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& Key message The position of trees in the canopy impacts xylem structure and its inter-annual variation. After canopy release, the increase in the hydraulic conductivity of growth rings was driven byanincreasein radialgrowthin largetrees,andbybothan increase in radial growth and changes in xylem structure in saplings. & Context Forest canopies are frequently subjected to disturbances that allow understory trees to access the upper canopy. The effect of canopy release on xylem anatomy has been assessed in juvenile trees and saplings, while the potential acclimation of larger trees remains poorly documented. & Aims We estimated the potential hydraulic conductivity of growth rings in large understory trees compared to overstory trees, and evaluated the responses to canopy release in large trees and in saplings. & Methods We recorded radial growth, wood density, and vessel structure in beech trees according to their position within the canopy and their size. Xylem traits were followed during 6 years after canopy release for large trees, and during 2 years for saplings. Vessel diameter and frequency as well as ring area were used to compute the potential annual ring hydraulic conductivity. & Results Large understory trees displayed lower radial growth increments and lower potential annual ring hydraulic conductivity than overstory trees. After canopy release, potential annual ring hydraulic conductivity increased in large trees, due exclusively to increased radial growth without any change in specific hydraulic conductivity. It increased in saplings due to both increased radial growth and increased specific conductivity. & Conclusion Tree size impacted xylem structure and resulted in plasticity of the potential hydraulic conductivity of the annual tree ring following canopy release.

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Pas dans WOSIn the present study, we investigated the reaction of small diameter but old beech poles to canopy opening with particular interest in occurrence of growth stresses which allows the tree to maintain or correct its spatial position. We studied the relationships between growth stresses and (i) thinning treatment and (ii) anatomical structure. Forty-two beech poles were used for the study, half of which were thinned in 2007. We measured the growth stresses indicators (GSI) at eight positions around the trunk periphery and wood anatomical characteristics including proportion of G-fibers and vessel characteristics. Surprisingly, thinning treatment did not affect the average growth stress level and intensity of reaction in old beech poles. This rather unexpected result may be related to the high age of these trees and/or the high reaction wood occurrence prior to thinning resulting from the growth in suppressed condition. Considering the relationship between the proportions of G-fiber and the level of growth stresses, a significant positive correlation was found in agreement with previous studies on other species. Further, a negative correlation was found between vessel surface area and GSI level. Vessel frequency was also decreasing with the increasing GSI level and proportion of G-fiber