Tree-ring structure determines the temporal coordination between xylem growth and the gain in hydraulic conductivity...

The study of seasonal xylem hydraulics has predominantly focused on embolism-induced losses, whereas growth-driven increases in hydraulic capacity have received little attention. We assessed the intra-annual dynamics of xylem formation and gain of conductivity in the current-year ring of three species with contrasting tree-ring structure, sessile oak (Quercus petraea, ring-porous), European beech (Fagus sylvatica, diffuse-porous) and Norway spruce (Picea abies, conifer), using micro-cores collected over 3 yr in north-eastern France. Ring conductivity loss (indicated by tylose-occluded vessels) was also examined in both angiosperms. Due to the differences in water-transport efficiency and cavitation vulnerability between earlywood and latewood vessels, oak presented distinct intra-annual dynamics of current-ring basal area increment (BAI) and the gain and loss of conductive area (CA), theoretical hydraulic conductivity (K_h) and specific hydraulic conductivity (K_s). Conversely, the intra-annual gain in BAI, CA and conductivity was largely coordinated in beech and spruce. Our results imply that the proportional gain in ring hydraulic capacity could potentially be estimated as a delayed BAI curve in diffuse-porous and conifer species, but not in ring-porous species; and the comparison of native conductivity measurements within or across species should account for their timing within the growing season, particularly in ring-porous species.