Effect of different stand densities on xylem and phloem formation in Norway spruce plantations

Summary Preliminary results of an investigation of the thinning effect on Norway spruce tree growth in terms of xylogenesis and phloemogenesis are presented. Three plots were selected where the stand densities were reduced by pre-commercial thinning to 1800 trees/ha (plot A; mild thinning) and 1300 trees/ha (plot C; heavy thinning) in February 2020. Plot B had no silvicultural intervention and represented a control variant (4500 trees/ha). Three dominant 14-year-old Norway spruce trees were sampled (micro-cored) for studying the xylem and phloem formation in the 2020 growing season. The total differentiation duration was determined to be around 169.7 ± 12.7–179.3 ± 4.0 days. The average number of xylem cells formed in control plot B was only 140 mature cells, plots C and A were determined to have 175 and 200 mature cells, respectively. Plot A had the fastest growth rate of all the plots studied. In all three plots (A, B, C), the beginning of the early phloem, late phloem sieve cells, and axial parenchyma cells coincided. Nonetheless, in terms of total phloem cell increase, plot C displayed the fastest growth rate among the three plots studied. The first results showed that the rate of total increment in both xylem and phloem cells, as well as the total number of produced tracheids and sieve cells, seem to be positively affected after the thinning application.