Effects of climate variability on secondary xylem formation and anatomy in <i>Fagus sylvatica</i> trees grown in Denmark

Tree-ring width chronologies of Fagus sylvatica L. have been studied to understand the growth–climate relationship in Denmark, but there is little evidence of a clear climate signal in such tree-ring width data. In this study, tree-ring width chronologies of F. sylvatica trees growing in an even-aged stand in Denmark over the period 1972–2012 were supplemented with wood density and quantitative wood anatomy variables using the SilviScan-3 combined set-up. The effects of moisture availability and temperature variations on xylem features were investigated. Some xylem features showed stronger associations with drought index (DI) and temperature than ring width. Specifically, the September drought was associated with an increased mean area of the individual fibers in the ring formed the following year. Drought in October resulted in a higher density the following year, probably via increased fiber wall thickness and decreased areas of individual vessels. Further, late summer and early autumn warming increased the vessel density in the following year, but drought reduced their cross-sectional area. By dividing tree rings into ten segments, relationships between wood density and DI could be evaluated at the intra-annual level. In the third segment, wood density decreased with previous year December drought, while the April drought increased the wood density of this segment. Together, the results indicated that xylem formation and anatomy in F. sylvatica seem to be linked to moisture availability and temperature, and that dry and warm conditions affect xylem features differently depending on when they occur relative to wood formation.