Tree‐ring structure determines the temporal coordination between xylem growth and the gain in hydraulic conductivity in the outermost ring

Summary 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.