cut tangential slices. The results revealed pronounced differences in tensile stiffness between earlywood and latewood cells but only minor differences in tensile stiffness between the cell walls of both tissue types. These comparatively small changes in cell wall stiffness across the growth ring were caused by slight changes in MFA. The findings suggest that trees mainly vary cell size to optimize water transport and mechanical stability during the growth period and that modification of the cell wall organisation plays a minor role.