Does elevated air humidity modify hydraulically relevant anatomical traits of wood in Betula pendula?

Key message: The xylem anatomical structure of Betula pendula is conservative regarding changes in atmospheric humidity, but it is plastic along the vertical canopy profile to optimise water supply among different foliage layers. We studied responses of wood anatomical traits in silver birch (Betula pendula) to artificially elevated air humidity to simulate a climate trend predicted for northern latitudes. The study was performed on saplings growing at Free Air Humidity Manipulation (FAHM) site, eastern Estonia, with the long-term mean relative air humidity (RH) increased by 7% over the ambient level. The humidification treatment did not significantly affect xylem traits or specific hydraulic conductivity, confirming a conservative nature of the wood anatomical structure with respect to air humidity. Only wood density decreased in response to elevated RH. The reduced atmospheric evaporative demand had a weak effect on the development of the plant water-conducting system in moderately humid climates in the northern temperate zone under conditions of ample soil water availability. The wood anatomical traits of both branches and main stems varied substantially with canopy position, with larger height effects in main stems. In main stems, vessel hydraulic diameter and theoretical (k) and specific conductivities (k) of the xylem decreased in the apical direction. In contrast, upper-crown branches had wider vessels and a bigger vessel relative area that resulted in k and k both increasing from the crown base upward. Vessel size and xylem hydraulic efficiency were positively associated with radial growth rate of stems.