Estimation of phloem conductance at tree level in young, middle-aged and old-aged Scots pine trees growing in different climatic conditions in boreal forests.

This article investigates the variation in phloem conductance at the whole-tree level (Ktree) in Scots pine (Pinus sylvestris L.) across different age groups and climatic conditions in boreal forests of northwestern Eurasia. Using anatomical measurements of conducting phloem and theoretical calculations based on the Hagen–Poiseuille equation, the study found that Ktree remains stable in actively growing trees aged 25–80 years but increases significantly in old trees (180–190 years), likely reflecting shifts in carbon allocation between aboveground and belowground parts. Trees of the same age growing under different climatic conditions exhibited similar Ktree values due to coordinated changes in phloem anatomy and tree height, with tree height negatively affecting Ktree and phloem width (or trunk diameter) and sieve cell diameter positively influencing it. An exception was observed in young trees in the transition zone from northern taiga to tundra, where Ktree was highest within their age group. The findings highlight the interplay of tree ontogeny, anatomy, and environment in maintaining phloem transport capacity in boreal Scots pine.