Xylem and phloem formation in sessile oak from Slovenia in 2007.

We studied the timing of xylem and phloem formation, and the widths and structure of completed xylem and phloem increments in sessile oak (Quercus sessiliflora Salisb.) from Slovenia in the growing season of 2007. Weekly samples of inner phloem, cambium and xylem were analysed using light microscopy. Cambial reactivation and, consequently, xylem and phloem started before our first sampling at the end of March 2007. Weekly measured xylem and phloem increments revealed that phloem production was faster in the first part of the growing season (i.e., from the beginning of April until the third week of May), but thereafter xylem production accelerated. The period of most intense xylem cell production was assessed to be in the period April-May and that of phloem in April. Production of cells in the cambium stopped in the first half of August on xylem and phloem sides. The average width of the xylem increment of the investigated sessile oaks was 1837.55 ± 951.88 μm, phloem was 286.81 ± 232.04 μm and the ratio between phloem and xylem increments was 0.197 ± 0.118. Trees with wider xylem increments also had wider phloem increments, and vice versa. Lower ratios between phloem and xylem growth rings of 2007 were found in trees with wider increments, indicating that cambial cell production favours xylem (specifically late wood) formation in more productive trees. Inter-tree variability in xylem increment was relatively high (± 951.88 μm), whereas intra-tree variability was much lower (± 254.15 μm), but increased with the width of increment. The portion of late wood increased with the width of the xylem growth rings, whereby xylem increments narrower than 1000 μm consisted of more than 50 % of early wood but this portion gradually decreased to 30 % in wider rings.