• The aim of this work was to examine the anatomy and functioning of secondary xylem in stems in relation to the decline of European ash (Fraxinus excelsior L.). • We tested the hypothesis that declining trees show changes in the structure of wood, which result in impaired water transport. • Anatomical analyses were carried out on wood samples (comprising all annual rings formed dur- ing the 30 years life of the analysed trees) collected at breast height from the main stem of healthy, weakened and dead ash trees. The width of annual wood increments, the diameter and density of earlywood vessels were measured and the theoretical hydraulic conductivity index through the sec- ondary xylem calculated by application of Hagen-Poisseuille’s formula. • Anatomical characteristics changed both with the age of trees and in response to unfavorable fac- tors. The largest vessels were observed in healthy trees, which implied that they had the highest hydraulic conductivity index, whereas trees considered to be in decline produce smaller vessels and hence had reduced conductivity.
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