Early response of phloem in naturally regenerated Norway spruce (Picea abies (L.) H. Karst.) after complete overstorey removal

Summary Removal of canopy over natural regeneration of Norway spruce is a common silvicultural practice. In response to release, several adjustments in the structure and function of above- and below-ground organs have been described in young spruce trees. However, changes in the anatomy of the conducting phloem have not been previously investigated. We conducted a study at two forest management sites in the boreal zone, comparing phloem traits in released and control (shaded) trees during the first two years after canopy removal. The trees ranged in age from 10 to 22 years old. Immediately after release, small phloem cells formed in treated trees, but phloem increments did not change, indicating a moderate level of water stress. In the second growing season after canopy removal, released trees formed larger phloem increments due to increased cell number and size compared to control trees. The above changes in phloem anatomy had a positive effect on the calculated values of phloem-specific conductivity, which may be related to the more active transport of photosynthates into the root systems of released trees. The effect was more pronounced in the variant with removal of the canopy of adult coniferous trees, compared to the variant of release from the canopy of young deciduous trees. The results are important for understanding tree responses to changing environmental conditions and for selecting the best silvicultural practices to promote the natural regeneration of Norway spruce.