Cellular and topochemical characteristics of secondary changes in bark tissues of beech (Fagus sylvatica)

Anatomical and histometrical investigations were carried out on bark tissues (non-collapsed and collapsed phloem, and periderm) of beech (Fagus sylvatica L.) trees growing at a forest site near Ljubljana, Slovenia (400 m a.s.l.). Secondary changes in bark, especially sclereid formation and lignification, were followed at cellular and sub-cellular levels by light microscopy (LM), UV microspectrophotometry (UMSP), and transmission electron microscopy (TEM). The average width of the whole bark was 5960 mm; non-collapsed phloem on average occupied 4%, collapsed phloem 89%, and periderm 7% of the bark. Secondary changes in mature phloem were characterized by a collapse of sieve tubes, inflation of axial parenchyma, and development of sclereids. The percentage of sclereid areas and the stage of their development elevated with increasing distance from the cambium. The most pronounced increment in the sclereid proportion was observed in the second- and third-fifth of the bark. Sclereid cell walls were thick and poly-lamellated and had similar spectral characteristics but distinctly higher UV absorbance values than xylem fibers. By means of a combination of LM, TEM and UMSP techniques, the structure and secondary changes in the bark could be precisely identified